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Swimmin' dinos, fish milk, avian polyphyly (was RE: Sheesh)



> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]On Behalf Of
> Mike Taylor
>
> Deinonychus47@aol.com writes:
>  > Of course, the fact that we have no evidence of aquatic or
>  > mole-like dinosaurs cannot be explained by the fact that we have
>  > just not discovered any representative fossils, can it?
>
> It's always possible, but since marine animals generally have better
> preservation potential than terrestrial, it seems unlikely.  We've
> found enough ichthyosaurs, plesiosaurs, nothosaurs and mosasaurs by
> now that it's hard to believe a robust clade of aquatic NON-AVIAN
> dinosaurs would have remained undetected till now.

Here are a few thoughts on why the first group of dinosaurs to become marine 
were hesperornithiforms:

1) Chance. The Null hypothesis. They others simply never got there.

2) Mechanical constraints. The limbs of most dinosaurs were pretty tightly 
locked into parasagittal motions: great for striding, but
not terribly good for swimming. Additionally, the major clades had mechanisms 
that tended to tighten up the flexibility of the
dorsal column (e.g., epaxial ossified tendons or hyposphene-hypantrum 
articulations): again, great for striding, not so good for
undulatory locomotion.
        --In this case, the reason that advanced birds might have been able to 
colonize the seas is that they were able to exapt their
already-transformed fore- and hindlimb orientations, bypassing as it were the 
phases that a stegosaur or compsognathid would have to
go through.

3) Physiological constraints. Okay, so this isn't really testable or what have 
you, but just suggesting it...

4) Ecological constraings. Not particularly happy with this one, but how about 
this: nearshore swimming niches were already occupied
by various other reptile clades (basal euryapsids, for instance), and when 
those spaces opened some other group (e.g., crocodilians)
happened to beat the dinosaurs to it.
        --Again, the reason that derived birds might have made it as sea dinos 
is that they were able to bypass the near-shore life habit
stage by having a novel method (i.e., diving from above) to make it as pelagic 
creatures. We already know of other Cretaceous
derived avialians (such as _Ichthyornis_) who may have been pelagic feeders.

And as for fish milk: a gland that leaks liquid into another liquid might not 
be the best method of delivering nourishment. However,
in a non-aqueous environment, glands already leaking liquid into air (e.g., 
sweat glands) might evolve, allowing for exaptation as a
source of nourishment.

A.P. Hazen: the skin-feeding animal you are likely referring to (the one 
announced earlier this year) was a caecillian (a member of
Lissamphibia, the modern amphibians), not a fish. However, there might well be 
skin-feeding fish for all I know.

Dora Smith: the idea that modern birds might have arisen out of multiple 
different groups has been proposed many times over the last
century. A good review of the older ideas is available in:
Witmer, L.M. 1991. Perspectives on avian origins. pp. 427-466, in H.-P. 
Schultze and L. Trueb (eds.), Origins of the Higher Groups
of Tetrapods. Cornell Univ Press.
Check the archives of the list for more recent incarnations.

However, the idea of avian polyphyly is a highly, highly, highly unlikely 
scenario, given the copious amounts of shared skeletal and
soft tissue specializations unique to this group (and the good record of those 
skeletal transformations in Cretaceous birds).

(For the record, mammalian polyphyly was also once fashionable, but now 
abandonded).

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796