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|Fish to Amphibian Transitional Forms
Copyright 1997 G.R.Morton.
This may be freely distributed as long as no change is made to the text and no charge is made.
Creationists claim that there are no transitional forms. This claim is made over and over as if it were a mantra. The plain fact is that there are transitional sequences but they never discuss the details. This is a sequence of fossils which occupy the transition from fish to amphibian.
378 MYR ago- Panderichthys--These are lobe-finned fish. Panderichthys was a rhipidistian,osteolepiform fish. The skull bones of these fish are bone for bone equivalents to the skull bones of the earliest tetrapods. (Carroll 1988, p. 160). These are the only fish whose fin bones fit the tetrapod pattern of humerus, ulna and radius in the forelimb and femur, tibia and fibula in the hindlimb. (Thomson, 1991, p. 488), Yet these limbs still have fins on them (Coates, 1994,p. 174). Their brain case is so much like that of the earliest tetrapod, they were originally classified as tetrapods until a complete skeleton was found. Then is was proven that they were really still fish. (Ahlberg and Milner, 1994, p. 508). This fish also had lungs and nostrils (Vorobyeva and Schulze, 1991, p.87) but also had gills. These things really looked like tetrapods until you see the fins. The teeth had infolding enamel which is identical to that of the earliest tetrapods. Unlike all fish but like the tetrapods, the Panderichthys have lost the dorsal and anal fins, leaving 4 fins in the place where legs would be in the Tetrapods.(Ahlberg and Milner, p.508). This contradicts Gish's claim that there is no fossil which shows loss of fins. (Gish, 1978, p. 78-79). Unlike fish, Panderichthys had a tail, like the amphibians with the fins stretched out along the top (Carroll, 1995, p. 389; Carroll, 1996, p. 19).
This is not a Panderichthys, but it is a related lobe-finned Devonian fish out of my personal collection. It gives some idea of what they looked like.
Panderichthyids and all other osteolepiform fish had a choana, a hole between the nasal passage and the mouth. This hole is missing in all other lobe-finned fish. It allowed air to pass from the nose into the mouth.. But Panderichthys also had external nostrils which were in the same position as those of the early tetrapods. (Schultze, 1991, p. 58). The lower jaws of panderichthyids had broad coronoids with fangs (Ahlberg 1991, p. 299)
370--Fish similar to Sauripterus. A very recent discovery in Pennsylvania by Daeschler and Shubin (1998, p. 133; Kinney, 1998) is of a fish which has fins, which is not unusual, except that inside of the fins were 8 fingers attached in a similar way to those of the earliest amphibians (see below). While many doubt that this creature is on the direct line of descent between fish and amphibians, the existence of fins with 'fingers' is illustrative of the fact that intermediate forms (broadly defined) do exist. Interestingly, as we shall see some of the earliest amphibians also had 8 digits on their hands.
368-Elginerpeton is a very primitive tetrapod found at Scat Craig, Scotland. Its lower jaw had coronoid fangs as did Panderichthys but they were smaller (Ahlberg 1991, p. 299). The very primitive limb bones found with it include an Ichthyostega-like tibia and an ilia and shoulder girdle comparable to the future Hynerpeton. There are no hands or feet found with the fossil so while the animal is quite tetrapod like in the parts which have been preserved, the final proof of its tetrapod status is missing. (Carroll, 1996, p. 19)
368 MYR- Obruchevichthys was found in Latvia and Russia but is only known from a partial mandible. The similarity between this mandible and Elginerpeton caused Ahlberg (1991) to reclassify this as a tetrapod. This creature also shows the coronoid fangs of the Panderichthys but they were also smaller than the panderichthyid fangs. Daeschler notes that this animal also has the parasymphysial fans of a tetrapod. (Daeschler, 2000, p. 307)
365-363 MYR -Hynerpeton-more advanced legs and pelvic girdle than Ichthyostega. (Carroll, 1996, p. 19) The coronoid fangs are not present. It lacked internal gills (Daeschler et al, 1994, p 641). There is no mention of feet having been found in Daeshler's report. The shape of the pectoral girdle implies both an aquatic and a terrestrial lifestyle.
365-363 MYR -Densignathus rowei--known only from the jaw but it is transitional between fish and amphibians. It has the parasymphysial fang of a stem tetrapod but also the coronoid fangs of a fish. As noted above Daeschler says this combination is also found in Obruchevichthys, Ventastega and Metaxygnathus. (Daeschler, 2000, p. 307). The earlier fish had a closed manidbular canal while the early amphibians had an open mandibular canal. Densignathus rowei is intermediate with a partially enclosed mandibular canal. Once again a transitional trait.
363 MYR-Ichthyostega-- Is the first animal with feet but the feet are different than most tetrapod feet. They are much like Acanthostega but has 7 digits on his hindlimb. His legs were only good for being in water. They could not support his weight. (Coates and Clack, 1990, p. 67) These are half evolved legs since they have more digits than the normal tetrapod but fewer bony rays than the fish and they are unable to support the weight. This contradicts Gish's statement that there are no half-evolved feet. (Gish, 1978, p. 79) . Ichthyostega had external nasal openings and a choana like that of the Panderichthys (Schultze, 1990, p. 35). He has lungs and gills. His tail was long with fins above and below like that Panderichthys and Acanthostega. (Carroll, 1992, p. 46). His legs were tetrapod having humerus, ulna and radius in the forelimb and femur, tibia and fibula in the hindlimb. (see diagram Carroll, 1992, p. 46).
363 MYR- Acanthostega- has four legs, lungs but still has internal gills. (Coates and Clack , 1991, p. 234) He has 8 digits on his front leg (see second picture below); seven on his back feet. (Carroll, 1995, p. 389) His legs could not support his weight either. (Coats and Clack, 1990, p. 66-67). Ahlberg (1991, p. 301) points out that the front legs were more fish-like than the back legs. He has fishlike lower arm bones (Coates and Clack 1990, p. 67). Once again, contrary to Gish (1978, p. 79), these are still half-evolved legs. He also retains a caudal fin (Coates, 1994, p. 175) and an elongated tail with fins stretched out along the top. (Carroll, 1995, p. 389). The stapes, the bone which eventually became part of the hearing apparatus in tetrapods was still used for ventilation of the gills (Clack,1989, p. 426).
Acanthostega served from http://www.sciencenews.org/Sn_arc99/5_22_99/bob1a.jpg
Reconstruction of Acanthostega gunnari is reproduced here by the kind permission of Dr. Jennifer Clack
One thing that the earliest tetrapods lacked were hands that could flex. We can curl our fingers and toes because of the arrangement of the tendons in our digits. None of the above tetrapods could do this simple trick because they lacked a notch in the flexor surface on the phalanges. Because of this, walking on a rocky surface, which requires the ability to curl the paws around various obstacles, would have been difficult for the early tetrapods. Acanthostega and Ichthyostega would only have been able to bend their hands slightly (Monastersky, 1999, p. 329). Thus, while they had hands, they were partially evolved hands.
It wasn't until the evolution of Casineria kiddi, that these notches are found on each phalange. (Paton et al, 1999, p. 512)
350 MYR ago. Pederpes finneyae- This creature was discovered at Dumbarton, Scotland. It has 5 toes on each foot with the exception of a small relict finger/toe on the forepaw. Because of this, this creature is transitional between the later amphibians and Acanthostega and Ichthyostega discussed above (Carroll, 2002, p. 35). This creature has a primitive stapes, the bone used in hearing and it resembles that of Acanthostega rather than those of the later amphibians. The expanded triangular flair on the ribs resemble those of Ichthyostega. (Clack, 2002, p. 74). But, unlike the early tetrapods this creature has a "clearly distinguishable metatarsals that are bilaterally and proximodistally asymmetric." (Clack, 2002, p.75). This is a trait which it shares only with the later terrestrially adapted amphibians. Thus, once again, this creature shows intermediate or transitional traits. Those who erroneously claim transitional forms don't exist, haven't looked at the data.
340 MYR ago. Fully evolved amphibians. Amniator, Crassigyrinus, Loxommatoidea, Temnospondyl, Colosteidae, Acanthracosauria.
ABC News http://www.abcnews.go.com/sections/science/DailyNews/link000405.html
Ahlberg,P. E. 1991, "Tetrapod or Near-tetrapod fossils from the Upper Devonian of Scotland," Nature, 354:298-301.
Ahlberg, P.E., 1995. "Elginerpeton pancheni and the Earliest Tetrapod Clade," Nature, 373:420-425.
Ahlberg, P.E. and Andrew R.Milner, 1994"The Origin and Early Diversification of Tetrapods," Nature April 7, 1994.
Carroll, Robert L. 1988, Vertebrate Paleontology and Evolution,(New York: Freeman).
Carroll, Robert L.,1992. "The Primary Radiation of Terrestrial Vertebrates," Annu. Rev. Earth Planet. Sci. 1992: 20: 45-84.
