Going to the sea! The "big turtle" in hot pants

During the Triassic period, a group of terrestrial vertebrates, perhaps under pressure from life, abandoned the land that their ancestors had built and “went to the sea”, becoming the largest vertebrate return to the ocean in the history of the Earth. These animals found that the ocean was vast and promising, so they rapidly evolved in a short period of time, and marine reptiles such as fin dragons, ichthyosaurs, and thalassos appeared. When their fossils are exhibited in museums, they are sometimes mistaken by uninformed viewers as dinosaurs in the sea, including ichthyosaurs that look like fish. However, among the fin dragons, there is a primitive and specialized group that is difficult to mistake for dinosaurs even if one has no knowledge of marine reptiles. Instead, they would be mistaken for turtles. They are placoderms.

The body length of placodontosaurs is generally 1 to 2 meters. Except for a few early members, most of them are covered with bony plates, so their appearance is very similar to that of turtles, such as Beantooth Dragon, Shield Dragon, and Pteranodon. However, if you look closely, you will find that they actually have an obvious difference from turtles. The carapace of turtles is made up of regular large pieces of scales, while the carapace of placodontosaurs is a pile of small and small scales. Moreover, the carapace of some members, such as Beantooth Dragon, is composed of two parts. The larger part protects the neck to the hips, and the smaller part protects the hips to the base of the tail. It looks like a big turtle wearing a pair of hot pants.

In addition, teeth are also the main feature that distinguishes placodonts from turtles. Early turtles had small and sharp teeth on their jaws, but today's turtles have mostly degenerated teeth, leaving only horny beaks. Except for the Pteranodon, all placodonts have a mouthful of stone- or bean-shaped teeth, which is why they are named placodonts and bean-toothed dragons.

Their thick carapace and flat teeth mean that they are destined to be unable to prey on fast-moving fish like the agile ichthyosaurs, but they are very suitable for carrying out dimensionality reduction attacks on mollusks protected by hard shells in shallow sea areas, and can be called professional shell crushers.

However, even such a group of creatures that value defense and have a slender development did not survive the fourth mass extinction of life in the late Triassic period. The reason is related to their highly specialized body structure and relatively single food source. However, the cool design that vertebrates can also stack bone plates to form thick armor has been cleverly inherited and carried forward in the countless repeated "convergent evolution" of nature. It is not limited to reptiles such as turtles, but also reflected in mammals, such as today's armadillos, and their extinct close relatives - glyptodonts. The above is the content shared in this issue. See you next time if you are interested.