Among-group transitions in the fossil record

As pointed out in the species-species transitions page of this Website, a claim repeated endlessly by those seeking to deny evolution is the supposed absence of transitional forms or intermediates. Here we will examine just a few examples of coarse-scale transitions between groups. Several of these involve major changes in morphology of organisms. As was the case for the species-species transitions, these are not iron-clad PROOF, since we cannot genetically link the suggested intermediate form to its predecessors or it descendents. What these ARE, however, are excellent examples of what the anti-evolution writers claim do not exist: fossils that are intermediate forms in a coarse-scale transition.

Almost every anti-evolution treatise that I’ve encountered makes the “no transitionals” claim. Here are a few examples. From Muncaster’s Dismantling Evolution: “First, there are no true transitional species in the fossil record at all. Only fully formed fossils with similar appearances are thought by some biologists to be transitions.” (p. 82)…..”The lack of a single example of a real transition is evidence enough that the fossil record does not support evolution.” (p. 87). From Geisler & Bocchino’s Unshakable Foundations: “The truth of the matter is that the fossil record shows no evidence of transitional fossils and consequently does not accurately describe a large class of observations.” (p. 160).

Of course, if one wants to claim that there are no transitional fossils, then it is incumbent to declare what counts as a valid transitional fossil and what doesn’t -- especially given that a great many fossils have been declared exactly that by the relevant experts, the paleontologists. The anti-evolution writers follow one of two paths on the issue: The easier and more disingenuous path is to simply ignore the imperative to define what you claim is lacking; perhaps half of the anti-evolution writers make no attempt at all. The second path is to make an uninformed and biologically ignorant attempt to define what a transitional should look like; this path is followed by Ralph Muncaster in Dismantling Evolution, and by Geisler & Bocchino in Unshakable Foundations. Both examples reveal the authors’ utter lack of understanding of evolutionary biology. Muncaster repeatedly claims that a transitional should have “stubs for wings”, and then declares that fossils such as Archaeopteryx lithographica are not valid transitionals because they have fully formed wings instead of stubs. Similarly, Muncaster thinks that a transitional fossil between aquatic and terrestrial animals should have stubby proto-legs and thus dismisses any fossil that does not have the stubs. Geisler & Bocchino include some silly illustrations, and offer only the vaguest hints that “The fossil record should be filled with some type of combination of a fish in a transition stage as it is becoming an amphibian (say, fishibian), or a mixture of a reptile in transition as it is becoming a bird (say, reptibird).” (p. 156). This empty statement provides no basis for what to expect in a transitional; their useless illustration shows a chimeric creature whose front half is totally fish-like and whose back half is totally frog-like.

Often, the anti-evolution make a few poorly-informed attempts to explain away one or two of the most well-known transitional fossils. Muncaster’s treatment of Archeaopteryx is illustrative. He declares that Archaeopteryx “doesn’t fit the model of a true missing link, because all of its components are fully formed. Its wings are perfectly suited to flight, and the structure of its feathers is perfect to the smallest detail. Just because it contains some characteristics of different species means nothing….A true missing link should show partial development of something that appears fully formed later on.” (p. 88). What Muncaster seems completely ignorant of is that Archaeopteryx is but one of dozens of fossil species that fill a spectrum from entirely earth-bound and featherless dinosaurs, to modern birds. In this spectrum, Archaeopteryx indeed shows a number of morphological traits that are part-way between earlier dinosaurs and modern birds. What it -- and all of the other actual transitional fossils -- does NOT show are stubs or their equivalent, since the call for stubs seems to actually require an embryo as a fossil rather than a fully-formed adult organism.

If Geisler, Bochino, Muncaster, Rhodes and their colleagues understood a bit about evolutionary biology, they would realize that most new morphological features do not spring de novo out of nothingness; most of the time evolution tinkers with some existing structure to fashion something new out of something old. The wings of birds did not simply sprout where there had been nothing; they are derived from the forelimbs of dinosaurs. Further back in time, the limbs of terrestrial animals were derived from the pectoral and pelvic fins of lobe-finned fishes like Osteichthyes. Indeed, the very idea of homology of limb bones between dinosaurs and birds makes this abundantly obvious to someone who would take the time to examine a fossil. The profound level of scientific ignorance displayed by these writers is deeply troubling, particularly given the frequent claims that essentially all of the evolutionary biologists are wrong -- and this from people with no scientific training or experience whatsoever. In one of many jaw-dropping ironies, several of the anti-evolution writers make an effort to point out the large number of fossils that are known. Besides the fact that I’ve been unable to find any claim by knowledgeable experts about how many fossils exist in the museums of the world, we find claims like “…. …. …”; where Dr. Rhodes found the information for this claim is unclear. But even more stunning is the irony that the same authors that point out hundreds of thousands of fossil species existing, go on to say that NONE of them are transitionals. One has to wonder how many of these fossils Mr. Muncaster or Dr. Rhodes or Dr. Geisler has actually examined. I suspect that the answer is “zero”. Yet with literally no firsthand knowledge of any of the fossils, these writers sweepingly declare that NONE of them are transitional. Astonishing!

