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Re: Platypuses may be older than we think...
So. Here is the surprisingly short SVP meeting abstract (p. 137A):
Timothy Rowe, Thomas Rich, Patricia Vickers-Rich, Mark Springer & Michael
Woodburne*: Fossil evidence on divergence timing of the platypus and echidna
"Monotremes have left a poor fossil record, and paleontology has been
virtually mute during nearly two decades of discussion about molecular clock
estimates of the timing of divergence between the platypus and echidna
clades. Molecular clock estimates of the divergence between platypus and
echidnas range from 17-80 [sic] Ma. New evidence from high-resolution X-ray
computed tomography indicates 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. Divergence of the two monotreme
clades therefore had occurred in or before the Early Cretaceous.
*Teinolophos* is a platypus in both phylogenetic and ecological aspects,
suggesting that monotremes have evolved at much slower rates than therian
mammals. Only relaxed molecular clock estimates of divergence times have
proven compatible with the fossil record."
* Institutional address given as "Museum of Northern Arizona, Flagstaff, AZ,
Doesn't seem to address the question of whether the echidnas, with their
reduced electro-sense, are secondarily terrestrial, perhaps having evolved
on some part of northern New Guinea when it started emerging from the sea in
the Miocene or so. In that case we'd have to expect that *Teinolophos* "is a
platypus in [...] ecological aspects" like *Steropodon* and *Kollikodon*.