Monotremes and you and me

Found this buried in among the drafts and I have no idea when it was…

It began as a post about a donkey in a hole. Farmer shovels in dirt, donkey takes a step up, farmer shovels in more dirt, donkey takes another step up. Finally the donkey is out of the hole and the hole isn’t even a hole, it’s full of dirt. The lady had thought maybe it was a real story–the picture of the donkey in the hole–and hated the farmer. Not sure why, but on Facebook you have to hate something. Me, I hate cat pictures. This lady hates farmers with donkeys in holes. I stopped paying attention. A day or two later I check in again, unintentionally. The thread had gone off topic, from donkeys to platypuses. Placental mammals to monotremes. The lady hated platypuses. Why do you hate platypuses, someone asked. They lay eggs, she said. Another guy said it’s because they’re monotremes, sort of a link between mammals and reptiles. The lady said eww….

Eww? Have you ever seen a platypus or an echidna, I asked. I mean, they’re adorable. Personally I love echidnas. I’ve never seen a real platypus…there aren’t any outside Australia, not even in zoos.  They have laws keeping them there. Echidnas, though, are in zoos all over the place. Some come from Australia, some from New Guinea, they look kinda like hedgehogs and trundle about looking for ants mostly and are cute as hell. The L.A. Zoo had a whole indoor display full of eucalyptus and koalas and echidnas. The koalas mostly slept, the echidnas trundled. I was being kind of Facebook obnoxious, I have to admit. But I was only beginning.

Because to be one of those really obnoxious Facebook people, I explained that a monotreme is actually a living example of one of the earliest stages of mammalian evolution, an early stage on the track which went from which from monotremes like echidnas and platypuses to marsupials like kangaroos and wallabies to placental mammals like you and me. And to be even more incredibly obnoxious, I explained that mammals didn’t evolve out of reptiles, synapsids did. The early synapsids are called  pelycosaurs, the most famous of which is the sail-backed dimetrodon.

Dimetrodon, about 275 million years ago.

Every child’s  favorite pelycosaur, the Dimetrodon, about 275 million years ago.

Most synapsids were much less exotic looking, however. And one, sans sail, evolved into therapsids, aka the mammal-like reptiles, from whence we eventually came.

The weirdly cat-like Pristerognathus, about 260 million years ago.

The weirdly cat-like therapsid Pristerognathus, about 260 million years ago.

Alas, nearly all the pelycosaurs and therapsids died out in the Permian extinction event (which took out 90% of all know species on earth, actually). Just before that, though, in the late Permian, a line of therapsids had evolved into the way more mammal like cynodonts, some of which somehow survived the Permian extinction to continue in the Triassic, a world virtually bare of species and ripe for dramatic evolutionary change, especially among reptiles. Hence all those wacky dinosaurs. Cynodonts, in comparison, seem much more mundane and functional. No cynodonts became tyrannosaurs or apatosauruses. None ever rose out of our imaginations to level Tokyo.

But one line of cynodonts that had survived the extinction eventually evolved into early mammals sometime in the latter half of the Jurassic, little shrew looking things that hid in the shadows from dinosaurs and were predecessors of the echidna and platypus and in the long run, you and me. But it would take another extinction event, this time a massive meteorite or comet, to end reptile dominance and enable our skulking little shrew-like predecessors to evolve into a zillion species and fill niches world wide. Actually not all were shrewlike, some may even have been downright cute, for egg layers:

At eighteen inches long, castorocauda was more like a platypus than a shrew, 160 million years ago.

160 million years ago the castorocauda, at eighteen inches long, looking more like a platypus than a shrew

However, you and me, being primates, probably did evolve out of the sneaky little shrew like creatures skulking about in the dark. But we didn’t evolve directly out of reptiles. And we are not related to dinosaurs at all. What did evolve directly out of reptiles, of course, were birds, which actually are dinosaurs, the only surviving dinosaurs who then radiated globally into a zillion species in the global emptiness follow mass extinction. And just as chickens are dinosaurs, we humans all still synapsids, like the dimetrodon. In fact, synapsids are much older than dinosaurs, which means that birds are actually a later creation than are mammals. Way later. Hence we keep them in cages and dice them into McNuggets. And when you look at a raven and think that’s a dinosaur? It could be looking at you and thinking and that’s a synapsid? Except all it’s really thinking is that it wants your sandwich.

What does this have to do with a donkey in a hole, someone said.

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