Carroll, Robert, 1995, "Between Fish and Amphibian", Nature, 373: 389-390.
Carroll, Robert L., 1966, "Revealing the Patterns of Macroevolution", Nature, 381,pp. 19-20.
Carroll, Robert L. 2002, "Early Land Vertebrates," Nature, 418:35-36.
Clack, J. A. 1989."Discovery of the Earliest-Known Tetrapod Stapes," Nature, 342:424-427.
Clack, J. A. 2002. "An Early Tetrapod from 'Romer's Gap'. Nature, 418:72-76
Coates M. I. and J. A. Clack, 1990.. "Polydactyly in the earliest Known Tetrapod Limbs," Nature, 347: 66-67
Coates and Clack, "Fish-like Gills and breathing in the earliest known Tetrapod," Nature, 352, July 18, 1991, p. 234-236
Coates,M.I., 1994. "The Origin of Vertebrate Limbs," Development 1994 Supplement, 169-180, p. 174
Daeschler, Edward B., et al, 1994, "A Devonian Tetrapod from North America," Science, 265:639-642.
Daeschler, Edward B., and Neil Shubin, 1998, "Fish with Fingers?" Nature, 391:133.
Daeschler, Edward B., 2000, "Early Tetrapod Jaws from the Late Devonian of Pennsylvania, USA,” J. Paleont. 74(2000):2:301-308, p. 307
Gish, Duane, 1978, Evolution: the Fossils say No! (San Deigo: Creation-Life Publishers).
Kinney, David, 1998, "Evidence of Finges in Fish?" AP wire, Jan 18, 1998
Monastersky, Richard, 1999.“Out of the Swamps,” Science News, 155:328-330.
Paton,R. L., T. R. Smithson and J. A. Clack, 1999. ”An Amniote-like Skeleton from the Early Carboniferous of Scotland,” Nature, 398:508-513, p. 512
Thomson, Keith Stewart 1991. "Where Did Tetrapods Come From?" American Scientist, 79(Nov/Dec 1991), p. 488-490, p. 488
Schultze, "Controversial Hypotheses on the Origin of Tetrapods," in _Origins of the Higher Groups of Tetrapods_, ed H.P. Schultze and L. Trueb, 1991, pp 29-67.
Vorobyeva and H.P. Schultze, "Description and Systematics of Panderichthyid Fishes with comments on Their Relationship to Tetrapods," in Schultz and Trueb, 1991. Origins of The Higher Groups of Tetrapods, Comstock Publ. Assoc., p. 68-109
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|Taxonomy, Transitional Forms,
and the Fossil Record
Keith B. Miller
Department of Geology
Kansas State University, Manhattan, KS 66506
The recognition and interpretation of patterns in the fossil record require an awareness of the limitations of that record. Only a very small fraction of the species that have lived during past geologic history is preserved in the rock record. Most marine species are soft-bodied, or have thin organic cuticles, and are essentially unpreservable except under the most extraordinary conditions. Furthermore, the destructive processes active in most marine environments prevent the preservation of even shelled organisms under normal conditions. Preservational opportunities are even more limited in the terrestrial environment. Most fossil vertebrate species are represented by no more than a few fragmentary remains. Because of the preservational biases of the fossil record, paleontologists must reconstruct evolutionary relationships from isolated branches of an originally very bushy tree.
The process of describing and classifying organisms introduces its own patterns into the taxonomic hierarchy. First, because organisms must be placed in one group or another, taxonomy gives the impression of discontinuity. Secondly, the placement of species into higher taxa is done retrospectively; that is, by looking backward through time. The evolutionary significance of particular morphologic transitions is only recognized because of the subsequent success of particular lineages. The defining characters of higher taxa are thus a consequence of history, and do not represent some objective scale of the magnitude of morphologic divergence. Closely-related species from two different higher taxa may actually be more similar in morphology than two distantly-related species belonging to the same group.
Because new character states are added over geologic time, the morphology of species within a higher taxonomic group becomes less divergent toward the point of origin of that group. In addition, species appearing early in the history of a taxon approach more closely the morphology of species from other closely related higher taxa, often to the extent that their taxonomic assignment is uncertain.
Transitional forms between higher taxa are thus a common feature of the fossil record, although continuous fossil lineages are rarely if ever preserved. Evidence from the fossil record is consistent with a wide range of proposed evolutionary mechanisms.
The fossil record provides persuasive evidence for macroevolutionary change and common descent. The pattern of appearance of fossil species through geologic time is critical for reconstructing evolutionary relationships. In addition, the fossil record may also contribute to our understanding of the tempo and mode of evolution, and help select between competing macroevolutionary theories.
However, before the fossil record can be applied to these questions, two critically important topics need to be addressed. The first concerns the completeness and resolution of the fossil record, and the second concerns taxonomic procedures. Taxonomy refers to the methods by which species are defined and grouped into a hierarchy of categories.
Nature of the Fossil Record
There are two opposite errors which need to be countered about the fossil record: (1) that it is so incomplete as to be of no value in interpreting patterns and trends in the history of life, and (2) that it is so good that we should expect a relatively complete record of the details of evolutionary transitions within most lineages.
What then is the nature of the fossil record? It can be confidently stated that only a very small fraction of the species that once lived on Earth has been preserved in the rock record and subsequently discovered and described by science. Our knowledge of the history of life can be put into perspective by a comparison with our knowledge of living organisms. About 1.5 million living species have been described by biologists, while paleontologists have catalogued only about 250,000 fossil species representing over 540 million years of Earth history (Erwin, 1993)! Why such a poor record?
Limits of the Fossil Record
Soft-bodied or thin-shelled organisms have little or no chance of preservation, and the majority of species in living marine communities are soft-bodied. Consider that there are living today about 14 phyla of worms comprising nearly half of all animal phyla, yet only one, the Annelida, has a significant fossil record. The inadequacy of the fossil record to preserve with any completeness the evolutionary history of soft-bodied organisms can be illustrated by the Conodonta. Originally assigned to their own phylum, they are now believed to belong to the cordates. These soft-bodied animals are represented by tiny tooth-like phosphatic fossils which are very abundant in sedimentary rocks extending over about 300 million years of Earth history, and have a worldwide distribution. Conodonts are a very important group of marine fossils for paleontologists, yet until only very recently the organism to which they belonged was completely unknown. Specimens of the worm-like conodont animal have now been discovered in Carboniferous, Ordovician, and Silurian rocks (Briggs et al., 1983; Mikulic et al., 1985; Aldridge & Purnell, 1996). Only a handful of specimens is now known from a very large and diverse group of marine animals known to be extremely abundant and widespread over a tremendous length of time!
The discovery of new soft-bodied fossil localities is always met with great enthusiasm. These localities typically turn up new species with unusual morphologies, and new higher taxa are built from a few specimens! Such localities are also erratically and widely spaced in geologic time between which essentially no soft-bodied fossil record exists.
Even those organisms with preservable hard parts are unlikely to be preserved under "normal" conditions. Recent studies of the fate of clam shells in shallow coastal waters reveal that shells are rapidly destroyed by scavenging, boring, chemical dissolution, and breakage. Rare events such as major storms appear to be required to incorporate shells into the sedimentary record. Getting terrestrial vertebrate material into the fossil record is even more difficult.
The limitations of the vertebrate fossil record can be easily illustrated. The famous fossil Archaeopteryx, occurring in a rock unit renowned for its fossil preservation, is represented by only seven known specimens, of which only two are essentially complete. Considering how many individuals of this genus probably lived and died over the thousands or millions of years of its existence, these few known specimens give some feeling for how few individuals are actually preserved as fossils and subsequently discovered. Yet this example actually represents an unusual wealth of material. The great majority of fossil vertebrate species are represented by only very fragmentary remains, and many are described on the basis of single specimens or from single localities. Complete skeletons are exceptionally rare. For many fossil taxa, particularly small mammals, the only fossils are teeth and jaw fragments. If so many fossil vertebrate species are represented by single specimens, the number of completely unknown species must be enormous!
In addition to these preservational biases, the erosion, deformation, and metamorphism of originally fossiliferous sedimentary rocks have eliminated significant portions of the fossil record over geologic time. Furthermore, much of the fossil-bearing sedimentary record is hidden in the subsurface, or located in poorly accessible or little studied geographic areas. For these reasons, of those once living species actually preserved in the fossil record, only a small portion has been discovered and described by science.
Because of the biases of the fossil record, the most abundant and geographically widespread species of hard part-bearing organisms would tend to be best represented. Also, because evolutionary change is probably most rapid within small isolated populations, species within rapidly evolving lineages are less likely to be preserved in the fossil record. In addition, the completeness of the fossil record improves up the taxonomic hierarchy (Erwin, 1993). A smaller proportion of once-living species is preserved than genera, of genera than families, of families than orders, etc. As a result we can better discern the general patterns of evolutionary change than the population-by-population or species-by-species transitions.
