Transitional Fossils FAQ
By Kathleen Hunt

I've recently been re-reading Colbert's Evolution of the Vertebrates, and was reminded of the old "there aren't any transitional fossils" complaint that pops up on t.o. every now and then. That argument has long been obsolete and inaccurate, as a brief glance at the fossil record shows. I thought it might be of interest to have a list of some of the transitional vertebrate fossils known, so that future t.o.discussions of the fossil record can be somewhat more up-to-date and interesting (I can dream, can't I?).

A couple people have asked me to post this as a f.a.q. file to t.o. So here goes. First, I'll present a partial list of known transitional fossils, compiled from Colbert's Evolution of the Vertebrates (ref at end). Also at the end I have a short note about the significance of "transitional fossils".

The fossils mentioned in this list are from species and / or genuses thought to represent transitions from one vertebrate group to another. This list is necessarily highly incomplete, because:

a) I skipped entire sections of Colbert's text (rodents, bovids, dinosaurs, teleosts, and more).

b) Colbert's text is not an encyclopedic list of all known fossils, but instead has detailed descriptions of particular fossils that Colbert thought were representative of that group at that time, or that were otherwise of special interest.

c) Colbert's text is from 1980 and thus somewhat outdated. I've added in some recently discovered bird, whale, horse, and primate fossils. Please let me know of other recent discoveries.

Now, on to some of the classes of mammals.

So, there's a partial list of transitional fossils.

This really only scratches the surface since I left out all groups that have no surviving relatives, didn't discuss modern amphibians or reptiles, left out most of the birds, ignored the diversity in modern fish, didn't discuss the bovids or elephants or rodents or many other mammal groups.... I hope this gives a taste of the richness of the fossil record and the abundance of transitional fossils between major vertebrate taxa.

By the way, 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 talked about. Evolution is not a ladder, it's a branching bush.

And now, for those of you who are still with me...

I have a few comments about "transitional fossils" in general. 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 occured, and that organisms are related to each other by common descent. (Whether evolution occurs by natural selection, rather than by some other mechanism, is another question entirely and is the topic of current evolutionary research.)

Since then many more transitional fossils have been found. Typically, the only people who still demand to see transitional fossils are creationists who have been reading 100-year-old anti-evolution arguments, and who are either unaware of the currently known fossil record or are unwilling to believe it for some reason. When presented with a transitional fossil, such creationists often then want to see the transitions between the transitions - - or, as Pilbeam complained, "as soon as you find a missing link, you've just created two more missing links". Alternatively, creationists will often state that the two groups being bridged by the transitional fossil are really the same "kind" (a term that has no meaning in modern biology) and that therefore "real evolution" hasn't occurred. This often leads to a weasely backtracking in which no transitional fossil, however dramatic, no matter what disparate groups it connects, will ever be accepted by a creationist. Biologists justifiably find this attitude irritating, and any creationist taking this tack can expect to have testy biologists demanding that he/she clearly define "kind" before the discussion goes any further.

Creationists also sometimes say "All right, so you have a transitional fossil from X to Y -- but you don't from Y to Z!" It is unreasonable to expect the fossil record to be absolutely complete. It is highly unlikely for any organism to get fossilized, and to demand a perfect sequence of fossils of all species from all times and all locations, perfectly preserved in rocks that are not plowed under or eroded away, and not taken by private collectors and sold for thousands of dollars at some auction or used as a doorstop or a paperweight, but instead are exposed just as one of the few working paleontologists in the world happens to walk by -- well, we're lucky that the known fossil record is as good as it is. Remember that even if only ONE transitional fossil were known, it would be a tremendous support for evolutionary theory. (Thus the tremendous impact of Archeopteryx in 1861). We now know of HUNDREDS of transitional fossils. It is logically absurd to demand that a particular gap be filled, and if it can't be filled to then say that evolution has been falsified -- meanwhile ignoring all the gaps that have been filled.

I'll leave it at that. This has been a partial list of transitional fossils among some of the major taxa of vertebrates. This list has been brought to you by the numbers 1 and 7 and the letter E.


"Chinese bird fossil: mix of old and new". 1990. Science News 138: 246-247 [this fossil was described at the 1990 annual meeting of the Society of Vertebrate Paleontology, so there's probably a paper on it in the collected meeting papers.]

Colbert, E. 1980. Evolution of the Vertebrates, 3rd ed. John Wiley & Sons, New York.

Gould, S.J. 1983. Hen's Teeth And Horse's Toes. W.W. Norton, New York. [The title essay discusses evidence that some species retain old genes for traits that they no longer express -- teeth in chickens, side toes in horses. ]

Feduccia, A. 1980. The Age Of Birds. Harvard University Press, Cambridge, Mass.

Gingerich, P.D., Smith, B.H., Simons, E.L. 1990. Hind limb of Eocene Basilosaurus: evidence of feet in whales. Science 249:154.

The Lonely Bird. 1991. Science News 140:104-105. [an article on the controversy surrounding Protoavis. A monograph on Protoavis's skull was published in June 1991 in Phil. Trans. Royal Soc. London, if anyone cares; this was the first publication on Protoavis, which was found years ago but has been jealously guarded by its discoverer for some time.]

Milner, A.R., and S.E. Evans. 1991. The Upper Jurassic diapsid Lisboasaurus estesi -- a maniraptoran theropod. Paleontology 34:503-513. [this is the bird-like archosaurian reptile]

Sanz, J.L., Bonaparte, J.F., and A. Lacassa. 1988. Unusual Early Cretaceous birds from Spain. Nature 331:433-435. [This is about the Las Hoyas bird. Also see Science News 133:102, "Bird fossil reveals history of flight", for a brief synopsis.]

Horse references will be in horse post.

Marsupial references (suggested by Peter Lamb):

[1] Mervyn Griffiths, "The Platypus", Scientific American, May 1988 pp 60-67.

[2] Mervyn Griffiths, "The Biology of the Monotremes", Academic Press, New York a.o., 1978

[3] Terence J.Dawson, "Monotremes and Marsupials: the other Mammals", Arnold, London, 1983

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