| Atopodentatus Temporal range:
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|---|---|
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| Life restoration in a swimming posture, with Dinocephalosaurus in the background | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Neodiapsida |
| Superorder: | †Sauropterygia |
| Genus: | †Atopodentatus Cheng et al., 2014 |
| Type species | |
| †Atopodentatus unicus Cheng et al., 2014
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Atopodentatus is an extinct genus of basal sauropterygian known from the early Middle Triassic (Anisian)[1] of Guanling Formation in Luoping County, Yunnan Province, southwestern China. It contains a single species, Atopodentatus unicus.[1] It is thought to have lived between 247 and 240 million years ago, during the Middle Triassic period, about six million years after the Permian extinction.[2][3][4] Atopodentatus was an herbivorous marine reptile, although marine reptiles are usually omnivores or carnivores.[3]
A near complete skeleton along with a left lateral portion of the skull were discovered near Daaozi village, Yunnan, China. The scientific name derives from the peculiar zipper-shaped morphology of the holotype specimen's jaws and unique dentition.[2] However, two fossil skulls discovered in 2016 indicate that the holotype skull was badly damaged, and that the living animal actually had a hammer-shaped head with shovel-like jaws.[5]
Atopodentatus was a medium-sized reptile measuring about 2.75 m (9.0 ft) long.[1] The geological strata in which the fossil was found, the elongated body, reduced neck, robust appendages and hips of Atopodentatus all suggest that the animal was probably semi-aquatic in nature.[2][6]
Originally, the upper mandible of Atopodentatus was believed to have small teeth running along the jawline, and then up along a vertical split in the middle of the upper jaw. This gave the upper jaw the appearance of a "zipper smile of little teeth". The upper jaw was believed to have hooked downwards.[6] Discoveries in 2016, however, overthrew these findings, and revealed that Atopodentatus actually had a hammer-shaped head, with a bank of chisel-shaped teeth, that was useful in rooting the seabed for food.[7][8]
The genus name Atopodentatus is derived from the Ancient Greek atopos (άτοπος), meaning "unusual",[note 1] combined with Latin dentatus, "toothed", referring to the unusual arrangement and shape of the teeth. The specific name "unicus" is also in reference to its unique anatomy.[2]
Due to its bizarre dentition, Atopodentatus was formerly considered to be a filter feeder which fed on invertebrates along the sea-bottom.[6] It was suggested that the morphology made Atopodentatus "capable of walking on land or tidal flats and sandy islands in the intertidal zone".[1] However, the 2016 findings reveal that Atopodentatus actually ate algae from the seabed, making it the second known Mesozoic herbivorous marine reptile after the sphenodontian Ankylosphenodon.[9] Atopodentatus is the earliest known herbivorous marine reptile by about 8 million years.[3][note 2]
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