If you need to craft something that needs Chitin or Keratin don’t get confused you can use both Chitin OR Keratin to build it.To get Chitin you can kill trilobites or any bug like thing for it.And to get Keratin you can get it from the big turtles.

Nobody mentioned scorpions yet, if you’re able to kill, they give a bit.

A way to easily get keratin is to kill big turtles and just kill bugs for chitin.