December 30 , 2016:

by Sarah Gibson

As we continue in our countdown of the winners of the Top 10 Open Access Fossil Vertebrates of the past year, we get to probably the most unusual of the group. Coming in at #5 is the herbivorous marine reptile Atopodentatus unicus, from the Middle Triassic of China. Atopodentatus was redescribed this year and published open access in the journal Science Advances by authors Li Chun from the Institute of Vertebrate Paleontology and Paleoanthropology, Olivier Rieppel from the Field Museum, Cheng Long from the Wuhan Centre of China Geological Survey, and Nicholas C. Fraser from the National Museums Scotland.

When Atopodentatus unicus was redescribed earlier this year, I was fascinated by its unusual, hammerhead-shaped skull and possible trophic feeding behavior. What I didn’t realize was this isn’t the first unusual interpretation of the jaws for this creature. When it was first described and published in 2014, the jaw configuration was interpreted to be more similar in shape to what is seen in a flamingo, with a downturned rostrum that was interpreted to be used to stir up sediment.

However, Chun and colleagues examined new, exquisitely preserved specimens for this study that were preserved in dorsal and ventral views. They realized that, instead of the strange downturned configuration, the jaws actually protrude laterally, creating a shape similar to a hammerhead shark. The processes are composed of the premaxillae and maxillae in the upper jaw, and the dentary in the lower jaw. Each jaw element is packed with dense, needle-like teeth. The roof of the mouth is also covered in tightly packed, small denticles.

Atopodentatus is not the only creature to evolve herbivory in the Early Mesozoic, this study notes. A second marine reptile, Henodus, from the Upper Triassic of Germany, has a series of flattened denticles that were interpreted by its author von Heune (1936) to have a herbivorous scraping function, though it lacks the lateral jaw processes of Atopodentatus. (And I might also be a bit biased towards Triassic herbivores because I recently redescribed the oldest herbivorous fish, Hemicalypterus weiri in PLOS ONE. Hemicalypterus possesses dentition specific to algae-scraping herbivorous fishes both past and present, which Andy recently covered for PLOS Paleo).

Regardless, Atopodentatus and its remarkable jaws represents the oldest known herbivorous marine reptile, and it and Henodus are unique, as no other cases for adaptation for herbivory have been recorded in the fossil record for marine reptiles for the rest of the Mesozoic.

Atopodentatus is interpreted to have lived in near-shore, shallow water habitats, where algae and aquatic plants would have flourished with ample sunlight penetrating the water column. What is informative about Atopodentatus‘ presence during the Middle Triassic is it helps researchers understand how life recovered after the end-Permian extinction event.

Reference:

Cheng et al. (2014) A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften. doi: 10.1007/s00114-014-1148-4

Chun L, Rieppel O, Long C, Fraser N (2016) The earliest herbivorous marine reptile and its remarkable jaw apparatus. Science Advances 2: 31501659.

von Huene F, (1936) Henodus chelyops, ein neuer Placodontier. Palaeontogr. Abt. A 84, 99–147.