Thermoregulation in Dinosaurs A Continued Controversy

Brian Laing
Bio Sci 136
Thermoregulation in Dinosaurs:
A Continued Controversy
Although dinosaurs often appear in documentary television and popular entertainment,
it must not be overlooked that these creatures were the most successful of all land vertebrates.
Spanning over 150 million years of terrestrial existence, dinosaurs far surpassed the duration
and dominance by any other group of tetrapods (Ruben 1998). These animals include some
of the most specialized herbivores and carnivores ever to have existed -- their extraordinary
diversity is estimated at 1500 species (Hadly 2001). Despite the quantity of research devoted
to dinosaurs, many questions of dinosaur biology remained unanswered.
No other facet of dinosaur research has raised more debate and controversy than the
attempts to determine their metabolic status. This concise overview of the ongoing debate
develops both sides of the argument in order to synthesize possible interpretations of
contradicting studies. Although a majority of paleontologists advocate that dinosaurs were
“warm-blooded” (endothermic), promoting scientific notions of extremely active dinosaurian
behavior, this theory cannot be concluded with empirical evidence. An accurate elucidation
of vertebrate evolutionary history requires objective analysis of appropriate data but this
notion, unfortunately,s has not been consistently followed by the scientific community.
Professor James Farlow remarks:
The strongest impression gained from reading the literature of the dinosaur [metabolic]
physiology controversy is that some of the participants have behaved more like
politicians or attorneys than scientists, passionately coming to dogmatic conclusions
via arguments based on questionable assumptions and/or data subject to other
interpretations (Farlow 1990).
Potential bias upholding endothermic dinosaurs should be identified and recognized as a risk
to scientific integrity. As Darwinian Evolution and many popularly accepted scientific theory
continues to be challenged, so must the theory concerning the metabolism of these highly
successful “terrible lizards.”
It is first essential to clarify the terms “warm-blooded” and “cold-blooded” to
understand what exactly is being debated. These two terms are popular phrases which refer to
three fundamental aspects of animal physiology: body temperature regulation, variation in
body temperature and level of resting metabolism (Poling 1996). Warm-blooded generally
refers to an organism that is endothermic, homeothermic and tachymetabolic. That is, an
organism that regulates body temperature internally, maintains a constant body temperature,
and whose resting metabolism remains high. In contrast, cold-blooded generally refers to an
ectothermic, poikilothermic and bradymetabolic organism. That is, an animal whose body
temperature is directly dependent on the temperature of their environment, maintains a widely
variant body temperature, and whose metabolism slows substantially when at rest. It must be
noted that the terms “warm-blooded” and “cold-blooded” may refer to an animal that exhibits
a mix of these characters rather than wholly one or the other. As this paper demonstrates,
dinosaurs most likely possessed the “mixed” physiology of ectotherms and homeotherms.
The argument for warm-blooded dinosaurs is multi-fold. One general argument is
based on their extensive terrestrial dominance. The ability to control body temperature and
maintain it at a constant value is a tremendous evolutionary advantage allowing a predatory
animal to hunt dependent of time of day and season. For every drop of 10 º C, metabolism is
twice as slow in cold-blooded animals and thus cannot be active at night or in cold
temperatures (Worth 1999). Evolutionary theory then demands that warm-blooded animals
will always win over cold-blooded animals. The question is then raised “If mammals had this
powerful evolutionary advantage, why did they maintain subservient position to the dinosaurs
throughout the Mesozoic?” In response, many paleontologists assert that dinosaurs must have
been warm-blooded to have ruled the land for such a long period in the history of terrestrial
As exemplified above, all evidence which supports the warm-blooded hypothesis is
largely inferential, that is, there is a lack of empirical evidence of dinosaur metabolism. This
additional inferential evidence includes: bone structure and histology, growth rates,
predator/prey ratios, speed and agility, rate of evolution, similarities with birds, parental care
and bone isotope composition. This paper will expand on the arguments of, bone histology /
growth rates, bone isotope composition and evolutionary links to birds to demonstrate the
two-sided and inconclusive nature of the dinosaur thermoregulation debate. Subsequent
discussion of studies of lung structure and respiratory turbinates will reinforce the possibilities
of ectothermic dinosaurs.
