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Like Finding a Unicorn: Why the Paleozoic Matters for Vertebrate Paleontology

For the general public, the view of the deep past is rather skewed towards dinosaurs and Ice Age mammals.  One gets the impression that the first life was bacteria, then bugs, then some fishy things, and then dinosaurs came around and changed the world.  To be fair, dinosaurs were an important and significant part of vertebrate history, but in many cases they were re-inventing ecosystems already established by our own distant ancestors, the Synapsids (or, as they were traditionally called, the “mammal-like reptiles”).

Dinosaurs lived during the Mesozoic Era (~245-66 million years ago), but our distant ancestors appeared during the previous Paleozoic Era, sometime around 300 million years ago.  In the public eye, the Paleozoic is unfortunately a forgotten Era, but it is possibly the most important time for vertebrate evolution: during this time period (~540-245 million years ago) the first vertebrates, all the major “fish” groups, the first tetrapods, and the ancestors of modern amphibians and modern amniotes (“reptiles,” birds, and mammal) all appear.  And during this time, the first major vertebrate terrestrial ecosystems comprised of herbivores and carnivores appeared.  The earliest representatives of these ecosystems were the aforementioned Synapsids and a group that is often referred to as the Parareptilia.

Many people have never heard of the parareptiles, a totally extinct group of reptiles (well, maybe: there is disagreement over whether turtles are living parareptile descendants). This group includes odd herbivores called procolophonids, stocky animals about 1 to 2 feet in length.  When I was doing fieldwork in South Africa, I worked alongside paleontologist Juan Cisneros who has traveled the world to study and unravel the evolutionary history of these peculiar parareptiles.  We were digging in the Elliot Formation which is right on the cusp of the Triassic-Jurassic boundary, and Juan was hopeful he would find a very early Jurassic or very late Triassic procolophonid amongst the dinosaur bones.  Alas, no new procolophonids were to be had.

Dr. Cisneros was to find luck later, though, with synapsids. In 2011, he and his colleagues discovered the totally weird and surprisingly cool synapsid named Tiarajudens eccentricus, an herbivore with huge sabertooth incisors (Cisneros et al., 2011)! Tiarajudens was a member of a synapsid group known as anomodonts, and most of these were toothless herbivores. Instead, here was an animal with huge saber teeth, which at first glance would lead to one to conclude it was carnivorous.  However, when Cisneros and his colleagues examined the rest of the teeth, especially the ones towards the back of the jaws and the palate, they discovered wear on the crowns indicative of grinding.  So, like other anomodonts, it was an herbivore. Apparently Tiarajudens was capable of grinding up vegetation in ways reminiscent of modern herbivorous mammals: its top and bottom sets of teeth could closely fit together, enhancing its ability to grind up vegetation. As Juan explained at the time, finding Tiarajudens was “like finding a unicorn.”

Tiarajudens is but one of the many intriguing vertebrates that flourished throughout the Paleozoic Era. Far from being an Era that simply set the stage for dinosaurs and mammals, the Paleozoic deserves and needs more recognition for the data and perspective it provides. Not only does the Paleozoic vertebrate fossil record show us how the major vertebrate groups originated, it provides us with a window into a past that is at once familiar and alien.

Submitted by Matthew F. Bonnan, Stockton University

References:

Cisneros, J. C., F. Abdala, B. S. Rubidge, P. C. Dentzien-Dias, and A. d. O. Bueno. 2011. Dental Occlusion in a 260-Million-Year-Old Therapsid with Saber Canines from the Permian of Brazil. Science 331:1603–1605.
Posted: 2/24/2016 5:05:02 PM by matthewbonnanadmin | with 0 comments
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