Potential of Fossil Record for Understanding Evolutionary Change
Given the limitations and biases discussed above, what should be expected from the fossil record? The situation is not as bleak as it may appear from my previous comments. Exceptional deposits, such as the Burgess Shale, Solnhofen Limestone, and Green River Shale, do provide surprisingly detailed glimpses of once living communities. These rare cases of exceptional preservation (fossil lagerstätten) are essentially snapshots in the history of life and are invaluable in gaining a more comprehensive picture of ancient communities. They also provide some of the most detailed anatomical data.
More commonly, thick sequences of fossiliferous rocks can enable selected skeleton or shell-bearing taxa to be examined at closely-spaced intervals. These localities provide opportunities to study patterns of evolutionary change within isolated lineages. Important information can be gained on morphologic change within species populations, and transitions between species and, rarely, even genera can be examined (Fig. 1). However, the time interval recorded by continuous series of closely-spaced fossil populations is limited because of changing environmental, depositional, and preservational conditions.
Figure 1. Changes in the shape of molar teeth of the Early Eocene mammal Hypsodus, showing evolutionary transitions from species to species within a genus. (From Gingerich , reprinted with permission of the American Journal of Science.)
Speciation events appear to take place primarily in small isolated peripheral populations. Therefore to catch a population "in the act" requires the fortuitous sampling of the particular geographic locality where the changes occurred. Even within well-preserved fossil series it is usually difficult to distinguish the record of speciation occurring within a particular depositional basin (or environment) from the effects of immigration of new species from outside that basin. For this and other reasons, well-documented and widely-accepted examples of speciation in the fossil record are few (for an example, see Gingerich, 1976).
The expectation, therefore, is for the preservation of isolated branches on an originally very bushy, evolutionary tree. A few of these branches (lineages) would be fairly complete, while most are reconstructed with only very fragmentary evidence (Fig. 2). While the details are missing, a general understanding of the large-scale patterns and trends in evolutionary history should be discernible. Evolutionary trends over longer periods of time and across greater morphologic transitions can be followed by reconstructing morphological sequences. Morphological transitions can be recognized in the fossil record that cross all levels of the taxonomic hierarchy.
Figure 2. The effects of an incomplete fossil record on the reconstruction of evolutionary relationships. (A) This branching tree (phylogeny) represents the actual pattern of evolutionary relationships. (B) The actual preserved record of species in the fossil record might look something like this. (C) This branching tree represents a possible reconstruction of the evolutionary tree based on the fossil evidence. Note that the general pattern of relationships is preserved, but that errors have been made with regard to specific ancestor-descendant relationships.
Taxonomy and Transitional Forms
Taxonomy, the process of classifying living and fossil organisms, produces its own patterns which order the diversity of life. It is thus important to recognize that names do much more than describe nature: they also interpret it. There is considerable ferment now within the field of taxonomy because of conflicting philosophies of classification, and different perceptions of which patterns in the history of life should be reflected in the taxonomic hierarchy (Eldredge & Cracraft, 1980; Schoch, 1986). Higher taxa can be either artificial groupings of species with similar morphologies (evolutionary grades), or "natural" groups sharing derived characteristics inherited from a common ancestor (monophyletic taxa or clades).
The Linnean classification system is hierarchical, with species grouped into genera, genera into families, families into orders, etc. This system reflects the discontinuity and hierarchy observed among living organisms. However, "this system leads to the impression that species in different categories differ from one another in proportion to differences in taxonomic rank" (Carroll, 1988, p. 578). This impression is false. Higher taxa are distinct and easily recognizable groups only when we ignore the time dimension of the history of life. When the fossil record is included, the boundaries between higher taxa become blurred during the major morphological radiations associated with the appearance of new higher taxa. Even in the modern world, discontinuity is not as great as it may appear superficially. In practice, species are often not easily recognized, and accepted species definitions cannot always be applied.
Another common misperception is that the origin of higher taxa does not take place at the level of populations and species. If the concept of common descent is accepted, then transitions between higher level taxonomic categories must also be species transitions (Fig. 3). This is recognized by all evolutionary paleobiologists, even those who stress the significance of the origin of phyla and classes (Valentine, 1992). Therefore, the more complete the fossil record of the origin and early radiation of higher taxa the more similar the transitional species, and the more difficult it is to determine their taxonomic assignments. Species placed into two different higher taxa may thus have very similar morphologies.
Figure 3. Pattern of phylogeny in which one clade (or higher taxon) emerges from another. In retrospect (time T2), the two clades are seen as being distinct, and the phylogeny is divided at the position of the heavy, dashed bar into taxa A and B. A taxonomist living at time T1, however, would have recognized only a single clade and would have grouped the entire phylogeny that had developed by that time into a single taxon (A). (From Macroevolution: Pattern and Process by Stanley © 1979 by W.H. Freeman and Company, used with permission. All rights reserved.)
The character states used to define higher taxa are determined retrospectively. That is, they are chosen based on a knowledge of the subsequent history of the lineages possessing those traits. They do not reflect the attainment of some objective higher level of morphologic innovation at the time of their appearance. Also, all the features subsequently identified with a particular higher taxon do not appear in a coordinated and simultaneous manner but as character mosaics within numerous closely-related species lineages, many of which are not included in the new higher taxon. In addition, as discussed above, the species associated with the origin and initial radiation of a new taxon are usually not very divergent in morphology. Were it not for the subsequent evolutionary history of the lineages, species spanning the transitions between families, orders, classes, and phyla would be placed in the same lower taxon (Fig. 3).
Based on the above discussion, a transitional form is simply a fossil species that possesses a morphology intermediate between that of two others belonging to different higher taxa.
Such transitional forms commonly possess a mixture of traits considered characteristic of these different higher taxa. They may also possess particular characters that are themselves in an intermediate state. During the time of origin of a new higher taxon, there are often many described species with transitional morphologies representing many independent lineages. It is usually very difficult if not impossible to determine which, if any, of the known transitional forms actually lay on the lineage directly ancestral to the new taxon. For this reason, taxonomists commonly have difficulty defining higher taxa, and assigning transitional fossil species to one or the other taxon. But, although the details may elude us, the patterns of evolutionary change are in many cases well recorded in the fossil record.
Examples from the Fossil Record
As stated above, the diversity of life appears much more discontinuous when viewed at any given point in time, than it does when viewed through time. For a given time slice through the tree of life, transitions between taxa are seen only where the slice intersects the branching points of lineages. Once a lineage is split, its branches continue to evolve and diverge such that their morphological (and genetic) distance increases and they become more readily distinguished taxonomic entities. When looking backward through time using the fossil record, it is found that representatives of different higher level taxa become more "primitive," that is have fewer derived characters, and appear more like the primitive members of other closely related taxa. As a result, for lineages with a good fossil record, the appearance of a new higher taxon is associated with the occurrence of species whose taxonomic identities are uncertain or whose morphologies converge closely on that of the new higher taxon. Such patterns are found repeatedly by paleontologists.
A longstanding misperception of the fossil record of evolution is that fossil species form single lines of descent with unidirectional trends. Such a simple linear view of evolution is called orthogenesis, and has been rejected by paleontologists as a model of evolutionary change (MacFadden, 1992). The reality is much more complex than that, with numerous branching lines of descent and multiple morphologic trends (Fig. 4). The fossil record reveals that the history of life can be understood as a densely branching bush with many short branches (short-lived lineages).
The well-known fossil horse series, for example, does not represent a single continuous evolving lineage (MacFadden, 1992). Rather it records more or less isolated parts of an adapting and diversifying limb of the tree of life. While incomplete, this record provides important insights into the patterns of morphological divergence and the modes of evolutionary change.
Figure 4. Comparison of a single direct line of descent (orthogenesis) with a branching phylogeny. Diversification is such an important feature of the history of life that orthogenesis is probably very rare. Fossils from a chronological series thus do not represent direct ancestor-descendant relationships, but individual branches. (From MacFadden , reprinted with permission of Cambridge University Press).
Interestingly, some critics of evolution view the record of fossil horses from "Eohippus" (Hyracotherium) to Equus as trivial (Denton, 1985). However, that is only because the intermediate forms are known (Fig. 5, 6). Without them, the morphologic distance would appear great. "Eohippus" was a very small (some species only 18 inches long) and generalized herbivore (probably a browser). Besides the well-known difference in toe number (four toes at front, three at back), "Eohippus" had a narrow elongate skull with a relatively small brain and eyes forward in the skull. It possessed small canine teeth, premolars, and low-crowned simple molars. Over geologic time and within several lineages, the skull became much deeper, the eyes moved back, and the brain became larger. The incisors were widened, premolars were altered to molars, and the molars became very high-crowned with a highly complex folding of the enamel (Evander, 1989; McFadden, 1988).
