Monotremes (Re: Walking With Dinosaurs Skin TExtures)

["Timothy Williams" <twilliams_alpha@hotmail.com>]

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...  ;-)


Tim




______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com