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RE: Fossil suggests platypus lived in dinosaur times



Looking at the Rowe et al. paper including the supplementary info (but
without re-running their analysis), I am not convinced at all. There are no
support values shown on their cladogram (suggesting they were nothing to
shout about), and the only characters mentioned in the text as supporting a
position of tachyglossids outside the (_Teinolophos_, _Ornithorhynchus_)
clade are the hypertrophy of the mandibular canal and presence of the medial
tubercle at its posterior opening (derived in platypuses), which are
presumably not independent (Can or would a tubercle be recognizable without
the large canal?).

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

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.

So, to argue that tachyglossids split from the ancestor of _Teinolophos_ and
_Ornithorhynchus_ one would expect to see a statistical test: with what
level of confidence does the evidence reject the hypothesis that
tachglossids are nested somewhere within 'platypuses'?  There is no such
test presented, and only one (or 'two') dubious characters mentioned to
support the conclusion.

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. 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. 

How would I know if I'm wrong?  Somebody finding Cretaceous 'echidnas' would
do it, but I accept that the data set analysed by Rowe et al. could also.
The published text, however, does not.


-----------------------------------------------
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
riversleigh@outbackatisa.com.au
http://tinyurl.com/f2rby

-----Original Message-----
From: Tim Williams [mailto:twilliams_alpha@hotmail.com] 
Sent: 25 January, 2008 9:51 AM
To: dinosaur@usc.edu; twilliams_alpha@hotmail.com
Subject: RE: Fossil suggests platypus lived in dinosaur times


Steve Walsh wrote:

> Being from the Antipodes I wanted to alert on this one...
>
> http://www.abc.net.au/news/stories/2008/01/22/2143811.htm


As a fellow antipodean, I'm happy to provide the reference for the article
(which I think is open access)...


Rowe, T., Rich, T.H., Vickers-Rich, P., Springer, M., and Woodburne, M.O.
(2008).  The oldest platypus and its bearing on divergence timing of the
platypus and echidna clades.  Proc. Nat. Acad. Sci. U.S.A. 105(4):
1238-1242.


ABSTRACT. "Monotremes have left a poor fossil record, and paleontology has
been virtually mute during two decades of discussion about molecular clock
estimates of the timing of divergence between the platypus and echidna
clades.  We describe evidence from high-resolution x-ray computed tomography
indicating that _Teinolophos_, an Early Cretaceous fossil from Australia's
Flat Rocks locality (121-112.5 Ma), lies within the crown clade Monotremata,
as a basal platypus.  Strict molecular clock estimates of the divergence
between platypus and echidnas range from 17 to 80 Ma, but _Teinolophos_
suggests that the two monotreme clades were already distinct in the Early
Cretaceous, and that their divergence may predate even the oldest strict
molecular estimates by at least 50%.  We generated relaxed molecular clock
models using three different data sets, but only one yielded a date
overlapping with the age of _Teinolophos_.  Morphology suggests that
_Teinolophos_ is a platypus in both
 phylogenetic and ecological aspects, and tends to contradict the popular
view of rapid Cenozoic monotreme diversification.  Whereas the monotreme
fossil record is still sparse and open to interpretation, the new data are
consistent with much slower ecological, morphological, and taxonomic
diversification rates for monotremes than in their sister taxon, the therian
mammals.  This alternative view of a deep geological history for monotremes
suggests that rate heterogeneities may have affected mammalian evolution in
such a way as to defeat strict molecular clock models, and to challenge even
relaxed molecular clock models when applied to mammalian history at a deep
temporal scale." 

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