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dinosaurs and endothermy
I couldn't resist... I forwarded one of Greg Paul's messages to John
Ruben to see if he would like to comment. He took me up on my offer:
Date: Tue, 12 Sep 1995 12:33:12 -0700 (PDT)
From: John Ruben <rubenj@BCC.ORST.EDU>
Subject: dinos again
Mickey - thanks for the note. Paul's assertions are both misleading and
1. We have NO (=none, zero, nada) way of knowing just what dinosaur
aerobic scopes (or active rates of oxygen consumption) were like.
Many were surely active animals (as are many extant varanid
lizards, some of which maintain almost mammal-like levels of
routine activity [e.g., komodo dragons and the Racehorse Goana
of Australia. See papers by Bennett]), but this says nothing
about resting metabolic rates or possible endothermy.
Some people (including Paul) have suggested that the pneumatized
(hollow) bones of some dinosaurs suggest they had a bird-like lung,
thus (so the story goes) they must have had a bird-like aerobic
metabolic capacity. However, a bird-like lung does not
necessarily imply a high aerobic capacity, and more to the point,
dinosaurs were highly unlikely to have had an avian lung: Bird rib
structure is related to respiration by being hinged in a
specialized way, such that on inhalation the breastbone is lowered
but lung volume is diminished (rather than expanded as in typical
mammals and reptiles). On exhalation, the process is
reversed. There is no evidence that ANY dinosaur possessed
hinged ribs even vaguely reminiscent of the avian condition.
The most parsimonious interpretation is that they possessed
alligator- or croc-like lungs. The hollow bones of some
dinosaurs seems more likely to be related to the need for
a less massive skeleton.
Occasionally, some workers (including Paul) have suggested
that dinosaurs must have mammal- or bird-like
hearts to maintain cardiovascular pressure sufficient for their
needs. Consequently (so the story goes), since they had
bird-like hearts, they must have had bird-like metabolism. This
is inaccurate-- it is generally not appreciated by
non-specialists that many living crocodiles maintain
systemic and pulmonary blood pressures not too discrepant from
mammalian values and some tree snakes achieve blood pressure
that EXCEEDS typical avian and mammalian numbers (see papers by
Grigg and others by Lillywhite).
Finally, Paul seems unaware that in some lizards, oxygen
consumption during exercise EXCEEDS that of many mammals during
at least routine levels of activity. Little wonder that some
varanid lizards have been clocked as running 30 kilometers/hr for
distances up to one kilometer. This compares closely with the
human record for the half-mile!!
1. Paul's assertions notwithstanding, there is not the slightest
bit of reliable evidence regarding the magnitude of dinosaur aerobic
2. Dinosaurs need not have been warm-blooded to have achieved
elevated aerobic scopes.
Significantly, in their forthcoming review entitled "Dinosaur
Biology," (Annual Review of Ecology & Systematics, 1995, 26:445-471,
Farlow, Dodson and Chinsamy conclude that "No evidence shows that
dinosaurs were endotherms, and some suggests that they were not."
I hope this is helpful. Feel free to copy/publish whatever
parts of this e-mail are of interest to you or your readers.