Bone histology and growth rates constitute an integral component for the argument
promoting endothermic dinosaurs. The two major histological types of compact bone are
“lamellar-zonal,” primarily found in reptiles and amphibians and “fibrolamellar,” found in
birds, mammals and dinosaurs (Reid 1997). Fibrolamellar bone, associated with endothermy,
often correlates with high growth rate and high metabolic rate. It is then thought, that
dinosaurs’ possession of fibrolamellar bone is indicative of the metabolism and growth rate in
birds and mammals. There are, however, exceptions to this correlation. For example,
fibrolamellar bone is known to be present in the skeleton of extant, rapidly growing turtles,
crocodilians, and lizards (Chinsamy 1995). These exceptions are sufficient to deconstruct the
argument linking dinosaur metabolism to bone type. Examination of juvenile dinosaur bones
indicate that they grew rapidly which is typical of endotherms but there are counter examples
for this correlation as well (Raven 1999). The American alligator has a growth rate that is
approximately four-fold that of marsupial and many placental mammals (Ruben 1995). The
author of this data, J.A. Ruben, also noted that alligator growth rates are virtually
indistinguishable from estimated growth rates for the bipedal theropod dinosaur, Troodon.
The above counter-examples invalidate the possibility of any definitive interdependence
between endothermy and growth rates.
Bone isotope ratios have also been cited as evidence for dinosaurs with internally
regulated body temperatures. The extremities (toes and tails) of ectothermic animals are
colder in comparison to the body’s core (the torso), whereas in endotherms the extremities are
usually the same temperature as the rest of the body (Raven 1999). Had dinosaurs been
ectothermic, bones in their extremities would have developed at lower temperature than the
rest of the body as in modern reptiles. This can be determined directly by examination of
bone composition which includes both oxygen-16 and oxygen-18. At high temperature 16O is
preferentially taken up into new bone. Consequently, the bones in toes and tails of reptiles
contain much less 16O than their rib bones. When six Cretaceous dinosaurs were examined in
1992, the proportion of 16O in toe, tail, and rib were equivalent indicating that they maintained
their own body temperature (Ibid). This research failed to recognize the abundance of data
demonstrating that many birds and mammals often maintain extremity temperatures well
below deep-body, or core, temperatures (Ruben 1995). It has also been established that fossil
bone isotope ratios may be strongly influenced by ground water temperatures (Kolodny 1996).
Unfortunately, fossilized bone oxygen isotope ratios yield no absolute conclusion regarding
dinosaur metabolic physiology.
