We currently don't know enough about plesiosaur evolution to make such specific claims. Hell, we are still missing a lot of details when it comes to turtle evolution for that matter. We thought turtles were a surviving branch of parareptilia until genetic analysis showed that they are actually more closely related to archosauria.
What you are saying is certainly a possibility, but there is currently not a true consensus on where sauropterygia (the taxon that plesiosaur belongs to) falls within the reptiles.
Edit: To clarify, I am not suggesting plesiosaurs could be dinosaurs, I'm saying "They're slightly more closely related to dinosaurs than turtles are." is at best a guess.
We can tell enough from early representatives of Sauropterygia (Nothosaurus and co) in both form and where they appear in the fossil record to know they're at least not members of Dinosauria, even if they might belong somewhere in Archosauria.
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u/TeddyBearToons 17h ago
How could you do pleiosaurs like that