Transitional Fossils Grouping Today's Creatures

The question here is not "can we find a change from one species to another ". This page takes the discussion up one level, and looks for fossil connections between dozens or hundreds of species. Obviously, the time spans are longer. We might be talking about 10 million years instead of one million: or perhaps 30 million instead of 3 million.

The Transitional Fossils FAQ lists a bunch of groupings - the bears, for instance. But there is a horse FAQ because we have such a huge collection of horse fossils, and because the pieces of the puzzle fit together so well.

Fifty five million years ago, there was an animal the size of a small dog, called Hyracotherium (sometimes incorrectly called Eohippus). Its front feet had four toes, and its back feet had three. Modern horse feet have a single hoof. So, are there transitional fossils that connect the two?

diagram of fossil horse feet The Horse Series

That suggests that the ancestors of the modern horse didn't have a hoof. So, do modern horses suggest any such ancestry?

diagram of fetal horse feet Horse Growth

That seems pretty clear. So, why have so many scientists been quoted as saying that something was wrong with the horse series?

They were actually complaining that the simple pictures are far too simple. It has in the past been shown as a "ladder of progress", a uniform and sure path from a starting point to an ending point. But in fact the horse family tree is enormous, and no one path through the tree is representative of all that went on. For example,

"Increase in size, for instance, did not occur at all during the first third of the whole history of the family. Then it occurred quite irregularly, at different rates and to different degrees in a number of different lines of descent. Even after a trend toward larger size had started it was reversed in several groups of horses which became smaller instead of larger."

Life of the Past, G.G. Simpson, Yale U. Press 1953. Page 125.

In fact, horse history was quite eventful, with four major migrations, and some mass extinctions. Also, the rate of evolution (measured in new species per million years) varied considerably. I don't have pictures of the transitional fossils - not because we don't have any, but because they are so numerous, and no one transition is the important one.
"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."

Evolutionary Biology, D. J. Futuyma, Sinauer Associates 1986, p.409

Last modified: 19 October 2000

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