Re: Re: Re: Dinosaur theories.

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

>In article  Thomas_R_HOLTZ@umail.umd.edu (th81) writes:
>>For example, Bakker does make a strong case that dinosaurs were more active
>>than modern reptiles.  This is something the majority of dinosaur workers
>>agree with.  However, he claims that the only means by which high activity
>>can be achieved is through mammalian-style endothermy.  There are other
>>mechanisms which have been proposed (such as gigantothermy) which would
>>allow a large dinosaur (but not small ones) to achieve "warm-bloodedness" on
>>an ectothermic engine.
>
>>Data which was not available to him when he wrote
>>"Heresies" show that dinosaurs had a mammalian growth style when juveniles,
>>then slowed down at adulthood.
>
>Huh? Doesn't that contradict gigantothermy?
>Jones


In fact, it doesn't per se.  The body of evidence out there now (including
the work of Dr. Chinsamy and her colleagues) strongly suggests ontogenetic
heterometabolism; that is to say, radically different metabolic  rates and
functions between juveniles and adults.  For example, a hadrosaur or
sauropod hatchling may have started out as a mammalian-style endotherm, then
shifted gears, as it were, become a gigantotherm after reaching a certain
size.  This is different from most large endotherms today, in which the
metabolic rates slow down at adulthood, but the metabolic "engine" remains
the same.  Interesting, work on modern elephants suggests that they may not
be true endotherms, but gigantotherms of a sort.

A quick observation, and
one which Bakker has noted less commonly now then he did decades ago, is that
"hot" and "cold" blood, or even endothermy and ectothermy, are not polarized
opposites but end members in a spectrum of various thermal regulation
mechanisms used by modern animals.  Many lizards, after all, are
ectothermic, bradymetabolic, homeotherms (homeothermy achieved by behavior,
rather than internal cellular means).

>
>

Thomas R. HOLTZ
Vertebrate Paleontologist, Dept. of Geology
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Phone:301-405-4084