Is Ichthyosaurus a Plesiosaur?

[Daniel Fischer]

Hello, I have a simple question but I can't find the answer and I think many of you probably know the answer.

Is the taxon Ichthyosauria  a part of the taxon Plesiosauria?

I want to start learning more about Ichthyosauria and this fact simply bothers me not to know.

[RuMert]

Nope, they are totally different. Ichthyosaurs' origin is not clear as the earliest known species were already aquatic. They lived at least from the early Triassic. Plesiosaurs are another order of reptiles, they are part of Sauropterygia taxon (with placodonts, etc). Sauropterygia also appeared in the early Triassic.  Both groups are very different from each other and the rest of reptiles, including archosaurs, dinosaurs, pterosaurs, lizards@snakes, turtles, etc. 

 

Ichthyosaurs were much more adapted for aquatic environment, having fish-like body, short amphicelous vertebrae (increased flexibility), big eyes, lots of verts, long powerful tail, reduced pectoral and sacral girdles (not connected to the spinal column), digits increased in number and totally converted so they looked more like small vertebrae, etc.

 

Plesiosaurs retained digits in more typical form, had short tails, platicelous verts (whale-like, better than those of terrestrial animals, but less flexible than those of ichthyosaurs). They looked more like typical reptiles (that's why they are called so after all, "more saurian"), had powerful legs and unique movement style (used both pairs of legs equally while other aquatic animals tend to lose hind legs akin to whales). They strangely survived longer than ichthyosaurs, till the end of the Cretaceous.

Edited August 22, 2021 by RuMert