''Atopodentatus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of basal

sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosau ...

n known from the early

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

(

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

) of

Guanling Formation The Guanling Formation is a Middle Triassic (Anisian or Pelsonian in the regional chronostratigraphy) geologic Formation (geology), formation in southwestern China. Geology The formation encompasses two members. The first member is primarily c ...

Luoping County Luoping County () is located in Qujing City, in eastern Yunnan province, China. It borders Xingyi, Guizhou to the east, Xilin County, Guangxi Zhuang Autonomous Region to the southeast, Shizong County to the southwest, and Luliang County, Qi ...

Yunnan Province Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

, southwestern

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. It contains a single species, ''Atopodentatus unicus''. 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 The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the sixth and last period o ...

. ''Atopodentatus'' was an herbivorous marine reptile, although marine reptiles are usually omnivores or carnivores. 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 A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

specimen's jaws and unique dentition. However, two fossil skulls discovered in 2016 indicate that the holotype skull was badly damaged, and that the living animal actually had a

hammer A hammer is a tool, most often a hand tool, consisting of a weighted "head" fixed to a long handle that is swung to deliver an impact to a small area of an object. This can be, for example, to drive nail (fastener), nails into wood, to sh ...

-shaped head with

shovel A shovel is a tool used for digging, lifting, and moving bulk materials, such as soil, coal, gravel, snow, sand, or ore. Most shovels are hand tools consisting of a broad blade fixed to a medium-length handle. Shovel blades are usually made ...

-like jaws.

Description

''Atopodentatus'' was a medium-sized reptile measuring about 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 animal was probably semi-aquatic in nature. 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. Discoveries in 2016, however, overthrew these findings, and revealed that ''Atopodentatus'' actually had a hammer-shaped head, with a bank of

chisel A chisel is a hand tool with a characteristic Wedge, wedge-shaped cutting edge on the end of its blade. A chisel is useful for carving or cutting a hard material such as woodworking, wood, lapidary, stone, or metalworking, metal. Using a chi ...

-shaped teeth, that was useful in rooting the seabed for food.

Discovery and naming

The genus name ''Atopodentatus'' is derived from the Ancient Greek (), meaning "unusual", combined with Latin , "toothed", referring to the unusual arrangement and shape of the teeth. The specific name "unicus" is also in reference to its unique anatomy.

Palaeoecology

Due to its bizarre dentition, ''Atopodentatus'' was formerly considered to be a

filter feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...

which fed on invertebrates along the sea-bottom. It was suggested that the morphology made ''Atopodentatus'' "capable of walking on land or

tidal flats Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...

and sandy islands in the

intertidal zone The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various ...

". However, the 2016 findings reveal that ''Atopodentatus'' actually ate

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

from the seabed, making it the second known Mesozoic

herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...

marine reptile after the sphenodontian ''

Ankylosphenodon ''Ankylosphenodon'' is an extinct genus of sphenodontian known from multiple specimens recovered from the Early Cretaceous deposits of the Tlayúa Formation, near Tepexi de Rodríguez, Tepexi de Rodriguez, Mexico. It is likely part of a sphenodo ...

''. ''Atopodentatus'' is the earliest known herbivorous marine reptile by about 8 million years.

Notes

References

{{Taxonbar, from=Q15642618 Fossil taxa described in 2014 Middle Triassic reptiles of Asia Sauropterygian genera Paleontology in Yunnan Anisian life Guanling Formation