Because placodonts were adapted for living in shallow near-shore ocean environments, their anatomy was optimized to help them plod along the seafloor looking for stationary prey such as hard-shelled mollusks and crustaceans.
Placodonts fall into two body types: the unarmored placodonts, informally known as the Placodontoidea, and the armored placodonts, known as the Cyamodontoidea. Body characteristics of the placodonts included:
Skull. The placodont skull was strong and heavily built. It was flat and wide in shape.
Sturdy jaws and crushing teeth. Placodonts ate very hard-shelled animals. Their ability to seize and crush these creatures with their jaws was the key to their survival. For grabbing, they had peglike, forward-pointing teeth at the front of their jaws and short, rounded teeth on the sides. For crushing, they had flat, tilelike tooth surfaces in the back of their mouths. Massive jaw muscles helped the placodont crush a mollusk's protective shell like a nut in a nutcracker. Once the shell had been pulverized, the placodont could eat the soft body of the mollusk hiding inside. The front teeth were weaker or absent in the armored placodonts, being replaced in at least one kind—Henodus ("one tooth")—by a toothless beak similar to that of a turtle.
Backbone. The backbone of the placodonts was thick and strong. The short neck and tail were flexible but the back portion was locked firmly together, making the trunk of the animal inflexible. This suggests that the placodont's only means of propelling itself through the water was by moving its short tail.
Short, stumpy legs. The legs of placodonts were similar to those of land-dwelling reptiles. The five fingers and toes on each limb may have been covered with webbing to give them a more paddlelike appearance, similar to the limbs of turtles. The legs were short and could be used to help the animal move through the water, but not with great speed. As above, the tail was probably a placodont's primary means of propulsion. The feet could have been used to steer and guide it over the sea floor.
Body shape. The unarmored placodonts had tall, long, rounded bodies. The armored placodonts were short and much flatter.
External armor. The Placodontoidea, or unarmored placodonts, had only a hint of external body armor in the form of a bony knob on top of each vertebral spine. In contrast, the Cyamodontoidea, or armored placodonts, developed a broad, turtlelike shell on their backs (the carapace) as well as (in some kinds) armor on the neck, skull, and tail. Some cyamodontoids also had an armor-plated underside similar to the plastron seen in turtles.
Belly ribs. The unarmored placodonts, especially Placodus, had a belly reinforced by extensive belly ribs. Belly ribs are common to reptiles, but in the case of Placodus, they were highly developed. They not only strengthened the belly of the animal but angled upward to protect its sides as well. This cage of bones would have provided protection from predators but also provided support for the body when the animal walked on land.
Unlike some other marine reptiles that were highly adapted for ocean life, placodonts appear to have been able to crawl back on land when they wanted. In this way, their lives may have been similar to that of a turtle that can walk on land as well as swim in the water. It is conceivable that placodonts spent much of their time out of the water, perhaps (for example) to lay eggs during the breeding season. While this would have allowed them to avoid larger marine predators such as ichthyosaurs and nothosaurs, the land was not without its dangers. In either case, the unarmored placodonts in particular were probably easy prey. They were a distinct kind of marine reptile, caught in an existence that was neither totally aquatic nor totally terrestrial.
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