''Henodus'' (from el, ἑνός , 'one' and el, ὀδούς , 'tooth') is an extinct

placodont Placodonts (" Tablet teeth") are an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were genera ...

of the Late

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

period during the early

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

age. Fossils of ''Henodus chelyops'' were found in the Estherienschichten Member of the Grabfeld Formation, near

Tübingen Tübingen (, , Swabian: ''Dibenga'') is a traditional university city in central Baden-Württemberg, Germany. It is situated south of the state capital, Stuttgart, and developed on both sides of the Neckar and Ammer rivers. about one in thr ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

. It was around in length. The single species within the genus is ''H. chelyops''. ''Henodus'' is the only placodont thus far found in non-

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

deposits, suggesting it may have lived in

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...

Description

''Henodus'', like many other placodonts, had a superficial resemblance to a

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...

. Like turtles, it had a shell formed from a plastron on the underside and a

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

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. The armor was fused to its

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolog ...

, 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'' also had a single tooth on each side of its mouth, though the remaining teeth were replaced by a beak. In addition, it had

baleen Baleen is a filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and ...

-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 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 ...

lifestyle. The head was squared-off at the front, just ahead of the eyes.

Paleo-ecology

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 ''Atopodentatus'' is an extinct genus of basal sauropterygian known from the early Middle Triassic (Pelsonian substage, Anisian to Ladinian stage) of Luoping County, Yunnan Province, southwestern China. It contains a single species, ''Atopoden ...

'', 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.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

References

. ''Handbuch der Paläoherpetologie'', Teil 12A. München, Friedrich Pfeil. * 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 * Darren Naish: ''Fossils explained 48. Placodonts.'' Blackwell Publishing Ltd, Geology Today, Vol. 20, No. 4, July–August 200
online
{{Taxonbar, from=Q194892 Placodonts Late Triassic reptiles of Europe Fossil taxa described in 1936 Taxa named by Friedrich von Huene Sauropterygian genera

Description

Paleo-ecology

References

Further reading