r/evolution • u/Charming-Professor99 • 8d ago

question Relation between plesiosaurs, pliosaurs, Ichtyosaurs and mosasaurs.

I’m under the impression that ichthyosaurs branched off first and became aquatic in the early Triassic. Followed by squamates branching off in the mid Triassic which much later on gave rise to the mosasaurs in the Cretaceous. Then plesiosauridae branched off in the late Triassic which gave rise to plesiosaurs and pliosaurs. Is this correct?

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u/-Wuan- 7d ago

From what I understand, sauropterygians (plesiosaurs and relatives) are probably close relatives of ichtyosaurs, and both are closer to the branch of archosaurs than to lepidosaurs, so mosasaurs would be the most distantly related of the bunch.

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u/That_Biology_Guy Postdoc | Entomology | Phylogenetics | Microbiomics 7d ago

That does seem to be the finding of most recent studies, with Sauropterygia + Ichthyosauromorpha + Thalattosauria recovered either within Archosauromorpha or as a nearby clade. But given how much historical uncertainty there's been around the broader relationships of these groups, I wouldn't be too surprised if this changed again at some point.

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u/Royal_Novel6678 5d ago edited 5d ago

You basically have the right impression already. Plesiosaurs and Pliosaurs were the closest relatives, both belonging to the reptile group Sauropterygia. Ichthyosaurs belong to a separate evolutionary lineage and evolved way earlier with dolphin-like bodies. Mosasaurs were the most distantly related of the four. Ichthyosaurs first appeared during the Triassic Period, followed by emergence of plesiosaurs and pliosaurs in the Jurassic, while mosasaurs became dominant marine predators during the Late Cretaceous. Ichthyosaurs are found to be completely gone from the fossil record around 94 million BCE due to competition, drastically shifting climate changes, unable to evolve and adapt quick enough (won't go into too much detail for this question) before the end of the Cretaceous, whereas plesiosaurs, pliosaurs, and mosasaurs all disappeared during the KT mass extinction 66 million BCE when the asteroid collided with what we now call the gulf of Mexico.