How about Kamen Rider Kyoryu! Japanese for dinosaur.

I like Paleo then maybe Jurassic, Triassic, and Cretaceous can be forms

Kamen Rider Kaseki (Fossil)..

Yep. Exactly what I was thinking. Jurassic, Triassic, and Cretaceous forms, and the final form could be Mesozoic. No idea how those would translate aesthetically.

One of the reasons I said "Paleon" instead of "Paleo" initially was to separate it entirely from all that "Paleo Diet" fad nonsense. Oh well.

I was thinking about the kaijin being based on....

- Jurassic
- Triassic
- Cretaceous

This one seems really good, especially if you think of the forms the each era can add

I was thinking the kaijins could be skeletal, sort of like dinosaur bones.

Let's not get ahead of ourselves, we're only trying to come up with a name. We'll talk about forms and villains later.
So far the names I've chosen are:
Paleo
Paleon
Jurassic
Triassic
Kyoryu
Kaseki

I'll take up to 4 more names, so give some more ideas.

As for the secondary rider, it needs to be based on prehistoric/extinct beasts.

Jumping ahead henshinzero..