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RE: Fossil suggests platypus lived in dinosaur times



John Scanlon wrote:


> It is known that tachyglossids have electroreceptive organs homologous to
> _Ornithorhynchus_, but with a much lower number of terminals; contrary to
> the assumption in Rowe et al., it is certainly plausible that this condition
> is vestigial (partial reversal) rather than an 'incipient' stage that has
> been retained for over 100 Ma!


Yes, I'll go along with that.  It appears plausible (to me) that the 
terrestrial/fossorial tachyglossids evolved from an aquatic or sem-aquatic 
monotreme which (as in the modern platypus) used its limbs for digging burrows. 
 In the process, the electroreceptive organs may have been secondarily reduced 
in number.  However, in arguing this, I'm not necessarily arguing that echidnas 
evolved from platypuses (ornithorhynchids - in this sense, the least inclusive 
clade comprising _Obdurodon_ and _Ornithorhynchus_).


> The tachyglossid mandible is edentulous and
> reduced overall, so reduction of one of its constituents (mand. canal) is
> arguably of no weight at all.


The tubular mandible of tachyglossids is highly specialized and associated with 
their insectivorous habits; so the edentulous condition has a separate function 
to that of the 'duckbill' of the modern adult platypus.  Personally, I don't 
think there is any compelling reason to argue that the edentulous condition 
evolved only once in monotremes, any more than it did in theropods (phew, 
finally managed to get a mention of dinosaurs in!)


> And given what we know about survival of other amniote lineages at the K/T
> (K/Pg, whatever), the prior expectation would be that one monotreme lineage
> survival is rather more likely than two.


I wouldn't have said so, but it's a difficult point to argue whichever side you 
take.


> The mandibular character(s) seem to me to be outweighed by: complete loss of
> adult dentition, and more or less complete fusion of cranial sutures in
> adults. 


Rowe &c argue that the hypertrophied mandibular canal is indicative of a 
'duckbill'.   As for the two characters you mention (complete loss of adult 
dentition; more or less complete fusion of cranial sutures in adults), you are 
in effect arguing that these characters are synapomorphic, whereas the 
characters recovered by Rowe &c are homoplastic.  In doing so, you're setting 
aside the character distribution (and polarities) offered by the cladogram.


Yes, I have reservations about the cladogram too.  (What is _Hadrocodium_ doing 
in the Monotremata, for example?)  But I have reservations about every 
cladogram because, as a cladogram, it's just one phylogenetic hypothesis that 
will stand or fall depending upon future data.  (This isn't a criticism of 
cladistics; it's just recognizing the limits of a cladogram.)  I doubt if the 
topology presented by Rowe &c is the last word.  Other fossil monotremes will 
turn up, and they'll spawn other hypotheses about the evolution of monotremes, 
which may or may not accord with that proposed by Rowe &c.


> These fairly uncommon and logically independent apomorphies unite
> Tachyglossidae and _Ornithorhynchus_ to the exclusion of _Obdurodon_ (and
> other known monotremes for the tooth character, skulls being unknown).
> Oligo-Miocene _Obdurodon_ is clearly a 'platypus', so I inferred a couple of
> decades ago that 'platypuses' were ancestral to Miocene-to-Recent echidnas.


I'm not sure if I'm conflating both sentences, but are you advancing the 
hypothesis that _Obdurodon_ is the sister taxon to an 
_Ornithorhynchus_-Tachyglossidae clade?  (If I'm misinterpreting your 
statement, then I apologize.)  If so, then I see problems; first among them is 
that this hypothesis requires a secondary loss of most (but not all) of the 
characters associated with the 'platypus' morphology in the line leading to 
echidnas.  Morphologically, both tachyglossids (including fossil taxa like 
_Megalibgwilia_) and ornithorhynchids (including _Obdurodon_) are highly 
derived, and very distinct.  


> How would I know if I'm wrong? Somebody finding Cretaceous 'echidnas' would
> do it, 


Yes, it certainly would.  _Kryoryctes_, alas, doesn't seem to be a tachyglossid 
(despite some press statements claiming otherwise).


BTW, I had the pleasure of seeing a real life platypus a few weeks ago.  Sydney 
Aquarium has three in captivity, and they were all up and about.  Not only are 
they beautiful creatures to behold, but they're also rather playful.  I'd even 
go so far as to say that their behavior often qualified as 'feisty'.


Cheers

Tim
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