The Collaborative International Dictionary of English v.0.48:

Eosaurus \E`o*sau"rus\, n. [NL., fr. Gr. 'hw`s dawn + say^ros
   lizard.] (Paleon.)
   An extinct marine reptile from the coal measures of Nova
   Scotia; -- so named because supposed to be of the earliest
   known reptiles.
   [1913 Webster]