The multitude of evidence suggesting evolutionary links between dinosaurs and birds
has often been utilized to further the argument of endothermic dinosaurs. These links include
erect posture, efficient hearts, intelligence, bone structure, food requirements per size,
feathers, and caring for their young (Worth 1999). It is now generally accepted within the
scientific community that birds are probably direct descendants of dinosaurs and because
birds are clearly warm-blooded it is not unreasonable to suppose that their ancestors were
already warm-blooded as well. Most recent data has had a powerful impact on the dinosaurbird theory. Fossils found in China in 1996, have revealed what many paleontologists believe
are “feathered dinosaurs” (Appenzeller 1999). The number of characters than uniquely define
birds has been rapidly declining as new discoveries have been made, but feathers have long
been considered one of the last supporting features of “bird-ness” (Worth 1999). It is
generally considered that the discovery of a genuine feathered dinosaur would be the
definitive proof of the dinosaurian origin of birds. The debate continues, however, if these
fossils are in fact the remains of feathered dinosaurs. Some scientists argue that one of the
specimens “proto-feathers” could actually be a collagen frill. The undoubtedly feathered
specimens also are scrutinized within a different debate – that is, whether these animals were
in fact feathered, carnivorous bipedal dinosaurs (theropods) or simply flightless birds. What
Appenzeller does not note in his 1999 Science article is that even if it were fact that feathers
evolved in theropod dinosaurs, this is not definitive evidence that birds evolved from this
lineage. The number of potential selective pressures promoting the evolution of feathers such
as insulation, display, or metabolic clearance system for excess sulfur indicate the possibility
that feathers may have independently in another dinosaur lineage other than theropods or
perhaps true birds evolved prior to the dinosaurs (Worth 1999). It is also essential to note that
Archaeopteryx, commonly referred to as the classic dinosaur-bird link, possessed a skeletal
structure almost indistinguishable from a small therapod. Current thinking has moved away
from this previously hypothesized link, and many authorities now believe that Archaeopteryx
lies on the path to the now extinct enantiornithine birds, while the true modern bird line
diverged from the enantiornithines well before Archaeopteryx (Worth 1999). It is also a
possibility that birds evolved prior to dinosaurs entirely, a theory popularized by George
Olshevsky. Analysis of bird and dinosaur lung structure brings further doubts to their
evolutionary relationship.
The notion of endothermic dinosaurs continues to be questioned due to the ambiguity
of the evolutionary link to birds. Examination of lung structure raises additional uncertainty
of endothermy and the relationship to birds. Reptiles and birds possess septate lungs (i.e. their
respiratory passages are separated by cartilage) rather than the alveolar-style lungs of
mammals. Only avian lungs, however, are capable of the oxygen-carbon dioxide exchange
rates that are typical of active endotherms due to air sacs which function to generate a
unidirectional flow of air and efficient cross-current of gas exchange at the pulmonary bloodair surface (Ruben 1997). The non-avian septate lung is probably unalterably constrained
from supporting respiratory exchange that is consistent with the aerobic metabolic rates
typical of active endotherms (Dunker 1989). Paleontological evidence consisting of some
soft tissue evidence and osteological similarities between crocodilians and theropods suggests
that theropod dinosaurs may have had relatively unmodified septate lungs that were ventilated
with the assistance of an active crocodilian-like diaphragm (Ruben 1999). This data promotes
an ectothermic status for theropod dinosaurs. Ruben also notes that this data poses a
fundamental problem in the dinosaur-bird relationship. The evolution of the avian air sac
system would require the selection of a diaphragmatic hernia in transition between dinosaurs
and birds. This debilitating condition would have greatly disturbed the entire respiratory
system and is thus unlikely to have been selectively advantageous. The question of
evolutionary timing of avian lung diversion reinforces the theory that true birds may have
evolved much earlier than Archaeopteryx or even earlier than theropods. Although Ruben
maintains the argument that lung structure indicates ectothermic dinosaurs, he does propose
that expansion of lung capacity might have allowed the septate lungs of dinosaurs to have
achieved rates of gas exchange that might have approached, or even overlapped, those a few
extant mammals. A compromise is then a distinct possibility – a theropod which maintained
ectotherm-like metabolic rates but were also capable of activity levels exceeding those of the
most active living reptiles.
Analysis of respiratory turbinates (RTs) also supports the theory of ectothermic
dinosaurs. RTs are folded structures of bone or cartilage in the sinus cavities considered to be
essential in conserving water and heat loss and thus necessary of warm-blooded creatures.