Figure 5. Fossil horse series from Hyracotherium ("Eohippus") to Equus showing changes in skull proportions associated with an adaptive shift from browsing to grazing. This sequence shows a chronological sequence of genera within the perissodactyl family Equidae from the Eocene to the Recent. (From MacFadden , reprinted with permission of Cambridge University Press).
Figure 6. Stages in horse evolution showing the reduction in the number of toes and foot bones. Forefeet above, hind feet below. (A) Hyracotherium, a primitive early Eocene horse with four toes in front and three behind, (B) Miohippus, an Oligocene three-toed horse, (C) Merychippus, a late Miocene form with reduced lateral toes, and (D) Equus.
(From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved.
This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)
The significance of the fossil record of horses becomes clearer when it is compared with that of the other members of the order Perissodactyla ("odd-toed ungulates"). The fossil record of the extinct titanotheres is quite good (Fig. 7), and the earliest representatives of this group are very similar to "Eohippus" (Stanley, 1974; Mader, 1989). Likewise, the earliest members of the tapirs and rhinos were very "Eohippus"-like. Thus, the different perissodactyl groups can be traced back to a group of very similar small generalized ungulates (Radinsky, 1979; Prothero, et al., 1989; Prothero & Schoch, 1989) (Fig. 8).
But this is not all; the most primitive ungulates (hoofed mammals) are the condylarths, which are assemblages of forms transitional in character between the insectivores and true ungulates (Fig. 9). Some genera and families of the condylarths had been previously assigned to the Insectivora, Carnivora, and even Primates (Romer, 1966). Thus, the farther you go back in the fossil record, the more difficult it is to place species in their "correct" higher taxonomic group. The boundaries of taxa become blurred.
Figure 7. Stages in the evolution of the extinct perissodactyl family of the titanotheres. (A) Eotitanops (early Eocene), (B) Limnohyops (middle Eocene), (C) Manteoceras (middle Eocene), (D) Protitanotherium (late Eocene), (E) Brontotherium (early Oligocene), and (F) Brontotherium. (From Stanley , reprinted with permission of the journal Evolution.)
Figure 8. Comparison of the early members of four perissodactyl families. (A) Hyracotherium (Equoidea), (B) Hyrachyus (Rhinoceratoidea), (C) Heptodon ("Tapiroids"), (D) Eotitanops (Titanotheriomorpha). (A and B from Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.) (C from Radinsky , and D from Stanley  both reprinted with permission of the journal Evolution.)
Figure 9. (A) The Eocene horse (Hyracotherium) and representatives of the condylarths, (B) Phenacodus (early Eocene) and (C) Mesonyx (middle Eocene). Note how very carnivore-like Mesonyx is although it possessed small hooves rather than claws and is classified with the ungulates. (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)
Moving further up the taxonomic hierarchy, the condylarths and primitive carnivores (creodonts, miacids) are very similar to each other in morphology (Fig. 9, 10), and some taxa have had their assignments to these orders changed. The Miacids in turn are very similar to the earliest representatives of the Families Canidae (dogs) and Mustelidae (weasels), both of Superfamily Arctoidea, and the Family Viverridae (civets) of the Superfamily Aeluroidea. As Romer (1966) states in Vertebrate Paleontology (p. 232), "Were we living at the beginning of the Oligocene, we should probably consider all these small carnivores as members of a single family." This statement also illustrates the point that the erection of a higher taxon is done in retrospect, after sufficient divergence has occurred to give particular traits significance.
Figure 10. Comparison of skulls of the early ungulates (condylarths) and carnivores. (A) The condylarth Phenacodus possessed large canines as well as cheek teeth partially adapted for herbivory. (B) The carnivore-like condylarth Mesonyx. The early Eocene creodonts (C) Oxyaena and (D) Sinopa were primitive carnivores apparently unrelated to any modern forms. (E) The Eocene Vulpavus is a representative of the miacids which probably was ancestral to all living carnivore groups. (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)
At the level of the class, the reptile/mammal transition is particularly well documented. Near the appearance of unquestioned mammals in the fossil record, a group of mammal-like reptiles called cynodonts included species that were exceptionally mammal-like in appearance (Hopson, 1994). In skeletal features the approach to the mammalian condition was almost complete (Fig. 11, 12).
The following mammalian characteristics were possessed by advanced cynodonts: (1) enlarged temporal openings with the loss of the post-orbital bar, (2) absence of the pineal eye, (3) differentiation of teeth, with front nipping teeth, canines, and molar-like back teeth, (4) a secondary palate permitting respiration while chewing, (5) a double occipital condyle which enlarges the hole for the spinal cord, (6) absence of lumbar ribs (possibly related to the presence of a diaphragm), (7) a nearly erect stance, and (8) an enlarged dentary bone in the lower jaw with an extremely close approach to the mammalian jaw articulation.
Furthermore, some workers argue persuasively that some mammal-like reptiles were endothermic (deRicqlés, 1974; Bakker, R.T., 1975; McNab, 1978). And a few exceptional fossils show evidence of glandular skin and horn (Hotton, 1991), features associated with the presence of hair.
Figure 11. Reconstructed skeletons of cynodont (advanced mammal-like reptiles) and early mammals. (A) The early Triassic cynodont Thrinaxodon and (B) the advanced cynodont Probelesodon from the middle Triassic. Note the very mammal-like erect posture of these skeletons. (C) The early mammal Megazostrodon from the early Jurassic. (All reconstructions taken from Carroll , A and C used by permission of Farish A. Jenkins, Jr., Museum of Comparative Zoology, Harvard University, and B used by permission of Arnold D. Lewis, Smithsonian Institution.)
Figure 12. Comparison of the skulls of cynodonts and early mammals. The cynodont skulls are (A) the late Permian Procynosuchus; (B) the early Triassic Thrinaxodon; (C) the middle Triassic Probainognathus; and (D) the early Jurassic Pachygenelus. Note the differentiation of the teeth and the reduction in the bones at the back of the lower jaw. The early mammal skulls are (E) the early Jurassic Sinoconodon; and (F) the early Jurassic Morganucodon. (A through D from "Systematics of the nonmammalian Synapsida and implications for patterns of evolution in synapsids" by J.A. Hopson , published in Origins of the Higher Groups of Tetrapods: Controversy and Consensus edited by H.-P. Schultze and L. Trueb. Used by permission of the publisher, Cornell University Press. This material is not to be printed or otherwise used without permission.) (E and F from Hopson  and used by permission of James A. Hopson.)
The complex of transitional fossil forms has created significant problems for the definition of the class Mammalia (Desui, 1991). For most workers, the establishment of a squamosal-dentary jaw articulation is considered one of the primary defining characters. The transition in jaw articulation from reptiles to mammals is particularly illustrative of the appearance of a class level morphologic character (Fig. 12). In reptiles, the lower jaw contains several bones, and the articular bone at the back of the jaw articulates with the quadrate bone of the skull. In mammals, the lower jaw has only one bone, the dentary, and it articulates with the squamosal bone of the skull. Within the cynodont lineage, the dentary bone becomes progressively larger and the other bones are reduced to nubs at the back. In one group of advanced cynodonts, the dentary bone has been brought nearly into contact with the squamosal, and in another, a secondary articulation exists between the surangular (another small bone at the back of the jaw) and squamosal (Hopson, 1991). The earliest known mammals, the morganucodonts, retain the vestigial lower jaw bones of the reptiles. These small bones still form a reduced, but functional, reptilian jaw joint medial to the new dentary-squamosal mammalian articulation. These reptilian jaw elements were subsequently detached completely from the jaw to become the mammalian middle ear (Crompton & Parker, 1978). Better intermediate character states could hardly be imagined!
As with most transitions between higher taxonomic categories, there is more than one lineage that possesses intermediate morphologies. Again, this is consistent with both the expectations of evolutionary theory, and the nature of the fossil record. The prediction would be for a bush of many lineages, many of which would be dead ends. Because of their objective to erect only monophyletic taxa (an ancestor is grouped with all of its descendants), some paleontologists have advocated including mammals with the advanced cynodonts, or even with the whole group of mammal-like reptiles, in a single higher taxon (Desui, 1991).
As in the case of the reptile-mammal transition, the distinctiveness of the classes also becomes blurred during the amphibian-reptile transition. The oldest known reptiles (Fig. 13) have been collected within the fossilized stumps of lycopod trees from the late Pennsylvanian in Nova Scotia (Carroll, 1970, 1991). Several groups of reptiliomorph amphibians occur near the appearance of these unquestioned reptiles. Some of these (the seymouriamorphs and diadectomorphs) were in fact previously regarded as reptiles (Carroll, 1988; Benton, 1991).
