Friday, October 4, 2013

Species that don’t get enough publicity #1- Diadectes sideropelicus

Ah, Dimetrodon. The only nonmammalian synapsid  (basically the ancestors of modern mammals) ever to become an honorary dinosaur.  It’s ubiquitous in art, toys, and museums. It’s better known than hundreds of dinosaurs, let alone members of its own group. Sometimes Dimetrodon lies right between lobe-finned fish and Stegosaurus in a march of history. That sail overshadows the Triassic, Permian, and Carboniferous periods. Sometimes other Permian animals show up in popular culture-Eryops, looking like nothing else but a crocodile frog, or Edaphosaurus, which is just the plant eating version of Dimetrodon (And so, not as popular), but it’s all Dimetrodon.
That’s why I’m not going to talk about Dimetrodon anymore.  Instead, I’m going to talk about an animal I find actually more interesting.  It was probably harmless (on the other hand, even Dimetrodon probably wouldn’t be any more dangerous than an alligator, Nile Monitor or Tasmanian devil), about the size of a large dog, and people have struggled for years whether it’s a reptile of an amphibian. The American, Field, and Harvard museums all have one right next to their Eryops, Dimetrodon, etc, but it’s probably overlooked by most visitors.

-Illustration by Robert Bakker
This animal is Diadectes. This animal has never been on film, or shown in a documentary. This is despite, or more likely because of, the fact that this is the first herbivorous tetrapod.  People like their dinosaurs, not what scuttled around the northern hemisphere during the Permian period. Diadectes is very much the opposite of the dinosaur posterchild Tyrannosaurus. It’s low slung, stocky, buck-toothed, sluggish, plant-munching,  and only the size of a Newfoundland (minus the tail). It didn’t have any spikes or horns or armor for defense, or frills or plates or fins for display. It went extinct fairly unspectacularly-the Permian became drier, and it was supplanted by pelycosaurs like Cotylorhynchus.   I only found out about this genus in a book featuring (surprise!) Dimetrodon, where it acted as a one-page dinner for the synapsid protagonist. 

So why should we know more about this animal? Ecology, taxonomy, and physiology.  On land, the only plant-eaters were species of insects for millions of years. They in turn were preyed upon by insectivorous arthropods, amphibians and reptiles. Amphibians and reptiles solely ate other animals-whether fish, tetrapod, or invertebrate. The two species of Diadectes (D. sideropelicus and D. tenuitectus) and its relatives in their own order, Diadectamorpha, were the earliest tetrapod to eat plants. The earliest genus found is Desmatodon, a small animal from the late Carboniferous, but Diadectes itself could grow up to 10 feet long and probably weighed as much as a human being.  

Mount of Diadectes at Field Museum

Instead of straight piscivorous or insectivorous teeth like the Pelycosaurs, or the curved teeth of predators like Dimetrodon or Sphenacodon, it had large, wide teeth for chopping plants. It even had a primitive chewing apparatus-the wide chopping front teeth, broad crushing cheek teeth, and one of the first secondary palates (the skull structure separating the nose from the mouth, allowing animals to breathe while eating) in nature.  Diadectes was soon followed by the equal-sized Edaphosaurus (they shared the Early Permian as prey species for Dimetrodon), the gigantic Cotylorhynchids (bucktoothed, tiny-headed blimp shaped pelycosaurs described aptly by Dr. Tom Holtz as “derpy”), and then all the synapsids, diapsids, dinosaurs, and mammals that developed herbivory to take advantage of Earth’s lush plant life.  Diadectes was the ur-cow, if you will.

The weirdest part isn’t just the size and diet; it’s their classification. They’ve been assigned to a group called reptilomorpha. These are amniotes (animals laying hard shelled eggs) that are neither amphibians nor true reptiles.  The Reptilomorphs (called Anthracosaurs by some) also include the small aquatic Lepospondyls, the Seymouriamorphs which spent their juvenile lives as amphibians and adult lives as reptiles, the crocodile like Chroniosuchids, the aquatic predator Embolomeres and the very small, early Gephyrostegids . All these animals have deep skulls, horizontally placed eyes, deep torsos with strong legs for crawling around, and a heavy build overall.

I recall reading many books, some of which placed Diadectes with the amphibians; others placed it as a reptile. Truth is, we’re still not sure. It’s one of the many, many problematic animals when it comes to organizing reptiles and amphibians. Both groups are astoundingly diverse today, and are even more so across geological history.

I guess I just have a fondness for this taxon. Maybe because you can almost spell my name with Diadectes. Maybe it’s my rooting for the underdog and affection for herbivores. Maybe I just like the big dumpy bucktoothed critter. If I were a Permian animal, I’d probably be one. Lumbering around, not a thought in my big dumpy head, munching on ferns and fermenting them in my gut, only having to worry about hungry Dimetrodons.  It’s not much, but it’s a nice, slow life, like any other in the early Permian.

So anyway,  Hollywood, give this guy a break. Museums, keep him on display. Toy companies, here’s a new fresh face. Authors,  think about the crap you’re writing and how this genus could perk up the place. And remember, shop Diadectes where you work or play.

For a related opinion, here’s palaeontologist David Button on this loveable critter

Coming soon-my first book review on the blog! Stay tuned!


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