Why There Were No Fully Marine Dinosaurs
No known dinosaur was a fully marine animal; famous ocean reptiles like ichthyosaurs, plesiosaurs and mosasaurs were separate reptile groups, not dinosaurs.
Details
The prehistoric oceans were packed with enormous reptiles, but scientists do not classify those famous marine animals as dinosaurs. Ichthyosaurs, plesiosaurs and mosasaurs evolved along different branches of the reptile family tree, even though they lived during the same broad era as dinosaurs. Dinosaurs were a distinct evolutionary group, recognized by anatomical features including their characteristic upright leg posture. The distinction can feel surprising because popular culture often groups many large Mesozoic reptiles under the single label dinosaur. Pterosaurs were also not dinosaurs, although they were close relatives within the larger archosaur group. There is an important modern twist: birds are dinosaurs, and some birds are strongly adapted to life in or around the sea. Penguins, for example, can spend substantial time in the water, but they still do not represent a fully marine dinosaur lineage in the same sense as whales or other obligate marine animals. The careful wording matters because the fossil record is incomplete, so the strongest scientific claim is that no fully marine dinosaur is currently known. The absence of a known example is different from proving that no such animal could ever have existed.
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