Figure 13. Skeleton and skull of the earliest known reptile Hylonomus from the early Pennsylvanian. Reptiliomorph amphibians placed in a group called the anthracosaurs converge closely on the reptiles in skeletal morphology (see reconstructions of the anthracosaur amphibians Bruktererpeton and Proterogyrinus in Carroll ). (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)
Fossil Transitions Associated with Major Adaptive Shifts
Of special interest in the history of life are the morphological transitions associated with the major adaptive shifts from water to land, land to water, and land to air. These major changes in mode of life opened up tremendous new adaptive opportunities for animal life. While the fossil evidence for some of these transitions is minimal, for others exciting parts of the puzzle have been uncovered.
The transition from water to land was one of the most significant events in animal evolution. Recent paleontological and systematic work has shed new light on this transition (Fig. 14). The most primitive amphibian yet known is the late Devonian Ichthyostega, a tetrapod with a flattened skull and bearing a tail fin. The limbs were until recently poorly known, but new fossil evidence has come to light. The hand, previously unknown, shows that these amphibians possessed seven to eight digits. The limbs also had a very limited range of movement and the animal was not as well adapted to terrestrial locomotion as previously thought (Ahlberg & Milner, 1994).
The rhipidistian fishes are widely considered to have given rise to the amphibians. One small group of late Devonian rhipidistians, the panderichthyids, appears to be closely related to the ichthyostegids (Schultze, 1991). These fishes have flattened skulls very similar to that of the early amphibians. In addition, the anal and dorsal fins are absent, and the tail is very similar to that of Ichthyostega (Vorobyeva & Schultze, 1991). The lobed pectoral and pelvic fins have bones that homologize with the limb bones of the tetrapods. Whether part of a single direct lineage or not, ichthyostegid amphibians and panderichthyid fishes are clearly transitional forms between class level taxa. The first known skull of a panderichthyid was in fact initially considered to be an amphibian (Vorobyeva & Schultze, 1991), again illustrating the taxonomic problems encountered during the appearance and early radiation of a new taxon.
Figure 14. The transition from fish to amphibian illustrated by body form and skeletons, with details of skulls and vertebrae. (A) Osteolepiform fish Eusthenopteron; (B) panderichthyid fish Panderichthys; and (C) labyrinthodont amphibian Ichthyostega. (From Ahlberg & Milner , reprinted with permission from Nature, copyright © 1994 Macmillan Magazines Limited, and from Per Ahlberg.)
Probably one of the most celebrated and mysterious transitions has been that of the origin of whales from a primitive condylarth (ungulate) ancestor. The earliest whales possessed skulls similar in many ways to those of a group of Eocene carnivorous condylarths called mesonycids.
Until 1993 the earliest fossil whales were only known from partial skulls with no postcranial material. However, several very important transitional fossils from Pakistan have been described over the last several years (Gingerich, et al., 1993) and more discoveries are certain to follow. The geologically oldest included enough of the skeleton to reveal that this otter-sized whale had short front limbs and longer hind legs with large feet apparently used in swimming (Berta, 1994; Thewissen, et al., 1994). The second, somewhat younger species had shorter hind limbs indicating a trend toward reduction in limb size (Gingerich, et al., 1994). Whales apparently evolved in what is now Pakistan since all the known fossil material for earliest whales has been found in that geographic area.
Because the evolution of new body plans is likely to occur in an isolated geographic area, the discovery of the fossil record of such transitions is dependent on the serendipitous sampling of the right locality.
The most famous of transitional fossils is the earliest known bird, Archeopteryx. Ostrum has described over 20 shared characteristics between Archeopteryx and coelurosaur theropods. Among these are: a theropod-like pelvis, the close similarities of the bones of the forelimbs including a swivel wrist joint, and the similarity of the hind limbs and feet with the presence of a reversed first toe (Hecht, et al., 1985; Dodson, 1985; Ostrom, 1994).
The similarities of Archeopteryx to theropod dinosaurs such as Velociraptor and Deinonychus are especially strong, and a newly discovered dinosaur called Unenlagia has features of the limbs and pelvis that are the most bird-like yet known (Novas & Puerta, 1997). As interesting as the similarities with the theropods are, the differences between Archeopteryx and modern birds are also significant: it has a long bony tail, a sternum is absent, its vertebrae are not fused together over the pelvis to form a synsacrum, and air ducts are absent in its long bones. In most respects, Archeopteryx is more of a flying feathered dinosaur than a bird.
In the last several years the discovery of new fossil birds from the Cretaceous has led to the erection of a whole new subclass of primitive birds called the enantiornithes (Chiappe, 1995). This new group includes several fossil species previously identified as theropod dinosaurs (e.g., Ornithomimus)! There are also some newly discovered fossils whose classification as theropod or bird is in dispute (Chiappe, 1995). The recent discovery in China of a theropod dinosaur with the possible preservation of fine feathers, even suggests that feathers may not be exclusively characteristic of birds (Morell, 1997). This again illustrates the taxonomic uncertainties that surround transitional forms.
From this brief survey of fossil vertebrates, it is clear that transitional forms between higher taxa are common features of the fossil record. The morphology of species within a higher taxonomic group becomes less divergent toward the point of origin of that group.
Morphological diversity and disparity increase with time. In addition, transitional species possess mixtures of morphologic characters from different higher taxa often to the extent that their taxonomic assignment is uncertain. This pattern is obscured by taxonomy which gives a false impression of discontinuity.
The fossil record thus provides good evidence for the large-scale patterns and trends in evolutionary history. Recognizing its limitations, the fossil record appears to be consistent with the wide range of evolutionary mechanisms already proposed. Any wholesale abandonment of present paradigms would be very premature.
Many critical gaps in our knowledge remain, but as evident from this review important discoveries are continually being made that intrigue, surprise, and enrich our understanding of the evolutionary history of life.
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Gingerich, P.D., Raza, S.M., Arif, M., Anwar, M., and Zhou, X., 1994, New whale from the Eocene of Pakistan and the origin of cetacean swimming: Nature, vol. 368, p. 844-7.
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"The man who follows is a slave. The man who thinks is free." Robert G. Ingersoll
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|This is just a sample of this exciting discovery.
Read the whole article at
“Scientists like Shubin, Gao, and Carroll say they are attracted to the study of salamanders because the amphibians give them a window to see how evolutionary mechanisms work.
Salamanders are more than 160 million years old, they have learned to live in a variety of environments, they have one of the largest genomes of any known animal, and researchers know quite a bit about their variation, said Shubin.
"Put all this together and it means we can understand how evolutionary changes to genes and development produce changes in anatomical features such as heads, limbs, tails," he said.
Of practical interest to humans, said Carroll, is the salamander's ability to regenerate limbs. This characteristic is unique among vertebrates. "It suggests the possibility that we may learn something of this capacity from salamanders that could be applied in the case of severe limb damage," he said”
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"The man who follows is a slave. The man who thinks is free." Robert G. Ingersoll
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|Here are some reptile to bird transitionals. There are obviously many thousands of transitional animals fish to amphibian, amphibian to reptile, reptile to mammal, reptile to bird. These are often called missing links.
(pronounces eye-BER-oh-mes-OR-nis) Iberomesornis (meaning "Iberian=Spanish intermediate bird") was a small, early, toothed bird that lived during the early Cretaceous period. It was capable of powered flight. It had tiny, spiky teeth in its beak and was the size of a sparrow. Its hip was primitive compared to modern birds; its ilium, ischium, and pubis were all parallel and directed backward. Iberomesornis was named by paleontologists Sanz and Bonaparte in 1992. Fossils were found in Spain. The type species is I. romeralli.
Ichthyornis (meaning "fish bird") were 8 inch (20 cm) long, toothed, tern-like, extinct bird that date from the late Cretaceous period. It had a large head and beak. This powerful flyer is the oldest-known bird that had a keeled breastbone (sternum) similar to that of modern birds. It lived in flocks nesting on shorelines, and hunted for fish over the seas. Ichthyornis was originally found in 1872 in Kansas, USA, by a member of paleontologist Othniel C. Marsh's Yale University expedition. Fossils have been found in Kansas and Texas, USA and Alberta, Canada. (Subclass Odontornithes, Order Ichthyornithiformes
Hespornis (meaning "western bird") was an early, flightless bird that lived during the late Cretaceous period. This diving bird was about 3 feet (1 m) long and had webbed feet, a long, toothed beak, and strong legs. Although it couldn't fly, it was probably a strong swimmer and probably lived near coastlines and ate fish. Fossils have been found in North America .
