Conclusion: What does the vertebrate fossil record show?
I've tried to present a reasonably complete picture of the vertebrate record as it is now known. As extensive as it may seem, this is still just a crude summary, and I had to leave out some very large groups. For instance, notice that this list mostly includes transitional fossils that happened to lead to modern, familiar animals. This may unintentionally give the impression that fossil lineages proceed in a "straight line" from one fossil to the next. That's not so; generally at any one time there are a whole raft of successful species, only a few of which happened to leave modern descendents. The horse family is a good example; Merychippus gave rise to something like 19 new three- toed grazing horse species, which traveled all over the Old and New Worlds and were very successful at the time. Only one of these lines happened to lead to Equus, though, so that's the only line I described. As they say, "Evolution is not a ladder, it's a branching bush."
A Bit Of Historical Background
When The Origin Of Species was first published, the fossil record was poorly known. At that time, the complaint about the lack of transitional fossils bridging the major vertebrate taxa was perfectly reasonable. Opponents of Darwin's theory of common descent (the theory that evolution has occurred; not to be confused with the separate theory that evolution occurs specifically by natural selection) were justifiably skeptical of such ideas as birds being related to reptiles. The discovery of Archeopteryx only two years after the publication of The Origin of Species was seen a stunning triumph for Darwin's theory of common descent. Archeopteryx has been called the single most important natural history specimen ever found, "comparable to the Rosetta Stone" (Alan Feduccia, in "The Age Of Birds"). O.C. Marsh's groundbreaking study of the evolution of horses was another dramatic example of transitional fossils, this time demonstrating a whole sequence of transitions within a single family. Within a few decades after the Origin, these and other fossils, along with many other sources of evidence (such as developmental biology and biogeography) had convinced the majority of educated people that evolution had occurred, and that organisms are related to each other by common descent.
Since then, many more transitional fossils have been found, as sketched out in this FAQ. Typically, the only people who still demand to see transitional fossils are either unaware of the currently known fossil record (often due to the shoddy and very dated arguments presented in current creationist articles) or are unwilling to believe it for some reason.
What Does The Fossil Record Show Us Now?
I think the most noticeable aspects of the vertebrate fossil record, those which must be explained by any good model of the development of life on earth, are:
1. A remarkable temporal pattern of fossil morphology, with "an obvious tendency for successively higher and more recent fossil assemblages to resemble modern floras and faunas ever more closely" (Gingerich, 1985) and with animal groups appearing in a certain unmistakable order. For example, primitive fish appear first, amphibians later, then reptiles, then primitive mammals, then (for example) legged whales, then legless whales. This temporal- morphological correlation is very striking, and appears to point overwhelmingly toward an origin of all vertebrates from a common ancestor.
2. Numerous "chains of genera" that appear to link early, primitive genera with much more recent, radically different genera (e.g. reptile- mammal transition, hyenids, horses, elephants), and through which major morphological changes can be traced. Even for the spottiest gaps, there are a few isolated intermediates that show how two apparently very different groups could, in fact, be related to each other (ex. Archeopteryx, linking reptiles to birds).
3. Many known species-to-species transitions (primarily known for the relatively recent Cenozoic mammals), often crossing genus lines and occasionally family lines, and often resulting in substantial adaptive changes.
4. A large number of gaps. This is perhaps the aspect that is easiest to explain, since for stratigraphic reasons alone there must always be gaps. In fact, no current evolutionary model predicts or requires a complete fossil record, and no one expects that the fossil record will ever be even close to complete. As a rule of thumb, however, creationists think the gaps show fundamental biological discontinuities, while evolutionary biologists think they are the inevitable result of chance fossilizations, chance discoveries, and immigration events.
Good Models, Bad Models (or, "The FAQ author rambles on for a while")
And now we come to the main question. Which of the many theories of the origins of life on earth are consistent with the known vertebrate fossil record, and explain its major features? I'll go back to the two main models I outlined at the beginning, creationism and evolution, and break them down further into several different possibilities. I'll try to summarize what they say, and whether or not they are consistent with the major features of the fossil record.
1. Evolution alone (with no God, or with a non-interfering God)
Evolution of all vertebrates by descent from a common ancestor, with change occurring both through punctuated equilibrium and gradual evolution, and with both modes of species formation (anagenesis and cladogenesis). These mechanisms and modes are consistent with (and in fact are predicted by) what is presently known about mutation, developmental biology, and population genetics According to this model, the remaining gaps in the fossil record are primarily due to the chance events of fossilization (particularly significant if evolution occurs locally or rapidly), in combination with immigration (the spreading of a new species from the site where it evolved out into different areas).
2. Evolution with a "Starting-gate God"
Evolution by common descent, as above, with God having set everything in motion in the beginning -- for instance, at the initial creation of the universe, or at the initial occurrence of life on earth -- and not having affected anything since.
3. Evolution with a "Tinkering God"
Evolution by common descent, as above, with God occasionally altering the direction of evolution (e.g., causing sudden extinctions of certain groups, causing certain mutations to arise). The extent of the "tinkering" could vary from almost none to constant adjustments. However, a "constant tinkering" theory may run into the problem that vertebrate history on the whole does not show any obvious direction. For instance, mammal evolution does not seem to have led inescapably toward humans, and does not show any consistent discernable trend (except possibly toward increased body size). Many lineages do show some sort of trend over time, but those trends were usually linked to available ecological niches, not to an inherent "evolutionary path", and the "trends" often reversed themselves when the environment or the competition changed.
