''Atopodentatus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of basal

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

n known from the early

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

(Pelsonian substage,

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic). ...

stage) of

Luoping County Luoping County () is located in Qujing City, in eastern Yunnan province, China, bordering Guizhou province to the east and the Guangxi Zhuang Autonomous Region to the southeast. The county of Luoping in eastern Yunnan is noted for its springtim ...

Yunnan Province Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the C ...

, southwestern

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

. 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 251.9 Mya. It is the last period of the Paleozo ...

. ''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 is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

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 nails into wood, to shape metal (as w ...

-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 of ...

-like jaws.

Description

''Atopodentatus'' was about long and weighed . 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. 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 tool with a characteristically shaped cutting edge (such that wood chisels have lent part of their name to a particular grind) of blade on its end, for carving or cutting a hard material such as wood, stone, or metal by hand, stru ...

-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 a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

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 fl ...

and sandy islands in the

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

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

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

from the seabed, making it the second known Mesozoic

herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

marine reptile after the sphenodontian ''

Ankylosphenodon ''Ankylosphenodon'' is an extinct species of sphenodontian known from Tepexi de Rodriguez, Mexico. It is known from Early Cretaceous sedimentary deposits from the Tlayua formation. Lifestyle ''Ankylosphenodon'' is thought to have been an aqu ...

''. ''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