"The man who follows is a slave. The man who thinks is free." Robert G. Ingersoll
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Evolutionary series with emphasis on invertebrate transitionals
The Cambrian explosion
The Burgess Shale fossils
Biodiversity: marine vs. terrestrial
The animals kingdom consists of appproximately 35 phyla. A phylum is a group of animals with the same body plan: they share a
fundamental pattern of organisation. The main criteria we use for defining the different body plans are:
Unicellular (protista, not considered animals)
Multicellular (true animals)
Without tissues (Porifera)
With tissues (all other)
Triploblastic (all other)
Pseudocoelomate (Nematoda…but now changed to coelomate)
Coelomate (mollusc, annelid, athropod, chordate, echinoderm)
protosomes (annelida, mollusca, arthropoda) (blastopore: mouth
plus (nematoda, platyhelminthes)
deuterostomes (echinodermata, chordata) (blastopore : anus)
---body wall consists of two cell layers: ectoderm and endoderm
---body wall consists of three cell layers: ecto-, endo- and mesoderm ---triploblasty is associated with bilateral symmetry
---Without a coelomic cavity
---Body cavity lies between the mesoderm and endoderm
---Fluid-filled body cavity within the mesoderm
---Coelom allows gut to function independently of rest of body, provides place for organs
---Coelom formed by splitting; blastopore becomes mouth; cleavage is spiral
---Coelom formed by folding, blastopore becomes anus; cleavage is radial
"pseudocoelomate": now thought to be a degenerative coelom rather than an alternative structure.
The nine largest phyla and their general body plans are listed below:
( * means 'and here under").
EXAMPLES (incomplete list)
FEATURES (incomplete list)
---Hexactinellids (glass sponges)
---Demosponges (bath sponges)
---cellular level organisation
---Tissue level organisation
---Medusa and polyp
---Turbellaria (free-living flatworms)
---Organ level organisation*
---No circulatory system
---No circular muscles in body wall
---high pressure body fluid
---Closed blood system
---Trochophore larvae in marine forms
---millipedes and centipedes
---Metamerism and tagmatisation
---Open circulatory system (haemocoel)
---Trachea, gills, booklungs
---Gastropods (snails, nudibranchs)
---Bivalves (clams, mussels)
---cephalopods (nautilus, squid, octopus),
Monomeric (not segmented)
---Highly variable forms
---Open blood system
---Echinoidea (sea urchins)
---Holothuroidea (sea cucumbers)
---Pentaradial symmetry (secondary)
---Without true head
---Watervascular system with tube feet
---Calcareous ossicles or plates in skin
---Urochordates (tunicates = sea squirts)
---At some stage:
---hollow dorsal nerve cord
---gill slits in pharynx
Diversity and disparity: in this course, diversity will be used to mean the number of species (i.e. arthropods are the most diverse phylum). Disparity will be used to mean the range of forms (i.e. molluscs are the most disparate phylum). The Cambrian explosion is about phylum disparity (i.e. the increase in range of body forms at the phylum level / the increase in the number of phyla present).
THE CAMBRIAN EXPLOSION
Life began over 3500 million years ago. Animals (multicellular heterotrophs) originated 700 million years ago. The most important period in animal evolution was the Cambrian: 505 - 540 million years ago. It was during the Cambrian that most invertebrate phyla originated. The Cambrian explosion was the nearly simultaneous appearance of virtually all invertebrate phyla in a very short period of time (5-10 million years).
What was life like before the Cambrian?
There was animal life before the Cambrian. Pre-Cambrian animals are called the "Ediacaran fauna" (after the Ediacara hills in southern Australia where an Australian geologist (Spriggs) discovered pre-Cambrian fossils in 1946. There are several thousand fossil specimens that represent a number of marine animals: cniderian-like animals (of many body forms, some like modern jellyfish, others like stalked, branched fronds); platyhelminth-like animals (elongated, flattened worms), and annelid-like animals (segmented worms of various shapes and forms).
The Ediacaran fauna is very controversial. Some scientists say these animals were the ancestors to the Cambrian fauna, others say they went extinct before the Cambrian: "failed experiments" that left behind no ancestors. Some even say they weren't animals at all, but were elaborately shaped lichens. Most believe, however, that the Ediacaran animals were the ancestors of the Cambrian animals.
What was the earth like in the Cambrian?
It was quite different to our modern world. The continents were not well-separated and mostly spread out along the equator or in the southern hemisphere. Canada straddled the equator, Australia was in the northern hemisphere. Life only existed in the sea, the land was sterile and desert-like. The sea was just as salty as it is now, and the atmosphere was similar to ours (possibly a little less oxygen). The earth rotated a little faster, one year would have been about 400 days.
What was life like in the Cambrian?
It was very different to anything that came before it. In a short span of evolutionary time (5 - 10 Ma), there was enormous radiation in body forms. Suddenly there were animals with exoskeletons and hunters and scavengers. There were molluscs and echinoderms and early chordates and arthropods and just about all the modern-day phyla. There were some weird animals that are nothing like modern species. It is easier to say what was not present than to list what was there: there were no insects, vertebrates or plants (they all evolved later, on land) and there were no Bryozoans (they are the only phylum to evolve after the Cambrian).
Was the Cambrian explosion a real burst of disparity in body forms or was it merely a burst in fossilisation?
Did the sudden increase in invertebrate fossils result from the evolution of new phyla in the Cambrian, or does it reflect the evolution of hard parts that fossilised more easily? It is true that the Cambrian saw the start of exoskeletal parts that fossilised well. Maybe we see the huge increase in diversity and disparity because these hard parts suddenly show up in the fossil record, but in fact soft-bodied animals were present before this, so there was no real explosion in body forms or species.
It is unlikely that it was merely an increase in fossils because:
Soft-bodied animals can fossilise. The Ediacaran fauna (soft-bodied) fossilised, and the soft-bodied animals of the Cambrian fossilised. So why don't we find any of the Cambrian soft-bodied animals in the pre-Cambrian fossil beds? The Cambrian explosion shows an increase in both the hard-bodied and soft-bodied fauna.
There are no pre-Cambrian trace fossils (i.e. tracks, scratches, burrows etc., trace fossils are common and easily preserved) that would suggest the Cambrian animals were present, so it is unlikely that the animals were present but just didn't fossilise.
What caused the Cambrian explosion?
This is unknown and hotly debated. There were 200 million years of only simple animals then- BAM!- nearly the full spectrum of the present-day phyla were there. What was the trigger?
Maybe there was an environmental trigger, like a change in climate; but there is no evidence of this. Possibly there was a genetic trigger, like the evolution of neural tissue. [the evolution of nerve tissue may have opened the door to a whole range of alternatives, like cephalisation, bilateral symmetry, movement etc.]. But neural tissue didn't evolve in the early Cambrian, it was already present in the Ediacaran fauna (cniderians have nerve networks, platyhelminthes have nerves, ganglia, bilateral symmetry etc). So it's unlikely that this was the trigger for the huge radiation in forms.
Maybe there was a biological trigger. I find the most plausible explanation to be that the diversity of form was caused by the evolution of predation. There was no evidence of predation prior to the explosion, and lots of evidence after it: suddenly there are weapons and defence systems (e.g. shields, mimicry, camouflage, behavioural protection and predator deterrents). Selection pressures generated on predators as well as on prey could cause massive and rapid evolutionary changes. There is no doubt that predation played a major role in the evolution of the Cambrian animals, but whether it was the trigger for the diversification of the fauna is still debated.
What fossils do we have from the Cambrian period?
There are many Cambrian fossil beds, but most of them contain only hard parts (e.g. exoskeletons, spicules). There are three sites that contain both hard and soft bodied Cambrian animals:
The Burgess Shale in Canada
The Sirius Passet in Greenland
The Chengjian in China.
These three sites show us that about 95% of Cambrian animals were soft-bodied. Typical Cambrian fossil beds (i.e. those with only hard-part preservation) contain only 5% of the Cambrian fauna. The three sites with soft-bodied preservation are very important sites because it is from them that we get a picture of Cambrian life. The most famous (and first documented) of these sites is the Burgess Shale.
THE BURGESS SHALE FOSSILS
In 1909 Charles Walcott discovered the Burgess Shale fossils in the Rocky mountains on the west coast of Canada.
Walcott was a high-school dropout who:
headed the Smithsonian (the big USA Museum/Research institute); become a world-famous geologist; became a world-famous palaeontologist; is called the grandfather of the space age because of his work as the chairman of the Aeronautics National Advisory Committee, and was very famous in WWII because he played a big role in applying science to warfare and his ideas influenced how battles were fought.
During the Cambrian, the Burgess Shale site would have been equatorial. The animals lived in a warm, shallow sea on a submarine escarpment at the base of a limestone cliff that was built by reef-forming algae.
The area of the seafloor where they lived was not stable. It lies on a fault and has a lot of tectonic activity, as evidenced by the fact that it is now the Rocky mountains. The sea floor in that region was prone to "slumping". There were big fissures on the sea floor and slumping occurred at the fissures when there was any tectonic activity, earthquakes or even big storms.
During slumping, the sediment breaks off at a fissure and starts sliding downwards into deeper waters. The blocks of sediment break up on the way down and when it hits the bottom of the slump it is a huge cloud of fine-grained sediment. Obviously all the animals that were living on or in (or even above) the sediment would have been carried along in the slump, and they would have been entombed in the settling dust afterwards. It is thought that the animals would have died during the slide (they were preserved in all sorts of directions, some on their heads, some upside down etc).
