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Monotremes (Re: Walking With Dinosaurs Skin TExtures)

Dann Pigdon wrote:

> Can't think just what _Steropodon_ is, but it ain't a dinosaur.

It's a marsupial (perhaps an early platypus) from the Early Cretaceous of
Lightning Ridge. It was one of the giants of Mesozoic mammals (about cat-sized).

Dann, I'm sure that's a typo (substitute "monotreme" for "marsupial").
Interestingly, apart from being very large (by Mesozoic mammal standards), _Steropodon_ and fellow Aussie monotreme _Kollikodon_ are the earliest evidence for aquatic mammals, at least as far as I know. (In both genera, an aquatic lifestyle is inferred from the design of the teeth and jaws - the only material known for both Mesozoic monotremes.)

_Steropodon_ certainly appears to be more closely related to modern platypuses than to the echidnas (spiny anteaters), although _Steropodon_ is distinct enough to be placed in a family (Steropodontidae) separate from the platypus (Ornithorhynchidae). Nevertheless, it does indicate that the platypus line (Platypodes) and echidna line (Tachyglossia) diverged well before the end of the Cretaceous. This is long before the time most placental orders are believed to have originated. Not surprisingly, many mammalogists now prefer to regard the platypus and echidnas as belonging to separate _orders_ of monotreme mammals (Platypodes and Tachyglossia), rather than lumping them (and their extinct kin) into a single order Monotremata.

But of course, "orders" are now passe. We've all moved on from those antiquated Linnean ranks... ;-)


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