The marine reptile Atopodentatus unicus basking in the sun.
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Art by Julius Csotonyi.
The marine reptile Atopodentatus unicus basking in the sun.

Atopodentatus Will Blow Your Mind

The fossil record is replete with wonders. Humungous fungus, dazzling dinosaurs, intricate ammonites, and perplexing protomammals just scratch the surface of such a wide array of fantastic organisms that sometimes it’s easy to become acclimated to the enigmatic and weird. Yet, even then, there are fossils so strange that they make me jolt upright in my seat and think “Wait, what the hell is that?” The latest prehistoric creature to leave me gobsmacked is Atopodentatus unicus.

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The skeleton of Atopodentatus with a close-up of the skull. From Cheng et al., 2014.

The roughly 245 million year old marine reptile is beautifully preserved. Uncovered in southwest China and described by Wuhan Institute of Geology and Mineral Resources paleontologist Long Chen and colleagues, the reptile’s nearly complete, nine-foot-long skeleton is laid out as charcoal-colored bones against gray rock. And while not as wholly adapted to an aquatic lifestyle like the eel-like ichthyosaurs found in the same deposits, the stout limbs, hips, and geological context of Atopodentatus hint that this reptile divided its time between land and sea. Then there’s the skull.

Preserved in profile, the cranium of Atopodentatus looks like a bony version of a Scotch tape dispenser.  In front of a rounded orbit, the creature’s snout is a downturned hook that creates an arc of tiny, needle-like teeth that are fused to the sides of the jaw rather than sitting in sockets. Stranger still, most of the teeth in the upper jaw faced each other in a split running between the two halves of the upper jaw. Head-on, Atopodentatus had a zipper smile of little teeth.

As Cheng and colleagues say in the title of their paper, this is a “highly specialized feeding adaptation.” But for what? Chomping down on fish seems unlikely. The reptile’s tiny teeth would have been too delicate for struggling prey, the researchers note, and muscle attachment sites on the bones suggest that Atopodentatus wasn’t capable of biting hard.

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Atopodentatus as envisioned by artist Julius Csotonyi.

Filter feeding seems a better option for the unusual Atopodentatus apparatus. In a fashion similar to living gray whales, Cheng and coauthors hypothesize, the Triassic reptile may have swum to the muddy bottom of the shallows and turned its head sideways to scoop sediment into piles. Then, scratching or grasping with that hooked upper jaw, Atopodentatus could have strained worms, small crustaceans, and other morsels through its toothy sieve. Nightmarish as Atopodentatus looked, only tiny invertebrates had much reason to fear.

Testing such a scenario is the tricky part. Gut contents and maybe even traces Atopodentatus left behind as the reptile dredged the seabottom could help paleontologists investigate how the “peculiar-toothed” reptile ate. For now, how the ancient swimmer employed such an unprecedented dental apparatus is still open to investigation. I’m grateful that such petrified puzzles exist. For as much as paleontologists have discerned about life’s past, there are likely even odder discoveries yet to be made. Ain’t evolution grand?

Top image by Julius Csotonyi. And for another take on Atopodentatus as an embodiment of a Lovecraftian horror, see this post at The Bite Stuff.

References:

Cheng, L., Chen, X., Shang, Q., Wu, X. 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaption. Naturwissenschaften. doi: 10.1007/s00114-014-1148-4