During the slide, the animals would have undergone large changes in temperature, huge amounts of silt that would have suffocated them and a rapid decrease in oxygen. The bottom of the slump was a deep and anoxic environment in which no animals lived (which we know because the fossils sediments show no trace fossils, e.g. burrows, footprints, scratch marks).
The dead animals were protected from scavengers and decomposing bacteria because the water was anaerobic (devoid of oxygen).The silt was so fine that it separated the appendages of the little animals and resulted in very clear and detailed fossils. These freak conditions resulted in some of the loveliest and most well-preserved fossils we have.
Today the fossils occur in sheets of easily separated shale. There are thousands and thousands of specimens covering a period of millions of years. Most of the specimens are arthropods (37%). Trilobites are extremely common. There are representatives of all the major phyla we know today and there are also fossils of animals that are so weird we don't know what they are.
I have picked out 7 of the fossil animals to show you here:
Marella splendens: The most common Burgess Shale fossil. About 2cm long. A strange head shield with four long, backward sweeping spines. Cylindrical, segmented body with 25 somites, each with a pair of biramous appendages (jointed leg + feathery gill branch). It is blind as it has no eyes. It is an arthropod.
Hallucigenia sparsa: Bizarre animals with seven pairs of movable spines below a cylindrical body. Seven vertical tentacles dorsally, each terminating in a pincer-like appendage. Globular head. Rare fossil. Up to 3cm long.
Aysheaia pedunculata: caterpillar-shaped with a pair of anterior appendages and 10 pairs of short, conical legs. The trunk is annulated and the mouth is surrounded by finger-like projections. It resembles the modern-day Onycophoran (Peripatus) and has characteristics of both annelids and arthropods: annelid-like eye and muscular wall with thin cuticle; arthropod-like tracheal system and coelom. (Extant Onycophorans are terrestrial, Aysheaia was marine). Rare fossil. 1-6 cm long.
Wiwaxia: a slug-like animals covered in dorsal sclerites. Double row of spines. The ventral surface was naked. Jaw with a scraper-like structure, ventral muscular foot. Crawled along the surface. Rare fossil. 3-50 mm.
Anomelocaris: The genus name means "strange shrimp". A small shrimp without a head. Up to 20 cm long.
Peytoia: a strange jellyfish-like animals that looks like a tinned pineapple ring.
Trilobites: Tri (three) lobe (lobes): three lobes to their bodies. Arthropod: chitinous exoskeleton, jointed limbs, segmented bodies. Marine, benthic. Extremely common fossils, over 500 species. Most were a few cm long, but some got up to 70 cm.
An interesting observation about the trilobites: many specimens had bite marks, but just about all the bites are on the RHS, so either their was one predator (probably Anomalocaris) that was of a single handedness; or trilobites (all species) had a tendency to escape in the same direction when threatened).
Why is the Burgess Shale interesting?
It allows us to picture Cambrian life, to examine the early representatives of the invertebrates. It raises some big questions about evolution, like 'Would life today be totally different if we replayed the tape?'. It shows how science works: how mistakes are made and ideas develop. To look at these issues, we'll go through the historic sequence of events.
First, Walcott found the fossils in 1909. He did an excellent job of collecting, documenting and writing about them. What he did (and what any of us would have done in those days) is that he shoehorned each animal into a modern day phylum, without considering the possibility that there were different phyla in the Cambrian. Modern invertebrate phyla need not have been present in the Cambrian, and there may have been other phyla then that have since gone extinct. But Walcott fitted each specimen into one of our modern categories.
Nothing much happened with the Burgess Shale fossils after Walcott died. Then in the 60s and 70s, a team of three scientists from Cambridge decided to have a new look at them: Harry Whittington and two graduate students: Simon Conway-Morris and Derek Briggs. They restudied Walcott's fossils and excavated more from the Burgess Shale deposits. They made very detailed studies of the fossils, even dissecting out very fine layers of shale to see hidden parts. Their basic conclusions were that the fossils were far less like modern-day animals than Walcott had alleged. They listed many species as "from unknown phyla" and started coining new phyla for many of them.
In 1989, Steven Jay Gould wrote a book "Wonderful Life" that was all about the Burgess Shale fossils and what they tell us about evolution and life. The three heroes of his book were the 3 Cambridge scientists and he even suggested they get the Nobel prize for their exceptional work because it profoundly changed the way we look at the world. His book had two main points:
The Burgess Shale fauna are much more diverse than modern-day animals…there were far more phyla then, and only some of these body plans have survived to the present. So disparity can be represented as a cone: lots of species in Cambrian, and far fewer now.
Contingency: He says that more than half the Cambrian phyla are now extinct (due to mass extinction events) and that mass extinction kills off animals indiscriminately. The phyla that survived were no "better" than the phyla that died off…it is total chance whether an animal dies in a mass extinction (it doesn't matter how well adapted animal is…if its in the path of the comet, that's it). Contingency therefore plays a huge role in evolution. History is unpredictable due to the effect of random, chance events. If you played back the tape of life on earth, say from just after the Cambrian explosion, a totally different world would result. For example, what if the trilobites had not gone extinct…. maybe they would have become the dominant life form and invaded the land. The life we see around us today is largely due to a number of chance events, and if some small changes had been made, the world would be a very different place.
Gould's book was popular with scientists and non-scientists alike (although some scientists criticised his views from the start). It had a big impact on the way many people think about evolution. Since then, more work has been done on the fossils, other Cambrian fossil beds (with soft body preservation) have been worked, and ideas have been changing. In 1998, Simon Conway-Morris (one of the Cambridge Three) wrote a book "The Crucible of Creation" which turns every thing on its head once again. It gives a very different picture to Gould's. It is very critical of Gould (causing some heated mud-slinging). It takes a new look at the fossils as well as a new look at the conclusions to be drawn from them.
New look at the fossils:
Hallucigenia: It was originally interpreted by Conway-Morris as an animal that walked on 7 pairs of rigid spines. Along its back was a row of 7 tentacles, each terminating in a pincer-like appendage. It was unlike any animal ever described and Conway-Morris suggested it be put into a new phylum. But there was always this nagging problem of how it walks on rigid spines. Then scientists working at the Chengjian site in China found a specimen that was intermediate between Hallucigenia and an Onycophoran: it had shorter spines and a double row of tentacles. On re-examining the Burgess specimens, they found a second row of tentacles hidden underneath the body. So, if you turn the specimen upside down, it becomes a lobopod.
Anomalocaris and Peytoia: The headless shrimp and the pineapple-ring jellyfish were found to be parts of a single animal (now called Anomalocaris canadensis). The shrimps were the front appendages and the jellyfish was the mouth (entire specimen found by Whittington in 1981). Anomalocaris was probably the largest Cambrian predator (about 60cm, but the ones found at Chengjian are up to 2m long). Some had pieces of trilobites in their guts. They were probably the terrors of the Cambrian sea.
It turns out that virtually every specimen can be placed in a modern-day phylum. It's possible that not a single new phylum needs to be raised to account for the entire Burgess Shale fauna. Walcott got it pretty much right. For example, Walcott called Wiwaxia an annelid: a polychaete worm. Everyone laughed at that one…it was obvious that it was similar to a mollusc (like a chiton, radula, foot, plates etc) but didn't quite fit there, so must be a new phylum. Now recent work has looked at the sclerites and found that they are very much like polychaete parapodia with setae etc. It IS very similar to polychaetes as Walcott had said.
New molecular work has shown that annelids and molluscs are in fact quite closely related. Also shows that nematoda and arthropoda are closely related. Originally it was thought that annelids and arthropods were close and Onycophorans were their common ancestors…now not so sure about this. The relationship of annelids, arthropods, molluscs and nematoda is still unclear.
So it seems like it is NOT true that there were more phyla in the Cambrian than there are today. The diagram of disparity is not like a cone, but more like a very rapid increase in the number of phyla in the Cambrian, and then stayed more or less the same thereafter (there is only one new phylum that has evolved since the Cambrian….Bryozoa). This has major implications because it means that the phyla did not become extinct in mass extinctions, and so contingency has maybe not played such a major role.
Note that this refers to the phylum level. At the species level, contingency obviously plays a major role. Many species become extinct at mass extinctions, and it is a random pattern of extinction. But at the phylum level this appears not to be the case, all the original phyla survived the mass extinctions.
Everyone agrees that the number of species has increased since the Cambrian. There are probably twice the number of species now than there were in the Cambrian (event though there were so many big mass extinctions along the way).
It seems like the major body plans were established in the Cambrian and have remained very conservative thereafter. E.g. mollusca: huge radiation in species and lots of variety in shape, but still stuck to the basic body plan of unsegmented, triploblastic, protostome, coelomate with an open blood system etc. So there has been an increase in diversity (number of species) but not in disparity (number of body plans).
