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Placodonts ("
Tablet Tablet may refer to: Medicine * Tablet (pharmacy), a mixture of pharmacological substances pressed into a small cake or bar, colloquially called a "pill" Computing * Tablet computer, a mobile computer that is primarily operated by touching the ...
teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, ...
") are an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
of marine
reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoceph ...
that lived during the
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 per ...
period, becoming extinct at the end of the period. They were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were generally between in length, with some of the largest measuring long. The first specimen was discovered in 1830. They have been found throughout central
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,
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, the
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and
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
.


Palaeobiology

The earliest forms, like ''
Placodus ''Placodus'' (meaning 'flat tooth') was a genus of marine reptiles belonging to the order Placodontia, which swam in the shallow seas of the middle Triassic period (c. 240 million years ago). Fossils of ''Placodus'' have been found in Central Eur ...
'', which lived in the early to middle
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 per ...
, resembled barrel-bodied lizards superficially similar to the
marine iguana The marine iguana (''Amblyrhynchus cristatus''), also known as the sea iguana, saltwater iguana, or Galápagos marine iguana, is a species of iguana found only on the Galápagos Islands (Ecuador). Unique among modern lizards, it is a marine rept ...
of today, but larger. In contrast to the marine iguana, which feeds on
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
, the placodonts ate
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...
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: the
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s. However, as time passed, other kinds of carnivorous reptiles began to colonize the seas, such as
ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...
s and nothosaurs, 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 as '' Henodus'' and '' Placochelys'', resembled the
sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...
s of the modern day more than their ancestors without bony plates. Other placodonts, like ''
Psephoderma ''Psephoderma'' (meaning "pebbly skin", from the Ancient Greek ''psepho'' (ψῆφος), "pebbly", and ''derma'' (δέρμα), "skin") is a genus of placodonts very similar to the related genera '' Placochelys'' and '' Cyamodus''. ''Psephoderma'' ...
'', developed plates as well, but in a different articulated manner that resembled the carapace of
horseshoe crab Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to ar ...
s more than those of sea turtles. All these adaptations can be counted as perfect examples of
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
, 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 their
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s, it is suggested that they lived in shallow waters and not in deep oceans. Their diet consisted of marine
bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
,
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 ...
s, and other invertebrates. They were notable for their large, flat, often protruding
teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, ...
, which they used to crush the molluscs and brachiopods that they hunted on the sea bed (another way in which they were similar to
walrus The walrus (''Odobenus rosmarus'') is a large flippered marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. The walrus is the only living species in the fami ...
es). The palate teeth were adapted for this durophagous diet, being extremely thick and large enough to crush thick shell. '' Henodus'', however, differs from other placodonts in having developed unique
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, which alongside features of the hyoid and jaw musculature suggest that it was a
filter feeder 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 ...
. Recent comparisons to '' Atopodentatus'' suggest that it was a
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
as well, bearing a similar broad jaw shape, albeit it obtained plant matter through filter-feeding it from the substrates. The group was once believed to be restricted to the western Tethys, but the discovery of ''Sinocyamodus xinpuensis'' in China overturned this view.


