My version:

Scout- Anchisaurus (Closest Massachusetts dinosaur I can find for Scout)

Soldier- Ankylosaurus (Don't have to explain it)

Pyro- Pyroraptor (Pyro's origins and reign is ambiguous, but fits)

Demoman- Megalosaurus (Since Megalosaurus was discovered from Scotland)

Heavy- Oloratitan (Oloratitan is discovered from Russia and are big dinosaurs (not by fat, but still)

Engineer- Pawpasaurus (An ankylosaur that was lived in Texas, I bet it be Engie's pet dino)

Medic- Plateosaurus (Not much to say about this one)

Sniper- Australovenator (Both Australian beings)

Spy- Liliensternus (Both agile and sneaky)

The reason these choices cause I was finding the prehistoric animals where they were discovered and these be the closest that can be as the mercs in my opinion.

Now it's your turn, what do you think each be their dinosaur variant?