What I Learned From Collecting Bones ...

[jpevahouse]

After I began collecting fossils of early mammals I found it very interesting that the basic skeletal anatomy of mammals hasn't changed very much in over 60 million years. I had always asumed that animals continued to evolve and over tens of millions of years natural selection would have produced diversity. But overall specific classes of animals seem to have evolved very early and despite tens of millions of years of changing environment have maintained an anatomical consistency.

Of course there are what I would call superficial changes but the basic anatomy which evolved at the end of the Cretaceous era has remained.

I have attached an example of astragalus dating from the Oligiocene (25 - 35 MYA) to the present era.

Some animals like the horse which evolved during this period show adaptation to environmental changes mostly in their feet and teeth. Even then there is an overall skeletal consistency with other mammals.

Edited August 22, 2014 by jpevahouse

[tmaier]

Read Stephen Gould's book "Wonderful Life". He discusses this subject and describes the the reduction of diversity in evolution. He uses the word "disparity" rather than "diversity" though. To put it in simple langauge, the tree is trimmed by random events and the remaining branches are just working off of pre-existing genetic traits, so what we see is a necking down of the pathways of diversity.

Diversity was maximum in the cambrian and has been following narrowing pathways ever since.

[Auspex]

Big changes in basic body plans should be the exception, and minor refinements should be the rule.

[jpevahouse]

Even though the examples I illustrate look alike they still tell us something. Size comparison tells us the hyracodon (one of the larger animals of the Oligiocene era) was smaller than a modern bison. The bison being smaller than other animals of it's time, like mastodon and mammoth, illustrates how much larger many animals became during the eras following the Oligiocene.

Seems mammals didn't need basic anatomical changes, just to be larger to survive. Mammals began small, became larger, then small again.

[jpc]

By the Oligocene, most mammals were fairly modern in skeletal appearances. Go backwards in time from there, and you get a little more variety. At the same time, except for a few obvious weirdos, (whales and bats) most mammalian postcranials from all of the tertiary are pretty conservative.

[siteseer]

The specimen identified as a Hyracodon astragulus belongs to something else (if Oligocene, maybe Archaeotherium?). Hyracodon was a rhino and had a "single-pully" shaped astragulus like horses and other perissodactyls. The bison, oreodont, and deer had the "double-pully" shaped astraguli - all artiodactyls.

After I began collecting fossils of early mammals I found it very interesting that the basic skeletal anatomy of mammals hasn't changed very much in over 60 million years. I had always asumed that animals continued to evolve and over tens of millions of years natural selection would have produced diversity. But overall specific classes of animals seem to have evolved very early and despite tens of millions of years of changing environment have maintained an anatomical consistency.

Of course there are what I would call superficial changes but the basic anatomy which evolved at the end of the Cretaceous era has remained.

I have attached an example of astragalus dating from the Oligiocene (25 - 35 MYA) to the present era.

Some animals like the horse which evolved during this period show adaptation to environmental changes mostly in their feet and teeth. Even then there is an overall skeletal consistency with other mammals.

[jpevahouse]

The issue with the hyracodon astragalus may be the difference between the hyracodon and subhyracodon which was a true rhino. These were two very different animals regardless of the similarity of their names. Reading suggest the hyracodon was the earliest ancestor of the rhino in North America. That doesn't necessarily make it a true rhino no more than a eohippus is a true horse.

I found examples of subhyracodon and hyracodon astragalus and they are quiet different.

[Harry Pristis]

siteseer posted Today, 05:43 PM

The specimen identified as a Hyracodon astragulus belongs to something else (if Oligocene, maybe Archaeotherium?). Hyracodon was a rhino and had a "single-pully" shaped astragulus like horses and other perissodactyls. The bison, oreodont, and deer had the "double-pully" shaped astraguli - all artiodactyls.

I think that Siteseer explained it quite succinctly: Hyracodon, like Subhyracodon, is a perissodactyl. Perissodactyls don't have 'double-pulley' astragali . . . they have 'single-pulley' astragali . . . just like horses. The bone in your image, Jpevahouse, is mis-identified.