Looking for advice: what can be a visible distinguishing feature for a fauna?
I'm looking for a feature of fauna that could distinct animals of one planet from the other. Most of animal on Earth are quadrupeds, with the exception of insects and arachnids, so I'd like to find a feature in a similar vain. Specifically something visible, like horns, or a skull shape, but more foundational.
I did come with the idea, that a lot of fauna on the planet should go through metamorphosis during growth, like tadpoles are very different from grown frogs, but I struggle to find a way to show it.
Please, anyone more knowledgeable in biology, point me to some examples of this kinds of characteristics!