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I am a politically-progressive, ethically-herbivorous anthropoid pursuing a paleontology education in the Los Angeles Basin. I am largely nocturnal, have rarely been photographed, and cannot thrive in captivity.

24 July 2010

Synapsid Song: The History Of Your Teeth, Part III

I'd like you to meet your cousin, several-million-times-removed, Archaeothyris florensis:

Image courtesy of Palaeos.com

She may look like a lizard, but she's not. In fact, she's more closely related to you -- and you to her -- than either of you are to any living reptile, bird or other non-mammal.

A. florensis is the earliest known pelycosaur in the fossil record, dating to Pennsylvanian time, about 307 million years ago. She is the first of a taxon of animals once known as "mammal-like reptiles" because of the several mammalian characters their fossils displayed. But that name has begun to fall out of favor in recent years, since it has become clear that the "mammal-like reptiles" never were reptiles to begin with.

You'll recall from my last post that the lines of the Synapsids and Sauropsids diverged from their common Amniote (animals capable of laying eggs outside the water) ancestor between 320 to 315 million years ago. While the earliest animals of each taxa have several superficial similarities, it is still inappropriate to class them together, because they each possess unique skeletal characters that profoundly affected the evolution of their descendants. As noted, the Sauropsids became reptiles, dinosaurs and birds. The Synapsids became mammals, but before that they were... something else. There's still a debate about what to call them, other than "mammal-like reptiles"; several terms have been suggested, such as "Permian synapsids," "stem-mammals," and -- the one that I prefer and will thenceforth use on this blog -- "proto-mammals."

The thing that separated the synapsids from other amniotes was -- and still is -- the number of temporal fenestrae in their skull. Synapsids possess only one on each side of their skull, and A. florensis was no different.

At top left, you can see a rendering of the generic synapsid skull, with the temporal fenestra in orange. This is the place where the protomammalian and mammalian jaw muscles are anchored. Contrast it with the distinct skull of the diapsids at bottom left. All synapsids, including you and your dog and your cousin A. florensis, display this pattern. All diapsids, including the dinosaurs and birds, have (or at one time had) two temporal fenestrae on each side.

A. florensis' fossils were found near Florence, Nova Scotia, in Carboniferous rocks indicating a humid, tropical, warm climate characterized by undergrowth of ferns and club mosses beneath a canopy of conifers. The lowland swamps of this time and place were full of masses of decaying vegetation, the origin of our modern-day coal beds. And like any swamp, it was full of insects, who in turn attracted your distant cousin.

She measured about 20 in./50cm. long, was very likely an insectivore, and possibly a predator of smaller insectivorous reptiles, like Hylonomous (the earliest known true reptile). In addition to her unique skull adaptations, A. florensis had several characters that peg her as an ancestor of mammals, and the first of the true proto-mammals; the most relevant of which (for our purposes here) were her teeth.

In A. florensis, we see the beginnings of mammalian heterodontism. Though her teeth were all the same shape, they were of varying sizes, including a pair of enlarged canines. The fact that clearly-differentiated teeth appeared so early in our evolution is important in understanding the context of the debate about anatomy and "ideal" or "evolutionary" diets for humans. It is often argued that because humans have canine teeth and sharp incisors, we are therefore natural carnivores. But, gazing back into deep time to our cousin A. florensis, we can see that distinct canines have been a part of our lineage for hundreds of millions of years.

In short, the reason humans have canine teeth is that our ancestors had them. It's that basic. Natural selection is conservative, and rarely produces entirely new designs. It works with what it has, and traits that aren't detrimental to a species' survival will be reproduced even if they serve no direct purpose for that species. Hence, we end up in modern times with deer fangs and primate canines. Various mammal species put their canines to various uses, but that doesn't mean the canines were "meant" for those uses. They weren't really "meant" for anything. They're just hand-me-downs from distant ancestors.

This is not to say that teeth are irrelevant. Far from it. A. florensis most likely used her teeth and flexible jaws to snatch and eat insect prey. But as we will see, this did not consign her ancestors to perpetual insectivory. She was already quite advanced, and her differentiated teeth were themselves inherited from as-yet undiscovered ancestors. We thus cannot know for certain what, if any, selection pressures produced heterodontism in the Synapsids. But we do know that such versatile teeth were part of the key to mammalian flexibility, and thus to our success.

As the earliest pelycosaur, A. florensis has many fascinating relatives later in the fossil record, who will be the focus of the next post in this series. She may be our distant cousin, but her descendents are very likely our direct ancestors. And they, too, have interesting teeth.

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