Behold the Platypus

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Why is the platypus such a mix of mammal and reptile? Where did this strange mix of characteristics come from? Are there any other animals like this that don’t seem to fit in?

In: Biology

Remember that mammals were once reptiles. Mammals are just a sub-category of reptiles that got warm and furry for some reason. Dinosaurs are another subset of reptile, and birds are too since birds are just a subset of dinosaurs. And of course, reptiles are just a subset of four-legged vertebrates, which are a subset of bony fish, which are a subset of vertebrates.

Categories of animals are colloquial. We group them together based on shared traits like having scaly skin, but if we were to categorise them properly, we’d do it by evolutionary relationship, and a species can never evolve out of its membership of its ancestral group, because this system of categorisation is one in which each segment always belongs to a larger class defined by its most recent common ancestor, like how humans are also primates, a category of species that all share a particular common ancestor.

Because this is evolution, these species don’t just spontaneously arise with different traits. They emerge from repeated mutation away from those common ancestors, and retain most of the same stuff those ancestors have – for example, every mammal still has the core body plan of a four-legged vertebrate, but in some cases, mutations have obscured this structure (in whales for example, the front limbs have become fused flippers instead of feet, and the back limbs have shrunk to the point of being just clusters of useless bone). We think that monotremes like the platypus probably diverged from the mammal line before the shift to live births in the mammalian mainstream, but after the development of some other notable mammalian traits. That is – the platypus and other mammals share a most recent common ancestor that laid eggs, with the therian mammals going “y’know what I’d rather squeeze out a weird tiny version of myself than an egg”.

Why are there so few animals left today that have traits that are common to two very distinct groups then? Why aren’t there more mammals that lay eggs, or have wings like bats and such? Is it just bcuz animals like that have evolved away/died out?