Models 1, 2, and 3 are all consistent with the known fossil record.
4. Standard "young-earth" creationism
Creation of separate "kinds" in the order listed in Genesis, in six days, followed by a cataclysmic flood.
The Flood model is completely falsified, since the fossils appear in a different order than can be explained by any conceivable "sorting" model. Note that this is true not just for terrestrial vertebrates, but also for aquatic vertebrates, pollen, coral reefs, rooted trees, and small invertebrates. For example, ichthyosaurs and porpoises are never (not once!) found in the same layers; crabs and trilobites are never found in the same layers; small pterosaurs and equal-sized modern birds and bats are never found in the same layers. In addition, countless geological formations seem to be the result of eons of gradual accumulation of undisturbed sediment, such as multi-layer river channels and deep-sea sediments, and there are no indications of a single worldwide flood. In addition, the Flood Model cannot account for the obvious sorting by subtle anatomical details (easily explained by evolutionary models), or for the phenomenon that lower layers of lava have older radiometric dates. These are only a few of the problems with the Flood Model. See the flood FAQ for further information.
Creation in six "metaphorical" days is also falsified, since the animals appeared in a different order than that listed in Genesis, and over hundreds of millions of years rather than six days.
5. "Separately created kinds", but with an old Earth.
Literal creationism won't fly, but could the concept of "separately created kinds" still be viable, with the creations occurring over millions of years? This would require the following convoluted adjustments:
First, if every "kind", (species, genus, family, whatever) was separately created, there must have been innumerable successive and often simultaneous waves of creation, occurring across several hundred million years, including thousands of creations of now- extinct groups.
Second, these thousands of "kinds" were created in a strictly correlated chronological/morphological sequence, in a nested hierarchy. That is, virtually no "kind" was created until a similar "kind" already existed. For instance, for the reptile-to-mammal transition, God must have created at least 30 genera in nearly perfect morphological order, with the most reptilian first and the most mammalian last, and with only relatively slight morphological differences separating each successive genus. Similarly, God created legged whales before he created legless whales, and Archeopteryx before creating modern birds. He created small five-toed horse- like creatures before creating medium-sized three-toed horses, which in turn were created before larger one-toed horses. And so on. This very striking chronological/morphological sequence, easily explained by models 1, 2, and 3, is quite puzzling in this model.
Third, God did not create these kinds in a sequence that obviously progressed in any direction, as discussed briefly under model 3. This is not necessarily a fatal flaw (mysterious are the ways of God, right?), but it is another puzzle, another unexplained aspect of the fossil record.
Fourth, what about those species-to-species transitions? They appear to show that at least some species, genera, and families arose by evolution (not necessarily all, but at least some.) How can a creationist model be reconciled with this evidence?
1. "Minor" evolution allowed.
In this model, the species-species transitions DO represent evolution, but of a minor and unimportant variety. Note, however, that during these bursts of "minor evolution", the evolution took place in an apparently non-directed manner, sometimes crossed genus and family lines, and resulted in just the same sorts of morphological differences that are seen between the other, presumably created, groups of animals.
2. Separately created fossils.
In this model, the "species-species transitions" do not represent evolution. This implies that every individual fossil in the species-to-species transitions must have been separately created, either by creation of the animal that later died and was fossilized, or by creation of a fossil in situ in the rock. I have heard this model called the "Lying God Theory".
In summary, models 1, 2, and 3 (slightly different versions of basic evolutionary theory) are consistent with the fossil record, and go further to explain its notable features with a coherent overarching framework. Evolutionary theory has made successful predictions about fossils that were discovered later (e.g. the whale fossils), about genetic patterns, and about numerous other aspects of biology such as the development of disease resistance. Model 4 (literal young-earth creationism) appears unsalvagable, as all of its predictions are wrong. Model 5 (nonliteral creationism, with separately created kinds on an old earth) can just barely be modified to be consistent with the fossil record, but only with bizarre and convoluted tinkering, and only, apparently, if God created the world to make it look like evolution happened. In my humble opinion, this still utterly fails to explain the record's notable features or to make any useful or testable predictions. It also raises the disturbing question of why God would go to such lengths to set up the appearance of evolution, right down to inserting the correct ratios of radioisotopes in the rocks.
Okay, having blathered on about that, now I'll quit pontificating and get to the main point.
The Main Point
Creationists often state categorically that "there are no transitional fossils". As this FAQ shows, this is simply not true. That is the main point of this FAQ. There are abundant transitional fossils of both the "chain of genera" type and the "species-to-species transition" type. There are documented speciations that cross genus lines and family lines. The interpretation of that fact I leave up to you. I have outlined five possible models above, and have explained why I think some of them are better than others. You might disagree with my conclusions, and you can choose the one you think is best, (or even develop another one). But you cannot simply say that there are no transitional fossils, because there are.
As Gould said (1994): "The supposed lack of intermediary forms in the fossil record remains the fundamental canard of current antievolutionists. Such transitional forms are scarce, to be sure, and for two sets of reasons - geological (the gappiness of the fossil record) and biological (the episodic nature of evolutionary change, including patterns of punctuated equilibrium and transition within small populations of limited geological extenet). But paleontologists have discovered several superb examples of intermediary forms and sequences, more than enough to convince any fair-minded skeptic about the reality of life's physical geneology."
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