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RE: Platypuses may be older than we think...

Dann Pigdon wrote:

> Given the limited number of extant montotreme morphotypes (namely two), it's
> not surprising that fossil monotremes have been shoe-horned into 'platypus'
> or 'echidna' camps. Who knows what variations in form extinct monotremes may
> have toyed with? It's like trying to classify fossil placental mammals if
> only primates and antelope still existed (anything with hooves was a
> proto-antelope, anything with fingers a proto-primate!). Our modern points
> of comparison for monotremes are some-what limited.

Good point.  Nevertheless, monotremes (both living and extinct) can be broadly 
divided into two categories: 'diggers' and 'swimmers'.  Modern echidnas are 
diggers, and so was _Kryoryctes_ (based on humeral morphology).  On the other 
hand, the platypus lifestyle has been extrapolated to _Steropodon_ and 
_Teinolophos_ based on the inferred presence of platypus-style 
electro-reception.  The teeth of _Kollikodon_ have been interpreted as being 
adapted to crushing shellfish, so this critter has been regarded as an aquatic 
predator too.  _Obdurodon_ (for which a beautiful skull is known) is certainly 
a bona fide platypus.  _Monotrematum_ is also regarded as a true 
ornithorhynchid (see below).  _Monotrematum_ is known from teeth that are so 
similar to those of _Obdurodon_ that some researchers even consider the two 
genera to be synonymous (or so I've heard - I don't have any references to back 
that up).  

ktdykes@arcor.de wrote:

<< Still worse: they got that minimal age by means of assuming that the 
Palaeocene *Monotrematum* was a platypodan. Problem is, *M.* is only known 
>> from two teeth, and not a single tachyglossan tooth is known, so we can say 
>> absolutely nothing about its phylogenetic position with respect to 
>> Tachyglossa.
> I've just had an e-mail from /Monotrematum/, and it demands I correct this 
> outrageous libel. 'Two teeth' indeed! Remains are vastly more extensive; 
> 50%. A third molar was reported by Pascual R, Archer M, Juareguizar EO, Prado 
> JL, Godthelp H, & Hand SJ, 1992. Two is merely the number of lowers.


This same paper also supports the referral of _Monotrematum_ to the 
Ornithorhynchidae, saying "...the lower molar morphology of _Monotrematum 
sudamericanum_ agrees with previous statements that it is a monotreme closely 
allied with _Obdurodon_."



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