Teeth.

Makes me think of this: https://en.wikipedia.org/wiki/Dentition#Dental_formula

Teeth can tell you a lot about an animal. Their size, shape, number etc can tell you what it eats or how it eats or how much it eats. Dental formula can give you clues for how animals are related to each other. If two different animals have the same dental formula then maybe they share a common ancestor or have a similar diet. If two animals are already very similar but have different dental formula then maybe they're only very distantly related or there's been some convergent evolution elsewhere.