Muppet [stem] Monotremes

Various depctions of stem-monotremes. Clockwise: Kollikodon by Cerri Thomas, Steropodon (or “SPEropodon” as the artist puts it) by Joschua Knüppe and Teinolophos by paleozoografica. Kollikodon might be a haramiyidan instead according to more recent papers, so just pretend its some other australosphenidan if you want.

Of all non-therian mammals, monotremes naturally are the best understood and well known given that they survived to present times in the first place. Crown group monotremes are a full Cenozoic phenomenon as echidnas evolved very recently from a platypus-like ancestor, but there is an abundance of Mesozoic stem-monotremes (collectively known as the australosphenidans or australosphenidan grade mammals due to teeth superficially convergent with those of boreosphenidans) known from Australia, South America and Madagascar.

Traditionally, these animals are depicted as platypus like, but it has since been found that at least Teinolophos (and likely all other Mesozoic stem monotremes) did not have a beak like modern platypodes do. One might presume that this would lead to stem-monotremes being depicted as more generic mammals, but instead this has lead to a wave of bizarre muppet faced creatures with plain muzzles devoid of whiskers or rhinaria.

I get the intent behind these depictions. It’s essentially like the echidna snooters, but thick and full of teeth. Unfortunately, I don’t think that’s a good model because as mentioned echidnas evolved from aquatic ancestors, and so while they lost the fleshy beak (kind off, their snouts have hardened skin that is still a bit beak-like) they couldn’t regain the lost characters like whiskers or wet noses.

Echidna (artist unknown). A red herring and it doesn’t even live on water!

Rather, I think it’s not at all weird to depict stem monotremes with “normal” mammalian snouts. After all, many of these species were rather opossum or shrew like in apparence and perhaps in habits. The platypus beak itself is basically an overgrown rhinarium and electroreceptors are known to derive from whisker pits in mammals, so we know for a fact early monotremes had both rhinaria and whiskers.

Henosferus depiction by Jorge L. Blanco. Note normal mammalian snout.

Platypodes and echidnas are essentially if we had desmans and moles to dictate how therian mammals looked like. It doesn’t take a genius to see that they’re not good analogues for the sheer diversity of their group.