what was the name of that book?

[rodlox]

I can remember a few things about it...but not the title or the author.

it features -
- a scientist recruited by a mysterious old man who gives the scientist part of a stegosaur as a gift.
- an explanation of "a type specimen" - in this case, the type specimen of human beings.
- time travel.
- students lost in the Mesozoic with their teacher.
- the mysterious old man keeps a skull on his desk, and everyone thinks it is the skull of his greatest enemy.
- smart birds.
- Tyrannosaurs and sauropods.
- time loops around on itself repeatedly for one character, providing an easy source of cheap labor.

if I can remember anything else, I'll add it. anybody ever come across anything like this?

thank you.