[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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



Basal crocodiliformes were quite similar to basal archosaurs, and some crocs 
did evolve to live in marine environments, so looking at what happened during 
their divergence might give some clues. Wasn't there a new marine or 
semi-marine crocodilian reported at SVP?  And as Tom said, crocodilians were 
the ones who settled in the right environments to expand their habitats into 
the ocean. 

At 8:55 AM -0400 10/25/06, Thomas R. Holtz, Jr. wrote:
>
>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).
>
-- 
Jeff Hecht, science & technology writer
jeff@jeffhecht.com  http://www.jhecht.net
525 Auburn St., Auburndale, MA 02466 USA
v. 617-965-3834; fax 617-332-4760