LAGs (was RE:Origin of feathers]

["Ralph Miller III" <gbabcock@best.com>]

> From: Jonathan <spockjr@email.msn.com>
> To: archosaur@usa.net
> >Interesting, but aren't there fossil birds that show lack
> >of blood channels in the bones. Thus indicating that early birds were
> ectothermic.
> >
> >Archosaur J
> 
> Actually, a few enantiornithine birds were found to
> have growth rings in a few bones which led (i believe)
> Chinsamy et el. to decide that they were ectotherms.
> This doesn't necessarily indicate ectothermy for them
> or in any other earlier birds which have not been studied
> histologically.

Ansuya Chinsamy stated in her Dinofest presentation that the presence of
vascularized bone with slow growth lines in the bones of dinosaurs and
early birds (_Patagopteryx_, _Enantiornithes_, _Gobipteryx_, and
_Archaeopteryx_) indirectly reflect what is likely to be a physiology
somewhat between endothermy and ectothermy.  Modern birds, _Cimilopteryx_,
_Ichthyornis_, _Hesperornis_, indeed the whole line of Ornithurine birds,
were judged to have had rapid, sustained growth on the basis of
vascularized bone that lacked LAGs (lines of arrested growth), suggesting a
modern endothermic physiology.

Jack Horner's talk on dinosaur bone histology went into considerable depth
in examining the microstructure of dinosaur bone as compared with reptile
and bird bone, and in sagittal section as well as the familiar plan
sections.  His conclusion: the growth plate of a dinosaur is identical to
that of a bird, but nothing like that of an alligator, and shows a high
degree of vascularization.  "I don't know how this impacts the warm-blooded
/ cold-blooded debate, but the complex fibro-lamellar bone shows that
dinosaurs grew like birds, not reptiles."

In his talk, _Dinosaurs, the Aerobic Powerhouses_, Gregory S. Paul pointed
out that the record of juvenile bone growth can be lost as animals mature,
so the adult bone may not retain any of the information required to
evaluate the early growth patterns of animals (including dinosaurs).  Of
course, he had a lot more to say than that, but this is the point he made
that is most pertinent to the proper application of bone histology in
determining metabolic status.

Kevin Padian's talk, _Problems in Avian Evolution and the Origin of
Flight_, included the question: "What is an intermediate physiology?"  He
felt that anyone who uses the term should explain just what it means.  Very
key to the issue of the metabolic implications of LAGs in dinosaur and
early bird bone, he pointed out: "Growth lines are found in some, but not
all dinosaurs.  Fourteen orders of mammals have growth rings."

So, suffice it to say, Chinsamy's conclusions are not universally accepted.
 Even she did not flatly state that the LAGs prove an ectothermic condition
in early birds and dinosaurs.  

-- Ralph Miller III     gbabcock@best.com

"Are you trying to tell me that Sue was a cold-blooded killer?"