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Re: Obdurodon (was Re: Coronosaurus, new ceratopsid genus for Centrosaurus brinkmani)



K and T Dykes <ktdykes@arcor.de> wrote:

> I was thinking along the lines of secondarily terrestrial echidnas, in my
> own utterly unqualified, amateurish way, for a different reason a few years
> ago, and that was spurred on by a paper proposing platypus-credentials for
> /Teinolophus/ from the Lower Cretaceous; that is the proposed placement of
> Teino in the family of Ornithorhyncidae.  That was at least in part based
> upon jaw similarities with a platy.


_Teinolophos_ certainly shares jaw similarities with a platypus; but
too little is known of it to be certain that it was a bona fide
ornithorhynchid.  It is perhaps more likely that the platypus-like
characters of _Teinolophos_ (and _Steropodon_, and _Monotrematum_
too?) were present in stem-monotremes, and are therefore primitive for
the crown group.


> It left me wondering how on Earth echidnas ended up with electro-receptors
> in the beak, albeit not all that many of them. These aren't an obvious
> blessing for terrestrial mammals, although they could presumably be of some
> help in the moist soils of humid mountain forests.  They could, I mused on
> the basis of nothing in particular, be a hang-over from semi-aquatic
> ancestors.


Based on current evidence, it does seem likely that the echidnas
(Tachyglossidae) evolved from amphibious/aquatic platypus-like
ancestors: echidnas are therefore secondarily terrestrial.  Among
mammals, the elephant lineage (Proboscidea) has also been proposed to
be secondarily terrestrial.


It is also possible that ornithuromorphs first evolved in water, and
all subsequent non-aquatic lineages are therefore secondarily
terrestrial or secondarily arboreal.  I base this on the preponderance
of basal ornithuromorphs that had aquatic adaptations.







Cheers

Tim