Now, with a bit of discussion behind us, let’s consider just a few among literally hundreds (perhaps thousands) of examples of transitional organisms. One particularly clear example is the recent description of a transitional fossil in the manatee lineage. Manatees are fully aquatic mammals, that like whales and dolphins, have lost their hind legs and live their entire lives under water. Prior to 2001, the fossil record was silent regarding their transition from terrestrial ancestors to an aquatic lifestyle. In 2001, Domining reported in the journal Nature on the first quadruped manatee ancestor; it was found in the West Indies, and was roughly 47-50 million years old; he named it Pezosiren. Not only does it have “terrestrial limbs”, but several other features are intermediate between those of terrestrial animals and modern manatees: the shape of vertebrae, rib structure, and pelvis. Surely if the anti-evolution writers had taken the time to look in the primary scientific literature, they would have found this striking intermediate. One gets the impression that Geisler, Bocchino, Muncaster, Rhodes and colleagues have not bothered to examine the one readily-available source of information on fossils -- the scientific literature!

A similar story exists in relation to a new discovery just reported in 2005. Until that time, the origins of lagomorphs (rabbits and hares) was unclear. The geneal opinion of the experts was that they derived from rodents, but there was no fossil evidence to support this. A major textbook on mammalian evolution (Kemp 2005) stated that the earliest clear lagomorphs appear in the fossil record 37-46 million years ago. Shortly after the publication of Kemp’s book, Asher and colleagues published their discovery of Gomphos elkema – a primitive lagomorph from 55 million years ago. Gomphos was a prime example of an intermediate: it had long hind legs and short front legs like a rabbit, but teeth & jaws that were more like a rodent. The tail was intermediate. Certainly this seems like a perfectly valid transitional form, yet the anti-evolution writers uniformly declare that such fossils either do not exist or are not valid transitionals.

Another clear example of a transitional fossil was reported in 2006 in the journal Science. Poinar and Danforth describe a fossil bee found in amber from 100 million years ago. The importance of this fossil lies in the fact that for many years, entomologists have thought that bees derived from primitive wasps -- but again, due to the scarcity of insect fossils, there was not fossil evidence to support this hypothesis. In their description of the fossil bee species Mellitosphex burmensis, Poinar & Danforth point out the numerous traits that are intermediate between wasps and bees -- the fossil was clearly of the appropriate age, and showed a transitional morphology. Yet another example of a fossil that the anti-evolution writers claim does not exist.

One of the newest powerful demonstrations of transitional fossils was just reported in February 2008 in the journal Nature by Nancy Simmons and colleagues. The fossil bat that they describe has been named Onychonycteris finneyi, and was found in 2003 in Wyoming. It is dated at 52 million years old, and it is the most primitive known bat. In fact, several features of Onychonycteris qualify it as intermediate between land-based ancestors of bats, and modern bats. The wings of Onychonycteris were shorter and broader than modern bats, indicating that while it could fly, it did not have flight capabilities to match modern bats. Its limb proportions -- forelimb and hindlimb lengths -- were intermediate between modern bats and terrestrial small mammals. It also had claws on all of its digits -- a trait no modern bats have, and most importantly, it lacked echolocation, the ability of modern bats to use aerial sonar to locate and catch insects. The graph at right shows an index (brachial index) of the ratio of forearm to upper arm length of all previously-known living and fossil bats (the circle at upper right) and a sample of land-dwelling mammals (circle at lower left), as well as Onychonycteris (the two red dots in the middle). In numerous respects, this fossil bat is an example of an intermediate form, and shows that achieved flight prior to their ability to echolocate.

Literature Cited

Asher, R.J., Meng, J., Wibble, J.R., McKenna, M.C., Rougier, G.W., Dashzeveg, D., and Novacek, M.J. 2005. Stem Lagomorpha and the antiquity of Glires. Science 307: 1091-1094.

Domning, D.P. 2001. The earliest known fully quadrepedal sirenian. Nature 413: 625-627.

Simmons, N.B., Seymour, K.L., Habersetzer, J., and Gunnell, G.F. 2008. Primitive early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451: 818-822.