''Placodus'' (meaning 'flat tooth') was a genus of marine reptiles belonging to the order

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 generall ...

ia, which swam in the shallow seas of the middle

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

period (c. 240 million years ago). Fossils of ''Placodus'' have been found in

Central Europe Central Europe is an area of Europe between Western Europe and Eastern Europe, based on a common historical, social and cultural identity. The Thirty Years' War (1618–1648) between Catholicism and Protestantism significantly shaped the a ...

(

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 ...

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan area ...

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) and China.

Palaeobiology

''Placodus'' had a stocky body with a long tail, and reached a total length of and body mass of . It had a short neck, and a heavy skull. They were specialized for a

durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil tu ...

diet of shellfish, such as bivalves. Chisel-like

incisors Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

protruded from the anterior margin of the snout, and were probably used to pluck hard-shelled benthic prey from the substrate. The back teeth were broad and flattened, and would have helped to crush the prey. Before the animals' anatomy was known, they were regarded as fishes' teeth. Similar smaller teeth were present on the palatine bones. ''Placodus'' and its relatives were not as well-adapted to

aquatic life An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The tw ...

as some later reptile groups, like the closely related plesiosaurs. Their flattened tails and short legs, which probably ended in webbed feet, would have been their main means of propulsion in the water. The

parietal eye A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

on top of the head assisted the animal with orientation, rather than its vision, and its presence is regarded as a primitive characteristic. The

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

l processes of ''Placodus'' dove-tailed into each other and were firmly connected, so that the trunk was rigid. The abdomen was covered with a special armor formed of the bent, right-angled abdominal ribs. Equipped with dense bones, heavy belly ribs, and a row of bony knobs above the backbone, ''Placodus'' was a heavily built and negatively buoyant creature that would have had no trouble staying on the seafloor to feed. This body armour would have offered protection from predators as well, but would have also hampered mobility on land, making ''Placodus'' slow and clumsy out of water. It was therefore most likely a terrestrial animal that ventured into the sea in search of food. Molluscs,

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s, and other inhabitants of the seabed would have formed its staple diet.

References

{{Authority control Placodonts Middle Triassic reptiles of Europe Sauropterygian genera