Let's see...
If we're talking about icthyosaurs....
Here's a good Wikipedia article on them... http://en.wikipedia.org/wiki/Ichthyosaur
And here's a website from an accredited college - Berkeley... http://www.ucmp.berkeley.edu/science/parks/berlin_ichthyosaur.php
Their presence in Nevada would probably be in the Triassic period's sedimentary layers.... Between 250 million and 202 million years ago...
Apparently a branch of the Pacific Ocean reached as far inland as Nevada - er, what would eventually become Nevada, at that time...
Let me see if I can find a description of Triassic-era Nevada...
brb...