Classification

* Class
Reptilia Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
** Superorder Sauropterygia *** Order Placodontia **** Genus '' Atopodentatus''? **** Genus ''
Pararcus ''Pararcus'' is an extinct genus of placodont marine reptile from the Middle Triassic of the Netherlands. The genus is monotypic and the type species is ''Pararcus diepenbroeki''. ''Pararcus'' is known from a holotype skeleton about long from th ...
'' **** Superfamily
Placodontoidea 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 ...
***** Family
Paraplacodontidae ''Paraplacodus broilli'' is an extinct placodont sauropterygian from the middle Triassic period, from the Anisian until Ladinian stages. The fossils were uncovered in Northern Italy and the species was named in 1931 by Bernhard Peyer. ''Parapl ...
****** Genus '' Paraplacodus'' ***** Family Placodontidae ****** Genus ''
Placodus ''Placodus'' (meaning 'flat tooth') was a genus of marine reptiles belonging to the order Placodontia, which swam in the shallow seas of the middle Triassic period (c. 240 million years ago). Fossils of ''Placodus'' have been found in Central Eur ...
'' **** Superfamily
Cyamodontoidea Cyamodontoidea is an extinct superfamily of placodont marine reptiles from the Triassic period. It is one of the two main groups of placodonts, the other being Placodontoidea. Cyamodontoids are distinguished from placodontoids by their large sh ...
***** Genus ''
Sinocyamodus ''Sinocyamodus'' is an extinct genus of placodont reptile from the Late Triassic (Tuvalian) Xiaowa Formation The Xiaowa Formation is a Carnian-age geological formation found in southern China. It is a sequence of limestone and marls from the Ca ...
'' ***** Genus ''
Psephosauriscus ''Psephosauriscus'' is an extinct genus of placodont reptile from the Middle Triassic of Israel and Egypt. It is known from bony armor plates that have been found from Makhtesh Ramon in Israel's Negev desert and Araif en Naqua on Egypt's Sinai P ...
'' ***** Genus '' Psephosaurus'' ****** Family
Henodontidae Henodontidae is an extinct family of superficially turtle-like 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 ...
******* Genus '' Henodus'' ******* Genus ''
Parahenodus ''Parahenodus'' is an extinct genus of henodontid placodont only known from a skull, discovered between 2008 and 2015 and described in 2018. It lived during the Late Triassic (Carnian–Norian). The skull, named and described as ''Parahenodus at ...
'' ****** Family
Cyamodontidae Cyamodontidae is an extinct family of superficially turtle-like placodonts belonging to the superfamily Cyamodontoidea Cyamodontoidea is an extinct superfamily of placodont marine reptiles from the Triassic period. It is one of the two main g ...
******* Genus '' Cyamodus'' ******* Genus '' Protenodontosaurus'' ****** Family
Placochelyidae Placochelyidae is an extinct family of placodonts belonging to the superfamily Cyamodontoidea. Genus *'' Glyphoderma'' *'' Placochelys'' *'' Psephosauriscus'' *'' Psephochelys'' *'' Psephoderma'' References Placodonts Prehistoric repti ...
******* Genus '' Glyphoderma'' ******* Genus '' Placochelys'' ******* Genus ''
Psephosauriscus ''Psephosauriscus'' is an extinct genus of placodont reptile from the Middle Triassic of Israel and Egypt. It is known from bony armor plates that have been found from Makhtesh Ramon in Israel's Negev desert and Araif en Naqua on Egypt's Sinai P ...
'' ******* Genus '' Psephochelys'' ******* Genus ''
Psephoderma ''Psephoderma'' (meaning "pebbly skin", from the Ancient Greek ''psepho'' (ψῆφος), "pebbly", and ''derma'' (δέρμα), "skin") is a genus of placodonts very similar to the related genera '' Placochelys'' and '' Cyamodus''. ''Psephoderma'' ...
'' Additionally, the name Placodontiformes was erected for the clade that includes '' Palatodonta'' and Placodontia. ''Palatodonta'', from the early Middle Triassic of the Netherlands, was a marine sauropterygian that was very similar to placodonts, but ''Palatodonta'' has teeth that are small and pointed instead of broad and flat. The clade Helveticosauroidea was previously considered to be a basal superfamily of placodonts with the sole member ''
Helveticosaurus ''Helveticosaurus'' is an extinct genus of diapsid marine reptile known from the Middle Triassic (Anisian-Ladinian boundary) of southern Switzerland. It contains a single species, ''Helveticosaurus zollingeri'', known from the nearly complete h ...
''. However, it is now thought that ''Helveticosaurus'' was not a placodont but possibly an unusual member of the
Archosauromorpha Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, lizard ...
.


Phylogeny

The cladogram below follows the result found by Rainer Schoch and Hans-Dieter Sues in 2015.


References


Sources

* * * * * * * * * * * * * * * *


External links


The Triassic World
Mike Everhart

{{Authority control Late Triassic extinctions Carnian first appearances