May 11 2016

Shaena Montanari

Two years ago, when paleontologists first described the skeleton of the croc-sized marine reptile Atopodentatus unicus, they knew it was weird, but not this weird. New specimens of this Triassic marine reptile from China reveal that its skull is even more bizarre than originally thought, indicating a whole new mode of feeding in marine reptiles from this time period. This updated description lead by Li Chun from the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing is out today in Science Advances.

In 2014, Atopodentatus was described from a crushed skull, but was reconstructed to have a funny downturned nose and tiny needle teeth studded near the opening of its jaw. This was hypothesized to be a filter-feeding mechanism for Atopodentatus, as its teeth are very fine and probably couldn’t have crunched down on any big fish. But a new find has turned this old theory upside down, because the previously reconstruction of its skull was incorrect.

Paleontologists can be wrong sometimes—fossils are poorly preserved and we need to do some guesswork when filling in the anatomical gaps that have been lost to millions of years of volatile earth history. We try to make our guesses as informed as possible, but if we are lucky, we get more information later and can rethink the parts we weren’t sure about. A new specimen of Atopodentatus from Yunnan Province fortunately shed some much needed light on the situation.

Instead of a downturned nose, the mouth of Atopodentatus was actually more like a “hammerhead” shape. It still has all of the fine, needle-like teeth that originally gave it its generic name of Atopodentatus—“strange teeth” —but they are arranged in a different fashion than previously thought. It also had thicker peg-like teeth on its jaws, indicating Atopodentatus had an affinity for chowing down on plants and not animals, making it the oldest herbivorous marine reptile ever discovered.

Co-author Olivier Rieppel at The Field Museum in Chicago emphasizes that this is not the status quo for marine reptiles: “It’s got a hammerhead, which is unique, it’s the first time we’ve seen a reptile like this.” Instead of filter-feeding for small marine animals on the ocean floor, it is hypothesized it was instead eating plants in a peculiar fashion, similar to marine mammals: “It used the peg-like front teeth to scrape plants off of rocks on the sea floor, and then it opened its mouth and sucked in the bits of plant material. Then, it used its needle-like teeth as a sieve, trapping the plants and letting the water back out, like how whales filter-feed with their baleen.”

Besides being extremely cool, Atopodentatus shows us how marine reptiles diversified after the biggest mass extinction this planet has ever seen, the Permo-Triassic extinction event. While herbivory is quite uncommon in marine reptiles, it did appear in another genus later in the Triassic after Atopodentatus. Henodus, while it does not have a hammerhead, has a similarly shaped flat, square head with tooth-like objects on the roof of its mouth that likely served to scrape plants like Atopodentatus. As Rieppel explains, “The existence of specialized animals like Atopodenatus unicus shows us that life recovered and diversified more quickly than previously though.”