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Ronald Orenstein wrote..
> Actually, I am not sure why it is any more of a problem than (say)
> the absence (as far as I am aware) of undoubted monotreme fossils
> from the Mesozoic
To my knowledge, there are undoubted monotreme fossils from the
Mesozoic. _Steropodon_ and _Kollikodon_ are monotremes, yes?
And on the subject of monotremes.. this just in.
FOX, R.C. and MENG, J. 1997. An X-radiographic and SEM study of the
osseous inner ear of multituberculates and monotremes (Mammalia):
implications for mammalian phylogeny and evolution of hearing. _Zool.
J. Linn. Soc._ 121: 249-291.
In quick summary.. (1) at some MTB cochlea have a legena (previously
known only from monotremes), (2) different MTB lineages convergently
evolved an enlarged vestibule, so it is not an MTB synapomorphy as
previously proposed, (3) MTB ears are poorly adapted for reception of
high-frequency airborne sounds as their cochlear canals lack osseous
laminae to support short, wide basilar membranes, (4)
bone-conducted hearing may have been important to some MTBs.
Some of this is perhaps supportive of subterranean, fossorial habits
for certain MTBs.
"Don't EVER ask me about Paris again!"
is for the view that these creatures represent an
earlier offshoot of the mammal stock than marsupials and placentals.
For one thing, how good is the fossil record generally for the upper
Triassic / lower Jurassic when protobirds presumably existed?
Compare (for example) the preservation of Protavis (whatever that is)
with that of Archaeopteryx or the new Chinese finds. Are there any
comparable beds to the Solnhofen in terms of preservational quality
for these critical periods? If not, this could be a reason why bird
ancestors have yet to be found.--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:firstname.lastname@example.org