Conway-Morris' ideas about contingency follow this idea through: he says that if you play the tape of life over again, contingency would not have such a profound effect. You would arrive at a modern world not very different to this one. This is because ecology and anatomy allow for only so many solutions to the problems of life (like breathing, moving, eating, reproducing etc) and these are going to evolve along fairly predictable patterns. The "real" range of possibilities is actually quite restricted and animals will stumble upon the same solutions to problems many times. Examples:
The eye evolved independently in many groups.
Both placental and marsupial mammals produced a sabre-toothed large cat carnivore on separate continents.
Dolphins evolved from dog-like animals but they are now shaped just like a fish because that's the best shape to be in order to move through the water
Intelligence arose separately in octopus and humans.
Given a new beginning, the re-emergence of life forms has a basic predictability. The details may differ, but the basics will be the same. You will always get gut endoparasite, there will always be small flying insect-like things, there will always be fish-like things. The differences between the two worlds would be trivial.
One contingency may have been very important:
It would not be a trivial effect if the dominant, intelligent, powerful, so-called 'superior' life form in the parallel universe were incapable of destroying the planet the way we do. Maybe a chance event that prevented it from wreaking havoc would be enough of a contingency to change the entire future of life on earth.
Our understanding of the relationship between animals has come from detailed examination of their body plans. Animals with a common anatomical body plan are put into a single phylum. We use features like the number of cell layers, the type of symmetry, patterns of embryonic development. Recently there has been a huge surge in research and understanding of body plans and phylogenetic relationships because of the discovery of HOX genes in the 80s.
What are hox genes?
How does a single fertilised egg develop into a whole, functioning, complex animal? What tells each of the cells in the developing embryo that it must switch on some of its genes and switch off others so that it becomes the appropriate cell type? What is organising the highly controlled and complicated network of well-timed events during embryogenesis? The answer is: Hox genes. They orchestrate development. They determine where the head of the animal grows relative to the rest of the body, and control where the body parts are positioned. They are developmental genes that switch other genes on and off. They are called selector genes because their expression within a region of the embryo causes its cells to select a particular route of morphological development.
How do they work?
The hox gene makes a protein that binds to DNA in a way that turns the DNA (gene) on or off. Hox genes code for proteins called homeodomain proteins (60 amino acid long sequence). These are regulatory proteins that recognise and bind to specific sequences of DNA and switch these genes on and off at specific stages.
How many are there and where are they?
All animals have hox genes, from sponges to kangaroos to snails. Hox genes occur in clusters on the chromosomes. Different species have different numbers of hox genes. More complex animals have more hox genes. Invertebrates have a single cluster of hox genes on a single chromosome. Nematodes have 4 individual hox genes within the cluster. Flies have 8 hox genes in the cluster. In the vertebrates, the clusters have duplicated so that mice have 4 clusters (each on a separate chromosome) and within each cluster they have up to 10 individual hox genes. The more complex the body of the animals, the more hox genes are needed to control its development. Different hox genes control different parts of the body, e.g. one of the hox genes controls head development, another controls the abdomen.
What happens when they mutate?
When mutated, hox genes produce dramatic effects on the structure of the animal, such as replacing body parts with a structure normally found elsewhere. They are master genes that affect and control gene expression of many subordinate genes. Experimental changes in these genes therefore cause major alterations, like legs growing out of eye balls. Some human birth defects are thought to be caused by hox gene mutations, e.g. cleft palate. A drug (retino acid) which is used to treat acne, seems to interfere with hox genes and cause human birth defects.
What is unusual about hox genes?
They have two unusual properties: they show colinearity, and they are highly conservative.
Colinearity: Genes are usually in a totally random order on chromosomes and there is very little rhyme or reason in the order they appear. However, this is not true for hox genes. There is a strict correspondence between the order of the hox genes on the chromosome and the order of the body section they control. In the fruitfly, the 8 hox genes line up in a cluster along the chromosome, and each one affects a different part of the body. They line up in the same order as the parts of the fly they affected: the first gene controls the development of the mouth, the second the face, the third the top of the head, the fourth the neck, the fifth the thorax, the sixth the front half of abdomen, the seventh the rear half of abdomen, and the eighth controls various other parts of abdomen. We don't know why hox genes are colinear, but it may have something to do with the fact that animals develop from the front to the back. The colinear expression of the hox genes follows a temporal sequence, so maybe each hox gene switches on the next one in the line.
Conservative: All animals have hox genes and, amazingly, all hox genes are very similar to each other. If you compare a single hox gene (MSX gene) from sea urchins and mice: both of them consist of about 180 nucleotides, and both code for a protein of about 60 amino acids. The sequence of amino acids in the sea urchin and the mouse differ only in a single amino acid. The rest have remained the same over 600 million years of evolution. Hox genes are so amazingly conservative that geneticists do all sorts of weird things like remove a hox gene from a fly and replace it with the equivalent human hox gene and produce an absolutely normal fly. This conservatism has a major implication, it means that hox gene evolved only once and so all animals evolved from a single ancestor.
How can we use hox genes to understand phylogeny?
It is difficult to determine the relationship between the various phyla because the body plans appeared in a very short time and have remained essentially unchanged since then. The evolutionary information in most genes is lost by random mutations over this evolutionary time scale. Hox genes, however, evolved very early on and are very conservative. Since the hox genes are what regulate the body plan, their evolution has paralleled the stasis of the body plans they created. The original function of hox genes was probably to specify the fundamental anterior-posterior axis of early animals. Once such a system of control had evolved, it could be modified to produce new body shapes for the different animal phyla. The more complex body plans needed more hox genes [achieved through gene duplication within a single cluster (all invertebrates) and through cluster duplication (vertebrates)]. So one can examine the number of hox genes at various points on the phylogeny. We are starting to get some interesting ideas about the relationships between the phyla.
Arthropods and vertebrates:
An interesting point to emerge from Hox gene research is that arthropods and vertebrates are upside down versions of each other. Looking at two of the hox genes: one makes cells become part of the back and the other makes them become part of the belly. In arthropods and vertebrates, they are the opposite way around. The same hox gene has the opposite effect in arthropods and vertebrates. The common ancestor of the arthropods and vertebrates must have split into two forms, one that took to walking on its back and one that took to walking on its belly. One line became the vertebrates the other became the arthropods. Supporting evidence is from the fact that the central nervous system (CNS) of arthropods is ventral while the CNS of vertebrates is dorsal.
The big question obviously is: what the #%^@ happened to hox genes during the Cambrian explosion?
BIODIVERSITY: MARINE VS TERRESTRIAL
There is an interesting pattern in biodiversity between the marine and terrestrial environments. Most phyla are marine but most species are terrestrial.
There are about 35 phyla, 28 of these are marine (13 exclusively so).
Eleven phyla are terrestrial (1 exclusively).
We do not knows how many species there are, but definitely most species are terrestrial (there are more species of insects than of every other animals species combined.)
Why is there greater phylum diversity in the sea but greater species diversity on land? This is hotly debated.
Possibly there are more phyla in the sea because marine conditions make it possible for a greater range of body forms to exist (no constraints on drying out, support, temperature control, oxygen supply etc. For example, a cniderian body plan could never exist in a terrestrial habitat). All phyla evolved in the sea and the invasion of land was secondary, so only those body plans that were preadapted for life on land could conquer it.
Why fewer species in the sea? There are fewer barriers to gene flow in the sea so maybe less chance of speciation. Marine species tend to be more widely distributed, so if something wipes them out in one area, they can recolonise the area from the remainder of their range. Possibly the marine environment is less variable, more homogenous (? questionable).
Stephen Jay Gould (1989) Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton & Co., New York.
Simon Conway Morris (1998) The Crucible of Creation. Oxford University Press, Oxford.
Derek Briggs, Douglas Erwin and Fred Collier (1994). The Fossils of the Burgess Shale. Smithsonian Institution Press, Washington.
Conway Morris, S & Gould, S.J. (1998). Showdown on the Burgess Shale.
Natural History 12/98-1/99: 48.
Gould, S.J. (1992). The Reversal of Hallucigenia. Natural History 1/92: 12.
Bell, M. A. (1997) Origins of metazoan phyla: Cambrian explosion or proterozoic slow burn? Trends in Ecology and Evolution: 12(1): 1-2.
Zakany, J & Duboule, D. (1999) Hox genes and the making of sphincters.
Nature 401: 761-762.
Martindale, M. & Kourakis, M. (1999) Size doesn't mater. Nature 399: 730-731.
Dr. Samuel Gon III
Dr. Samuel Stubbs
http://www.bi.bbsrc.ac.uk/WORLD/Sci4Alll/Gaunt/Gaunt2.html (hox genes)
http://biology.uoregon.edu/classes/bi355f00/topics/topic13.00.html (hox genes)
Sorry it is so long. You can download it from the website at the beginning or print out this one.
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