These structures are found in both birds and mammals but not modern reptiles. In the
absence of respiratory turbinates, continuously high rates of oxidative metabolism and
endothermy would exceed tolerable levels (Ruben 1996). RTs have never been found in
fossilized dinosaur nasal cavities. Ruben has also determined that the presence of respiratory
turbinates in extant endotherms is inevitably associated with marked expansion of the
proportional cross-sectional area of the nasal cavity. Following this association, computed
axial tomography (CT scan) of theropod dinosaur skulls demonstrate that they were unlikely
to have possessed respiratory turbinates. The nasal passage cross-sectional dimensions are
nearly identical to those of extant ectotherms indicating that these animals probably did not
attain metabolic rates equivalent to those of modern endotherms (Ruben 1998). Although it is
possible that potential dinosaurs RTs could have deteriorated in fossilization, many RTs have
been found in the fossils of at least two groups of Permo-Triassic mammal-like (therapsid)
reptiles (Hillenius 1994). It is not likely that RTs would have deteriorated in every dinosaur
specimen of the current paleontological record. These indicators of ectothermic dinosaurs are
also consistent with the theory that ancient birds such as Archaeopteryx had not achieved
endothermy and thus may not have been then the true ancestors of modern birds. Ruben,
again, proposes a compromise in dinosaur metabolism and behavior. He postulates that
because of the mild climate of the Mesozoic, most dinosaurs were probably large enough to
have been homeothermic regardless of metabolic status and the dynamic skeletal structure of
many dinosaurs strongly suggests that they possessed bird- or mammal-like capacity for at
least burst activity.
The idea of dinosaurs ectothermic homeotherms (body temperature constant but also
dependent on environment) is also supported by direct comparisons made to the
thermoregulation of modern crocodiles. Zoologists from the University of Queensland,
Australia, gathered data describing the daily and seasonal cycles in body temperature of free-
ranging Crocodylus porosus. Using mathematical models they were able to predict the body
temperature of a very large, dinosaur-sized crocodile and concluded that some dinosaurs must
have had an essentially high and stable body temperature without any requirement for
endothermy (Seebacher 1998). Seebacher also notes that access to shade or water must have
been crucial for the survival of dinosaurs in warm tropic conditions. It is then a distinct
possibility that dinosaurs may have relied on their environment to maintain a relatively high,
constant body temperature. Another recent study used a physical, model-based approach
incorporating variables such as operative environmental temperatures and ratio of surface area
to volume of metabolically heated tissue to explore the range of thermoregulatory strategies
that were available to dinosaurs (O’Connor 1999). This research presents several conclusions:
the tremendous range of body sizes within dinosaurs likely resulted in very different thermal
problems and strategies; the smallest dinosaurs were unlikely to benefit from high metabolic
rates, and their body temperatures would respond rapidly enough to changes in environmental
temperature to make behavioral thermoregulation possible; and endothermic metabolic rates
may have put large dinosaurs at risk of overheating without adaptations to shed excess heat. It
is essential to note that when discussing thermoregulation in dinosaurs it must not be assumed
that there is one universal physiology for all dinosaurs. This paper has focused on theropod
physiology due to their proposed evolutionary link to birds. O’Conner’s study also indicates
through mathematical models that endothermy would not have been the ideal
thermoregulatory strategy for many dinosaurs. Although a majority of paleontologists
advocate that dinosaurs were warm-blooded, the evidence of ectothermy continues to build.
After over 150 years, the debate concerning thermoregulation in dinosaurs continues
to be a source of heated controversy in the scientific community. The reality of the opposing
arguments is that there exists no empirical evidence to prove that these animals were either
“warm-blooded” or “cold-blooded.” Data supporting either side is purely inferential and thus
no theory can be proven. The fact that endothermy is almost exclusively an attribute of “soft
anatomy,” which leaves a poor or usually nonexistent fossil record, contributes to the
difficulty of proving dinosaurs were warm-blooded. The excitement raised over a fossilized
“dinosaur heart” last year turned out to be equivocal at best and far too unclear to yield
conclusive data (Stokstad 2001). Paleontologists can only hope that perhaps discovery of
more definitive dinosaur soft tissue or respiratory turbinates will assist in the debate.
Although a substantial amount of recent studies indicate that dinosaurs were ectothermic
homeotherms, these studies have already been disputed and publications continue to criticize
the abundance of current theories. The scientific community should also recognize that the
debate over dinosaur metabolic physiology may continue indefinitely.
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