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'''Placodus''' was a genus of 2.5 metre (6.5 foot) long marine reptiles, belonging to the order Placodontia, which swam in the shallow seas of the early to middle Triassic period (210 million years ago). The robust skull and teeth of ''Placodus'' show that they were specialized for a durophagous diet of shellfish such as bivalves. These animals would have plucked their hard-shelled benthic invertebrate prey from the substrate with protruding front teeth, and once in their mouths, crushed them with their back teeth. Before the animals' anatomy was known, they were regarded as fishes' teeth. Similar smaller teeth were present on the palatine bones. Chisel-like incisors protruded from the anterior margin of the snout.
''Placodus'' and its relatives were not as well-adapted to aquatic life 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 on top of the head assisted the animal with orientation rather than its vision and is presence is regarded as quite a primitive characteristic. The vertebral 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'' were heavily built and negatively buoyant creatures that would have had no trouble staying on the seafloor to feed. This body "armour" offered protection from predators as well, but would have also hampered mobility on land, making ''Placodus'' slow and clumsy out of water. It was actually a terrestrial animal, but ventured to the sea in search of food. Molluscs, brachiopods, crustaceans and other inhabitants of the seabed formed its staple diet.
Fossils of ''Placodus'' have been found in Europe (the Alps).

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A modified Placodus (along with a more inaccurate Metriorhynchus) appeared in the 2006 movie Ice Age 2.