Placodonts were a fascinating group of marine reptiles that lived during the Triassic Period in the Mesozoic Era, between around 235 and 200 million years ago (MYA), and thought to have a common ancestor with the long-necked plesiosaurs and the long-necked nothosaurs, all of which are in the large group called the saruopterygians. As a group, the placodonts are sometimes referred to as "walrus turtles," due to their diet of molluscs and other shelled invertebrates and, of course, their general appearance.
Like the turtles and tortoises of today, the placodonts would not have been all that quick and nimble. Their weight would have made them negatively buoyant, akin to the manatees and sea cows of today, meaning that they would have had no trouble swimming along the bottom of the ocean, snapping up molluscs and other shelled invertebrates, and using their large, flattened cheek and palatal teeth to crush them down. It's also possible that they scraped algae off of marine rocks and swallowed it whole, letting it slowly digest in their massive guts! Like modern sea turtles, the placodonts are thought to have been amphibious, spending most of their time (sleeping, chillaxing, and other activities) on land, but dipping into the water to feed, akin to the extant marine iguana of the Galápagos Islands.
Although younger placodonts would have been especially vulnerable to predation from many different types of animals, many paleontologists puzzle over why the ponderous adult placodonts would have needed this armor, as there don't seem to be any marine predators capable of making a meal out of them that lived at the same time. Of course, as we mentioned before, they probably spent much of their time on land and, as we also mentioned before, their awkward build would have left them particularly vulnerable to attack. So to counter this, some of the placodonts started to evolve in a very turtle-like appearance.
Placodus, the animal whose name is lent to the entire group, displays a single row of dermal ossifications above its neural spines down its back. This dermal armor (a fancy way of saying "skin armor," in which the skin hardens into an armor like structure, as seen in the armadillo) would have helped to protect the animal from attack. Earlier, more primitive genera, such as Paraplacodus, lack this dermal ossification. However, other, more derived placodonts take this ossification of the dermals to a whole new level.
Remember Henodus, the first picture in this post? Henodus is one of those dermal armorers (I don't think that's actually a word) that took the ossification of the dermals to a whole new level. Outwardly similar to the turtle shell, the placodont armor was composed of a number of polygonal ossicles, while the shells of turtles are composed of large plates. Two other placodonts that were extremely well armored include Cyamodus, as well as the VERY turtle-looking creature, Placochelys.
Like many other animals, the placodonts became extinct at the end of the Triassic Period. Many groups did survive, however, and one of those groups that survived through to the Jurassic Period was actually the turtles! 220 MYA, the placodonts were sharing the seas with what would one day become the hard-shelled reptiles adored by so many people!
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