Henodus

Henodus
Temporal range: Late Triassic, 228–220 Ma
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: †Sauropterygia
Order: †Placodontia
Family: †Henodontidae
Huene, 1936
Genus: †Henodus
Huene, 1936
Species: †H. chelyops
Huene, 1936
Artist's reconstruction

Henodus chelyops ("Turtle-Faced Single Tooth") was a placodont of the Late Triassic period during the early Carnian age. Fossils of Henodus chelyops were found in Tübingen, Germany. It was around 1 metre (3.3 ft) in length.

Henodus was the placodont that had the greatest (albeit superficial) resemblance to a turtle. Like turtles, it had a shell formed from a plastron on the underside and a carapace on top. The carapace extended well beyond the limbs, and was made up of individual plates of bony scutes covered by plates of horn. However, the shell was composed of many more pieces of bone than that of turtles, forming a mosaic pattern.[1] The armor was fused to its spine, and its limbs were situated in normal positions, unlike the turtle, where they are located inside the ribcage. The weak limbs of Henodus suggest it spent little, if any time on land.

Henodus chelyops also had a single tooth on each side of its mouth, though the remaining teeth were replaced by a beak. Instead, it had baleen-like denticles along the jaws, which combined with a unique feature of the hyoid and musculature indicative of rapid jaw closing indicate a filter feeding lifestyle.[2][3] The head was squared-off at the front, just ahead of the eyes.

More recently, however, it has been suggested that this placodont was an aquatic herbivore, scraping off vegetation from the bottom with its broad jaws. This suggestion has been brought up in a paper discussing the habits and morphology of Atopodentatus, another Mesozoic aquatic reptile formerly suggested to be a filter-feeder and bearing strongly convergent jaw adaptations, including a similar "hammerhead" jaw tips, though unlike Atopodentatus it's still believed that Henodus relied on filter feeding to obtain plant-matter from the substrates.[4]

Henodus is the only placodont thus far found in non-marine deposits, suggesting it may have lived in brackish or freshwater lagoons.

References

  1. ↑ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 71. ISBN 1-84028-152-9.
  2. ↑ Rieppel, O. (2002). Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas Zoological Journal of the Linnean Society, 135, 33-63
  3. ↑ Naish, D. 2004. Fossils explained 48. Placodonts. Geology Today 20 (4), 153-158.
  4. ↑ Li Chun, Olivier Rieppel, Cheng Long and Nicholas C. Fraser, The earliest herbivorous marine reptile and its remarkable jaw apparatus, Science Advances 06 May 2016: Vol. 2, no. 5, e1501659 DOI: 10.1126/sciadv.1501659
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