Use the past tense to refer to extinct animals like dinosaurs.
Pterosaurs are extinct, so you should write:
“Pterosaurs were cold-blooded like modern reptiles.”
“Pterosaurs are cold-blooded like modern reptiles.”
The sea cow is extinct, so you should write:
“Kelp was the main food source of the sea cow.”
“Kelp is the main food source of the sea cow.”
You might think this is a very obscure point, but I fix this sort of error every week when I review student essays. ETS loves to create integrated writing questions about dinosaurs and other ancient animals.