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 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]

Atopodentatus
Temporal range: Middle Triassic, 247–240 Ma
Life restoration in a swimming posture, with Dinocephalosaurus in the background
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Pantestudines (?)
Superorder: †Sauropterygia
Genus: †Atopodentatus
Cheng et al., 2014
Type species
†Atopodentatus unicus
Cheng et al., 2014

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]

Description
Restoration

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][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]

Discovery and naming

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]

Paleoecology

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]

Notes
  1. Compare "atopic".
  2. The paper describing the herbivorous habits of Atopodentatus also suggests that the placodont Henodus was a herbivore as well.[10]

References
  1. Cheng, L.; Chen, X. H.; Shang, Q. H.; Wu, X. C. (2014). "A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation". Naturwissenschaften. 101 (3): 251–259. doi:10.1007/s00114-014-1148-4. PMID 24452285.
  2. Prostak, Sergio (17 February 2014). "Atopodentatus unicus: Bizarre New Fossil Reptile Discovered in China". Sci-News.com. Retrieved 2 March 2014.
  3. Geggel,LiveScience, Laura. "Fearsome Dinosaur-Age "Hammerhead" Reptile Ate... Plants?". Scientific American. Retrieved 2016-05-10.
  4. "Ancient Reptile Ate Like an Underwater Lawn Mower". National Geographic News. 2016-05-06. Retrieved 2016-05-10.
  5. Davis, Nicola (2016-05-06). "Atopodentatus was a hammerheaded herbivore, new fossil find shows". the Guardian. Retrieved 2016-05-06.
  6. Switek, Brian (5 February 2014). "Atopodentatus Will Blow Your Mind". Laelaps (blog). National Geographic Society. Retrieved 2 March 2014.
  7. "Ancient hammerhead creature may have been world’s first vegetarian sea reptile". Science, By Sid Perkins. May 6, 2016.
  8. "Nailing the reconstruction of a hammerhead reptile". Toronto Star, May 14, 2016, page IN5. by Ben Guarino
  9. Reynoso, V. H. (2000). "An unusual aquatic sphenodontian (Reptilia: Diapsida) from the Tlayua Formation (Albian), central Mexico". Journal of Paleontology. 74: 133–148. doi:10.1017/s0022336000031310.
  10. Chun, Li; Rieppel, Olivier; Long, Cheng; Fraser, Nicholas C. (2016). "The earliest herbivorous marine reptile and its remarkable jaw apparatus". Science Advances. 2 (5): e1501659. doi:10.1126/sciadv.1501659. PMC 4928886. PMID 27386529.
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