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Warm- vs. Cold-blooded Dinos
Dear Gilfredo --
>I would like to ask for an opinion from the group on whether dinosaurs
>were endo or ectothermic.
I'm not a professional paleontologist; I'm just someone who has followed
the dinosaur metabolism debate since I was knee-high to a Troodon; so
at the risk of provoking outrage from the skeptics, I'll give my humble
opinion that all dinosaurs where entirely endothermic. Here are the
evidence acquired by paleontologists over the last thirty-odd years that
I believe backs up my conviction:
1. Upright Gait: Almost all dinosaurs had long, limber, upright limbs held
underneath the body like most modern endotherms. This gait implies
high levels of activity and are unlike sluggish ectotherms like lizards and
crocodiles whose limbs are sprawling or semi-sprawling at best.
2. Predator-Prey ratios: Endothermic animals need much more to eat than
ectotherms. Therefore, in animal communities whose principal members
are ectotherms, the predators make up a relatively large fraction
(20% or more) of the population. But in endothermic communities, the
predators make up 10% or less of the population. Dinosaurs had
endothermic predator-prey ratios, the predators frequently making up
less than 1% of the population as implied by fossil finds. Admittedly,
however, there are other explanations for the rarity of predator dinosaur
fossils, and in any case this argument only implies endothermy in the
predatory theropods and indicates nothing about the herbivorous phyto-
dinosaurs, except for the argument that the phytodinosaurs all
evolved from endothermic theropods and that it is unlikely that
evolution would have worked backwards and phytodinos would have lost
3. Rapid Evolution: The fossil record indicates that ectothermic
creatures evolve very slowly, individual genera remaining virtually
unchanged for tens of millions of years. Endothermic groups evolve at a
much more rigorous pace, individual genera lasting only five or so
million years. Dinosaurs exhibit this kind of rapid evolution, e.g.
"primitive" ceratopsians like _Centrosaurus_ and _Chasmosaurus_ were
rapidly replaced in just a few million years by the "advanced"
_Triceratops_ and _Torosaurus_. By contrast, ectothermic genera like
the crocodile _Leidyosuchus_ remained unchanged throughout the Late
4. "Pole-to-Pole" Dinos: Ectotherms, especially the large ones, are
restricted to temperate-to-tropical climates, but dinosaur fossils
have been found nearly everywhere on the globe. How could dinosaurs
have survived at cold latitudes unless they were endotherms? Of course,
the common rebuttal to this is the assertion that during the Mesozoic
the world was balmy and tropical everywhere, but fossils of deciduous
plants seem to indicate a significant cooling trend throughout the
Cretaceous, just when dinos were most successful.
5. Growth Rings: Animals with low, ectothermic metabolism grow slowly
and leave marks of their slow growth in rings in bone cross-sections,
almost like rings in a tree trunk. High-metabolism endotherms like
mammals grow quickly and build up their bone tissues much faster and
more homogeneously, resulting in a lack of "growth rings" in their bone
cross sections. Examinations of dinosaur bones reveal a lack of "growth
rings," implying fast growth, and a high endothermic metabolism.
6. Therapsids and Suppression of Mammals: Before dinosaurs appeared, the
land was dominated by the mammals' ancestors the therapsids. Advanced (and
probably endothermic) therapsids lived side-by-side with the earliest
dinosaurs. If dinosaurs were ectotherms, the therapsids should have had the
evolutionary advantage and their immediate descendants the first mammals
should have inherited the Earth in the late Triassic, while dinosaurs
should have gone out with the rhyncosaurs. But instead, dinosaurs rose to
domination and forced mammals to remain small and in hiding for 150 million
years. Only after the non-avian dinosaurs' extinction were mammals allowed
to come out of their holes and "adaptively radiate". The only way dinosaurs
could have maintained such domination over endothermic mammals was if they
were endothermic as well.
Of course, skeptics believe that all of the above evidence is circumstantial
and attempt to "explain away" each individual piece of evidence. But if
there is one thing we have learned in 400 years of modern science, it is
that in general, given a choice of theories, it is usually the SIMPLEST
theory that is correct. I support the idea that dinosaurs were warm-blooded
because it is much SIMPLER to explain all the above evidence by saying that
yes, indeed dinosaurs were warm-blooded than to have to provide one
explanation for growth rings, another, disconnected explanation for
predator-prey ratios, yet another explanation for upright gait, etc., etc.
Much simpler it is to provide a single, unifying explanation for all of
the above--Dinosaurs were warm-blooded endotherms.
But this is just my humble opinion...