Marine Lawnmowers of the Triassic

Sometimes you read a paper about a new fossil animal and just shake your head in disbelief. That was the posture I adopted back in 2014, when Atopodentatus unicus was unveiled to the world in the pages of Naturissenchaften. It’s a pretty good-sized marine reptile with a long tail and body, stout limbs, and a very small skull. From the neck down, this is a pretty nondescript critter that, according to its description, seems to have a close relationship with the Sauropterygia.

Perhaps suffering from the Triassic version of the Zika virus (too soon?).

But let’s turn our gaze back to that seemingly inappropriately small skull.

I, uh...hmmm.

Gaze upon it, and be amazed. The maxilla appears to curve down sharply in front of the mouth, and the structure of the premaxillary bones implies a hare-lipped appearance in which the hare itself was studded with needle-like teeth. The unbeatable Julius Csotonyi threw together this quick sketch of Atopodentatus: basically a nightmarish marine iguana. 

I NEED AN ADULT.

Jamie Headden compared (appropriately) it to a Lovecraftian monstrosity. The authors, Chen et al., suggested it must have been a bottom-feeding filter-feeder, grubbing around in the sediment and sifting water and non-food particles out through its hare lip, where food particles would get stuck on its inwardly-directed teeth.

This is almost worse than that deep-sea squid with teeth.

Oh what hath mad evolution wrought?

Atopodentatus came and went, representing little more than a highly specialized case of what the fuckery in the great pantheon of Triassic marine reptiles. We all collectively said “well, THAT was a thing” and moved on with our lives. 

But just last week, dear readers, new specimens have come to light which show that Atopodentatus is both more normal than we thought but also way weirder.

Oh the snout is HORIZONTAL. That's TOTALLY FINE.

For you see, the original skulls of Atopodentatus were preserved on their sides, crushed flat. The two new fossils show what the skull looks like from the top and the bottom.

This really isn't any less disturbing.

Words can only describe so much. Here’s a very nice model to illustrate what the fossils now show:

Atopodentatus has a vacuum cleaner mouth. And it’s not like the lawnmower mouth of Nigersaurus. Similar, but with important differences.

"I just want things to work properly."

In Nigersaurus, the snout doesn’t turn into a T-shape. It’s just an extension of the already-wide skull, and the teeth are restricted to the front. In Atopodentatus, the teeth start prior to the T-junction, then the skull (and mandible) do their T-shape thing and the teeth continue the whole way. That isolated premaxilla above doesn't sit vertically--it's horizontal.

Nigersaurus: totally normal by comparison. Nothing to see here.

The authors suggest that Atopodentatus scraped algae off rocks with its "hammerhead" rostrum, sucked everything into its mouth, then filtered the water out through its back teeth. That means it’s still a nightmarish marine iguana, but not quite the Lovecraftian horror we originally thought. Atopodentatus is from the Middle Triassic of China--it shows up around the same time as saurosphargids but after hupehsuchians.

The Triassic was a weird time for everyone.

References:

Cheng, L., Chen, X., Shang, Q. & Wu, X. (2014). A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenchaften 101(3): 251-259.

Chun, L., Rieppel, O., Long, C. & Fraser, N. C. (2016). The earliest herbivorous marine reptile and its remarkable jaw apparatus. Science Advances 2(5): 1-4. OPEN ACCESS!