Thursday, March 26, 2015

Triassic Thursday IV: Giant temnospondyls

By both diversity and popular opinion, reptiles ruled the Triassic. I’ve said it again and again on this blog and I’m sure you’re sick of it. So how about we change things up and talk about big gregarious flat-headed amphibians?

Before I begin, let me clear something up about what we like to call “amphibians.” You mention the word to most people and they will get the general idea of what an amphibian is: a frog or a salamander, for example; something that has moist skin and likes to swim around and lay little squishy eggs. Based on this diagnosis, lots of extinct critters fall into the category of amphibians. The animals I am about to discuss, however, are not the same as the amphibians we’re familiar with. Rather, they are labyrhinthodonts, specifically temnospondyls: members of an extinct clade of large, often crocodile-like amphibians which were among the dominant semi-aquatic predators from the Carboniferous to the early Jurassic. My point, I suppose, is that it is taxonomically incorrect to refer to these big guys as amphibians per se, but they aren’t far off and I won’t judge you for referring to them as such. Whether or not they would take offense remains to be seen.

Across the floodplains of Late Triassic Pangaea, which was slowly drifting apart, huge vernal pools and ponds were home to groups of these large temnospondyls. Metoposaurs and mastodonsauroids were the biggest in the Triassic, growing to a couple of meters long. They also have some ridiculous-looking skulls. The first time I heard someone refer to metoposaurs as “toilet seat-headed,” I was surprised I hadn’t thought of it myself. The resemblance is uncanny. 

Metoposaurus bakeri, a toilet-headed temnospondyl. Reconstruction by DiBgd, from Wikipedia.

These giant toilet-headed beasties probably ate whatever they could fit into their big mouths. Fish were probably key menu items; however, with eyes located dorsally on their skulls and an overall dorsoventrally compressed bodyplan, it was likely that they could ambush riparian animals as well. Living in a time of phytosaurs, rauisuchians, and dinosaurs, I’m sure that temnospondyls were much more often the prey of reptiles than the other way around.

Some of the best-preserved metoposaurs come from mass bone beds. For a while, it was thought that these animals had gathered together during a drought, congregating in rapidly-dissipating pools which were essential for their survival. However, taphonomy of these bone beds suggests that, while they were likely in close association when alive, they were probably carried from other locations rather than being preserved in a single pool. Perhaps these metoposaurs lived as crocodiles do in resource-abundant areas, passively gathering together when times were plentiful. Or, some suggest, they could have gathered to spawn, which would have been an awesome spectacle: imagine a vernal pool on a Pangaea floodplain full of dozens of writhing, spawning, two-meter temnospondyls depositing tens of thousands of eggs. Beautiful. And gross.

1 comment: