[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Echidnas evolved from amphibious ancestors

John Scanlon <riversleigh@outbackatisa.com.au> wrote:

> You may recall there was some
> discussion of the previous Rowe et al. paper in January
> 2008; I made a long comment on the DML (and an edited
> version at 
> http://scienceblogs.com/afarensis/2008/01/23/the_platypus_may_be_older_than/).
> Unfortunately, Phillips et al. didn't discuss the idea
> (which I argued for in those comments) that the Miocene
> _Obdurodon_ lies outside the _Ornithorhynchus+Tachyglossus_
> clade. Can't think of a reason why not, I guess I'll ask the
> authors.

The cladogram figured in Phillips et al. recovers _Tachyglossus_ as outside an 
_Obdurodon_+_Ornithorhynchus_ clade (Ornithorhynchidae).  So I guess, given the 
authors' confidence in this topology, they thought there was no need to discuss 
alternatives.  Plus, as I'm sure you know, the earliest known tachyglossid 
material comes from the Miocene (referred to _Megalibgwilia_), roughly 
contemporary with the Oligo-Miocene _Obdurodon_.  Having _Obdurodon_ outside 
crown Monotremata therefore requires that _Obdurodon_ be a 'relict' taxon.  
(BTW, this is not inherently unlikely, given _Obdurodon_'s apparent similarity 
to _Monotrematum_... see below.)

(I would add that an Ornithorhynchidae+Tachyglossidae clade to the exclusion of 
_Steropodon_ has been found before, by Flannery et al. (1995).  That particular 
analysis was prompted by the discovery of _Kollikodon_, and so before the 
discovery of _Teinolophos_.)

To take up your point John, it would be interesting to see what effect 
_Monotrematum_ (Paleocene) has on monotreme phylogeny.  _Monotrematum_ is 
regarded by Phillips et al. as a "late stem monotreme", meaning it's the 
youngest monotreme inferred to be outside the crown clade.  But this position 
is not tested; and _Monotrematum_ is said elsewhere to be very similar to 
_Obdurodon_ (e.g., Pascaul et al., 2002).  

Because _Monotrematum_ has "teeth and femora very similar to those of undoubted 
fossil and living platypuses" (Phillips et al.), you and Phillips et al. would 
seem to be in agreement that echidnas evolved from ancestors that had a 
distinctly platypus-like morphology.  If _Monotrematum_ is found to be outside 
the crown Monotremata, this hypothesis is upheld.  But if _Monotrematum_ is 
recovered inside the crown clade (as a bona fide ornithorhynchid, like 
_Obdurodon_), then it upsets the entire applecart: echidnas would still be 
secondarily terrestrial, but the ornithorhynchid-tachyglossid divergence would 
be pushed back into the Cretaceous, and there'd be no reason to think that 
echidna forbears were especially platypus-like (aside from being semi-aquatic).

I hope some of that made sense.