Platypuses have traits of birds and reptiles, but also have traits of mammals. Rather puzzling to scientists picking over pickled specimens was their internal reproductive system was more like a bird than a mammal.

Female platypuses don't have vaginas. They have a cloaca, which is a single hole for pee, poo, sex and eggs. Eggs. Which they lay.

The female platypus has two ovaries, but only the left one is functional. They also have two organs which can be called uteruses or oviducts. The embryo isn't in there very long, and the uterus puts a shell on it before laying the egg.

The weird inner junk aside, platypuses also don't have nipples, even though they produce milk, like other mammals. They secrete it through the pores in their skin, which then pools on their belly. Their young lap it up from these little milk bowls.

These discoveries throughout the nineteenth century were quite an existential crisis for the burgeoning field of taxonomy, who were trying to neatly define and group animals, then this pesky creature with its peculiar birdy inner junk and no nipples but milk comes along.

There were arguments. Lots of arguments about what to do about platypuses. Also, huge questions about whether they were using their bird-like internal organs to make eggs or live young because nobody had seen it in action yet.