Atopodentatus

Atopodentatus is an extinct genus of marine reptile, possibly basal sauropterygian, known from the early Middle Triassic (Pelsonian substage, Anisian stage) of Luoping County, Yunnan Province, southwestern China. It contains a single species, Atopodentatus unicus.[1] It is thought to have lived between 247 and 242 million years ago.[2]

A near complete skeleton along with a left lateral fossil of the skull were discovered near Daaozi village, Yunnan, China. The scientific name derives from the peculiar zipper-shaped morphology of its jaws and unique dentition.

Etymology
The genus has been named Atopodentatus from Ancient Greek atopos (άτοπος), signifying "unplaceable, strange, extravagant, absurd, eccentric, disturbing",[3] combined with Latin dentatus, "toothed", referring to the unusual form of arrangement and shape of the teeth. The specific name "unicus" reinforces the uniqueness of the reptile's morphology.

Morphology
Atopodentatus is 3 metres (9.8 ft) long. The geological strata in which the fossil was found, the elongated body, reduced neck, robust appendages and hips of Atopodentatus all suggest that the reptile was probably semi-aquatic in nature.[2][4]

The most bizarre feature of Atopodentatus is its upper mandible which has the teeth run along the jawline and then up along a vertical split in the middle of its jaw. This gives the upper jaw the appearance of a "zipper smile of little teeth". The upper jaw hooked downwards.[4] The needle-like teeth of Atopodentatus are covered in enamel and are not fixed in sockets but instead are fused to the sides of the jaw.[2][4] There are 35 small teeth in the front of the upper jaw on each side of which around 35 are placed in the vertical stretch of the "zipper" while a hundred or so teeth are lined along the horizontal stretch of jawline. The lower jaw is shovel-shaped and hooked downwards and has over 190 small teeth along the horizontal jawline.