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The Talk.Origins Archive: Exploring the Creation/Evolution Controversy

Transitional Vertebrate Fossils FAQ
Part 2A

Copyright © 1994-1997 by Kathleen Hunt

[Last Update: March 17, 1997]

Part 1B


Part 2B


Overview of the Cenozoic

The Cenozoic fossil record is much better than the older Mesozoic record, and much better than the very much older Paleozoic record. The most extensive Cenozoic gaps are early on, in the Paleocene and in the Oligocene. From the Miocene on it gets better and better, though it's still never perfect. Not surprisingly, the very recent Pleistocene has the best record of all, with the most precisely known lineages and most of the known species-to-species transitions. For instance, of the 111 modern mammal species that appeared in Europe during the Pleistocene, at least 25 can be linked to earlier European ancestors by species-to-species transitional morphologies (see Kurten, 1968, and Barnosky, 1987, for discussion).


Pleistocene 2.5-0.01 Ma Excellent mammal record
Pliocene 5.3-2.5 Ma Very good mammal record
Miocene 24-5.3 Ma Pretty good mammal record
Oligocene 34-24 Ma Spotty mammal record. Many gaps in various lineages
Eocene 54-34 Ma Surprisingly good mammal record, due to uplift and exposure of fossil-bearing strata in the Rockies
Paleocene 67-54 Ma Fair record early on, but late Paleocene is lousy

For the rest of this FAQ, I'll walk through the known fossil records for the major orders of modern placental mammals. For each order, I'll describe the known lineages leading from early unspecialized placentals to the modern animals, point out some of the remaining gaps, and list several of the known species-to-species transitions. I left out some of the obscure orders (e.g. hyraxes, anteaters), groups that went completely extinct, and some of the families of particularly diverse orders.


I'll outline here the lineage that led to humans. Notice that there were many other large, successful branches (particularly the lemurs, New World monkeys, and Old World monkeys) that I will only mention in passing. Also see Jim Foley's fossil hominid FAQ for detailed information on hominid fossils.

GAP: "The modern assemblage can be traced with little question to the base of the Eocene" says Carroll (1988). But before that, the origins of the very earliest primates are fuzzy. There is a group of Paleocene primitive primate-like animals called "plesiadapids" that may be ancestral to primates, or may be "cousins" to primates. (see Beard, in Szalay et al., 1993.)

The tarsiers, lemurs, and New World monkeys split off in the Eocene. The Old World lineage continued as follows:

GAP: Here's that Oligocene gap mentioned above in the timescale. Very few primate fossils are known between the late Eocene and early Oligocene, when there was a sharp change in global climate. Several other mammal groups have a similar gap.

GAP: There are no known fossil hominids or apes from Africa between 14 and 4 Ma. Frustratingly, molecular data shows that this is when the African great apes (chimps, gorillas) diverged from hominids, probably 5-7 Ma. The gap may be another case of poor fossilization of forest animals. At the end of the gap we start finding some very ape-like bipedal hominids:

Known species-species transitions in primates:

Phillip Gingerich has done a lot of work on early primate transitions. Here are some of his major findings in plesiadapids, early lemurs, and early monkeys:

And here are some transitions found by other researchers:


GAP: One of the least understood groups of modern mammals -- there are no known bat fossils from the entire Paleocene. The first known fossil bat, Icaronycteris, is from the (later) Eocene, and it was already a fully flying animal very similar to modern bats. It did still have a few "primitive" features, though (unfused & unkeeled sternum, several teeth that modern bats have lost, etc.)


GAP: few miacoid skulls are known from the rest of the Eocene -- a real pity because for early carnivore relationships, skulls (particularly the skull floor and ear capsule) are more useful than teeth. There are some later skulls from the early Oligocene, which are already distinguishable as canids, viverrids, mustelids, & felids (a dog-like face, a cat-like face, and so on). Luckily some new well-preserved miacoid fossils have just been found in the last few years (mentioned in Szalay et al., 1993). They are still being studied and will probably clarify exactly which miacoids gave rise to which carnivores. Meanwhile, analysis of teeth has revealed at least one ancestor:

From the Oligocene onward, the main carnivore lineages continued to diverge. First, the dog/bear/weasel line.



The transitions between each of these bear species are very well documented. For most of the transitions there are superb series of transitional specimens leading right across the species "boundaries". See Kurten (1976) for basic info on bear evolution.

Raccoons (procyonids):

Weasels (mustelids):

Pinniped relationships have been the subject of extensive discussion and analysis. They now appear to be a monophyletic group, probably derived from early bears (or possibly early weasels?).

Seals, sea lions & walruses:

Now, on to the second major group of carnivores, the cat/civet/hyena line. Civets (viverrids):



Species-species transitions among carnivores:


Lagomorphs and rodents are two modern orders that look superficially similar but have long been thought to be unrelated. Until recently, the origins of both groups were a mystery. They popped into the late Paleocene fossil record fully formed -- in North America & Europe, that is. New discoveries of earlier fossils from previously unstudied deposits in Asia have finally revealed the probable ancestors of both rodents and lagomorphs -- surprise, they're related after all. (see Chuankuei-Li et al., 1987)





GAP: No cavy fossils are known between Paramys and the late Oligocene, when cavies suddenly appear in modern form in both Africa and South America. However, there are possible cavy ancestors (franimorphs) in the early Oligocene of Texas, from which they could have rafted to South America and Africa. Known species-species transitions in rodents:

Part 1B


Part 2B

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