Atopodentatus

Atopodentatus Temporal range: Anisian PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration in a swimming posture, with Dinocephalosaurus in the background
Scientific classification
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

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]}