[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.
Cheers
Tim