Plesiosaurs are among the most charismatic reptiles that ever lived. That's why at least a cameo by one of their snaggletoothed members is obligatory in just about any book about prehistoric life. But how did they became such iconic parts of the Mesozoic seas? Until now, it was thought that plesiosaurs originated in the early days of the Jurassic and proliferated for the next 169 million years until their extinction at the end of the Cretaceous. But a new find from Germany draws the plesiosaur story further back into the past.
Rhaeticosaurus mertensi wasn't very big as plesiosaurs go. Paleontologist Tanja Wintrich and colleagues estimate that this marine reptile was about 7.7 feet long in life. But size isn't everything. What makes this fossil so significant are its age and who it's related to.
As you might guess from the animal's name (if you're a bigtime Triassic fan, at least), Rhaeticosaurus lived during the Rhaetian stage of the Triassic. That's about 205 million years ago, and this was thought to be a time before plesiosaurs when the closest thing to them were their pistosaur ancestors. Yet Wintrich and coauthors not only place Rhaeticosaurus among the plesiosaur family, but within a subgroup of large-headed forms called pliosaurs.
So what does this mean? Well, not only did plesiosaurs evolve earlier than thought, but their initial diversification - the split between the long-necked, small-headed forms and the big-headed pliosaurs - must have already happened by 205 million years ago. Rather than springing up after the mass extinction that marked the end of the Triassic, plesiosaurs evolved before that disaster and persisted through it. In other words, these marine reptiles had quite a reach.
Name: Rhaeticosaurus mertensi
Meaning: Rhaeticosaurus means "Rhaetian lizard", after the stage of the Triassic it lived during, and mertensi honors Michael Mertens for finding the fossil..
Age: Triassic, about 205 million years ago.
Where in the world?: North Rhine-Westphalia, Germany.
What sort of organism?: A plesiosaur, being closer to the subgroup known as pliosaurs.
How much of the organism’s is known?: A skeleton, mostly still in articulation.
References:
Wintrich, T., Hayashi, S., Houssaye, A., Nakajima, Y., Sander, P. 2017. A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan. Science Advances. doi: 10.1126/sciadv.1701144
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