Placodontoidea
| Placodonts Temporal range: 235–201 Ma Triassic |
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| Placodus, type genus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Clade: | †Placodontiformes |
| Order: |
†Placodontia Cope, 1871 |
| Families | |
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Cyamodontidae |
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Cyamodontidae
Henodontidae
Paraplacodontidae
Placochelyidae
Placodontidae
Placodonts ("Tablet teeth") is an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. It is believed that they were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were generally between 1 to 2 m (3.3 to 6.6 ft) in length, with some of the largest measuring 3 m (9.8 ft) long.
The first specimen was discovered in 1830. They have been found throughout central Europe, North Africa, the Middle East and China.
The earliest forms, like Placodus, which lived in the early to middle Triassic, resembled barrel-bodied lizards superficially similar to the marine iguana of today, but larger. In contrast to the marine iguana, which feeds on algae, the placodonts ate molluscs and so their teeth were flat and tough to crush their shells. In the earliest periods, their size was probably enough to keep away the top sea predators of the time: the sharks. However, as time passed, other kinds of carnivorous reptiles began to colonize the seas, such as ichthyosaurs 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 turtles of modern-day more than their ancestors without bony plates. Other placodonts, like Psephoderma, developed plates as well, but in a different articulated manner that resembled the carapace of horseshoe crabs more than those of sea turtles. All these adaptations can be counted as perfect examples of convergent evolution, as placodonts were not related to any of these animals.
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