Placodonts ("tabletteeth") are anextinctorder of marinereptiles that lived during theTriassic period, becoming extinct at the end of the period. They were part ofSauropterygia, the group that includesplesiosaurs. Placodonts were generally between 1 and 2 m (3 ft 3 in and 6 ft 7 in) in length, with some of the largest measuring 3 m (9.8 ft) long.
The earliest forms, likePlacodus, which lived in the early to middleTriassic, resembled barrel-bodied lizards superficially similar to themarine iguana of today, but larger. In contrast to the marine iguana, which feeds onalgae, the placodonts atemolluscs and so their teeth were flat and tough to crush shells. In the earliest periods, their size was probably enough to keep away the top sea predators of the time: thesharks.[citation needed] However, as time passed, other kinds of carnivorous reptiles began to colonize the seas, such asichthyosaurs andnothosaurs, and later placodonts developed bony plates on their backs to protect their bodies while feeding. By the Late Triassic, these plates had grown so much that placodonts of the time, such asHenodus andPlacochelys, resembled thesea turtles of the modern day more than their ancestors without bony plates. Other placodonts, likePsephoderma, developed plates as well, but in a different articulated manner that resembled the carapace ofhorseshoe crabs more than those of sea turtles. All these adaptations can be counted as perfect examples ofconvergent evolution, as placodonts were not related to any of these animals.
Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean and would have used a lot of energy to reach the water surface. For this reason, and because of the type of sediment found accompanying theirfossils, it is suggested that they lived in shallow waters and not in deep oceans.
Their diet consisted of marinebivalves,brachiopods, and other invertebrates. They were notable for their large, flat, often protrudingteeth, which they used to crush the molluscs and brachiopods that they hunted on the sea bed (another way in which they were similar towalruses). The palate teeth were adapted for thisdurophagous diet, being extremely thick and large enough to crush thick shell.
Henodus, however, differs from other placodonts in having developed uniquebaleen-like denticles, which alongside features of the hyoid and jaw musculature suggest that it was afilter feeder.[1][2] Recent comparisons toAtopodentatus suggest that it was aherbivore as well, bearing a similar broad jaw shape, albeit it obtained plant matter through filter-feeding it from the substrates.[3] The group was once believed to be restricted to the westernTethys, but the discovery ofSinocyamodus xinpuensis in China overturned this view.[4]
Additionally, the namePlacodontiformes was erected for the clade that includesPalatodonta and Placodontia.Palatodonta, from the early Middle Triassic of the Netherlands, was a marine sauropterygian that was very similar to placodonts, butPalatodonta has teeth that are small and pointed instead of broad and flat.
The cladeHelveticosauroidea was previously considered to be a basal superfamily of placodonts with the sole memberHelveticosaurus. However, it is now thought thatHelveticosaurus was not a placodont but possibly an unusual member of theArchosauromorpha.[citation needed]
^Wang, Wei; Li, Chun & Wu, Xian-Chun (10 December 2018). "An adult specimen of Sinocyamodus xinpuensis (Sauropterygia: Placodontia) from Guanling, Guizhou, China".Zoological Journal of the Linnean Society.185 (3):910–924.doi:10.1093/zoolinnean/zly080.
^De Miguel Chaves, Carlos; Ortega, Francisco & Pérez-García, Adán (2018). "A new placodont from the Upper Triassic of Spain provides new insights on the acquisition of the specialized skull of Henodontidae".Papers in Palaeontology.4 (4):567–576.Bibcode:2018PPal....4..567D.doi:10.1002/spp2.1218.S2CID133679210.
Bardet, N. (1995). "Evolution et extinction des reptiles marins au cours du Mesozoique".Palaeovertebrata (24):177–283.
Huene, F. von (1933). "Die Placodontier. 4. Zur Lebensweise und Verwandtschaft vonPlacodus".Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft (in German) (38):365–382.
Mazin, J.-M. (1986). "Negevodus ramonensis n. g. n. sp., un nouveau placodonte du Trias moyen du Negev (Israel)".302. Comptes Rendus de l'Académie des Sciences, Série II:927–929.{{cite journal}}:Cite journal requires|journal= (help)
Mazin, J.-M. & Pinna, G. (1993). "Palaeoecology of the armoured placodonts".Paleontologia Lombarda. New Series.2:83–91.
Nosotti, S. & Pinna, G. (1993a). "Cyamodus kuhn-schnyderi n. sp., nouvelle espèce de Cyamodontidae (Reptilia, Placodontia) du Muschelkalk supérieur allemand".Comptes Rendus de l'Académie des Sciences, Série II.317:847–850.
Nosotti, S. & Pinna, G. (1993b). "New data on placodont skull anatomy".Paleontologia Lombarda. New Series.2:109–114.
Westphal, F. (1976). "The dermal armour of some Triassic placodont reptiles". In Bellairs, A. d’A. & Cox, C. B. (eds.).Morphology and Biology of Reptiles. London: Academic Press.
Zanon, R. T. (1989). "Paraplacodus and the diapsid origin of Placodontia".Journal of Vertebrate Paleontology.9: 47A.doi:10.1080/02724634.1989.10011786.
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