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

Transitional Vertebrate Fossils FAQ
Part 2B

Copyright © 1994-1997 by Kathleen Hunt

[Last Update: March 17, 1997]

Part 2A


Part 2C


Known species-to-species transitions in lagomorphs:

Condylarths, the first hoofed animals

Within a few million years the condylarths split into several slightly different lineages with slightly different teeth, such as oxyclaenids (the most primitive), triisodontines, and phenacodonts (described in other sections). Those first differences amplified over time as the lineages drifted further and further apart, resulting ultimately in such different animals as whales, anteaters, and horses. It's interesting to see how similar the early condylarth lineages were to each other, in contrast to how different their descendants eventually, slowly, became. Paleontologists believe this is a classic example of how 'higher taxa" such as families and orders arise.

Says Carroll (1988, p.505): "In the case of the cetaceans [whales] and the perissodactyls [horses etc.], their origin among the condylarths has been clearly documented....If, as seems likely, it may eventually be possible to trace the ancestry of most of the placental mammals back to the early Paleocene, or even the latest Cretaceous, the differences between the earliest ancestral forms will be very small -- potentially no more than those that distinguish species or even populations within species. The origin of orders will become synonymous with the origin of species or geographical subspecies. In fact, this pattern is what one would expect from our understanding of evolution going back to Darwin. The selective forces related to the origin of major groups would be seen as no different than those leading to adaptation to very slightly differing enviromments and ways of life. On the basis of a better understanding of the anatomy and relationships of the earliest ungulates, we can see that the origin of the Cetacea and the perissodactyls resulted not from major differences in their anatomy and ways of life but from slight differences in their diet and mode of locomotion, as reflected in the pattern of the tooth cusps and details of the bones of the carpus and tarsus." (p. 505)

Species-to-species transitions among the condylarths:

Cetaceans (whales, dolphins)

Just several years ago, there was still a large gap in the fossil record of the cetaceans. It was thought that they arose from land-dwelling mesonychids that gradually lost their hind legs and became aquatic. Evolutionary theory predicted that they must have gone through a stage where they had were partially aquatic but still had hind legs, but there were no known intermediate fossils. A flurry of recent discoveries from India & Pakistan (the shores of the ancient Tethys Sea) has pretty much filled this gap. There are still no known species-species transitions, and the "chain of genera" is not complete, but we now have a partial lineage, and sure enough, the new whale fossils have legs, exactly as predicted. (for discussions see Berta, 1994; Gingerich et al. 1990; Thewissen et al. 1994; Discover magazine, Jan. 1995; Gould 1994)

In the Oligocene, whales split into two lineages:

  1. Toothed whales:
  2. Baleen (toothless) whales:

Perissodactyls (horses, tapirs, rhinos)

Here we come to the most famous general lineage of all, the horse sequence. It was the first such lineage to be discovered, in the late 1800's, and thus became the most famous. There is an odd rumor circulating in creationist circles that the horse sequence is somehow suspect or outdated. Not so; it's a very good sequence that has grown only more detailed and complete over the years, changing mainly by the addition of large side-branches. As these various paleontologists have said recently: "The extensive fossil record of the family Equidae provides an excellent example of long-term, large-scale evolutionary change." (Colbert, 1988) "The fossil record [of horses] provides a lucid story of descent with change for nearly 50 million years, and we know much about the ancestors of modern horses."(Evander, in Prothero & Schoch 1989, p. 125) "All the morphological changes in the history of the Equidae can be accounted for by the neo-Darwinian theory of microevolution: genetic variation, natural selection, genetic drift, and speciation." (Futuyma, 1986, p.409) "...fossil horses do indeed provide compelling evidence in support of evolutionary theory." (MacFadden, 1988)

So here's the summary of the horse sequence. For more info, see the Horse Evolution FAQ.

GAP: There are almost no known perissodactyl fossils from the late Paleocene. This is actually a small gap; it's only noticeable because the perissodactyl record is otherwise very complete. Recent discoveries have made clear that the first perissodactyls arose in Asia (a poorly studied continent), so hopefully the ongoing new fossil hunts in Asia will fill this small but frustrating gap. The first clue has already come in:

SMALL GAP: It is not known which Merychippus species (stylodontus? carrizoensis?) gave rise to the first Dinohippus species (Evander, in Prothero & S 1988).

Compare Equus to Hyracotherium and see how much it has changed. If you think of animals as being divided into "kinds", do you think Equus and Hyracotherium can be considered the same "kind"? Tapirs and rhinos:

Species-species transitions:


GAP: Here's that Oligocene gap again. No elephant fossils at all for several million years.

Meanwhile, the elephant lineage became still larger, adapting to a savannah/steppe grazer niche:

The Pleistocene record for elephants is very good. In general, after the earliest forms of the three modern genera appeared, they show very smooth, continuous evolution with almost half of the speciation events preserved in fossils. For instance, Carroll (1988) says: "Within the genus Elephas, species demonstrate continuous change over a period of 4.5 million years. ...the elephants provide excellent evidence of significant morphological change within species, through species within genera, and through genera within a family...."

Species-species transitions among the elephants:

Sirenians (dugongs & manatees)

GAP: The ancestors of sirenians are not known. No sirenian-like fossils are known from before the Eocene.

Part 2A


Part 2C

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