'Ichthyosaur' definitions:

Definition of 'ichthyosaur'

noun

Any of several marine reptiles of the Mesozoic having a body like a porpoise with dorsal and tail fins and paddle- shaped limbs

From: GCIDE

Ichthyosaur \Ich"thy*o*saur\, n. [Cf. F. ichthyosaure.] (Paleon.) One of the Ichthyosaura. [1913 Webster]