WERE DINOSAURS WARM-BLOODED? (88)

from http://www.isgs.uiuc.edu/dinos/de_4/5c51d90.htm

NOTE: The terms 'warm-blooded' and 'cold-blooded' are used for simplicity throughout this essay. In fact
these terms are something of an over-simplification. Virtually all animals, if examined at the proper time, will
appear to be 'warm-blooded' ie their internal body temperatures will be about the same. What is more
important is the mechanism by which the body temperature is maintained, and in this sense the terms
endothermic and ectothermic are more appropriate; an ectothermic animal relies on heat from the
outside ie the environment, to maintain body temperature, whereas an endothermic animal relies on heat
generated within its own body by metabolic processes, and will therefor have a higher metabolic rate. There
are even more precise technical terms for processes intermediate between full endothermy and full
ectothermy which will not be discussed here.

Once it was thought that all dinosaurs were cold-blooded - now many suggest that at least some were
warm-blooded, which would help to explain why they became so plentiful and dominant for so long. If we
examine today's fauna , we find no large land predators that are cold-blooded, except for crocodiles that
occupy only one very specific ecological niche and are basically water dwellers. The same is true of the
entire Cenozoic era - virtually all large predators were warm-blooded.

The reason is not difficult to find. The position of top predator is a very competitive one. The ability to
control body temperature and maintain it at a constant value (ie warm-blooded) is a very large advantage.
Not only does it mean that the animal is not dependent on the environmental temperature but can hunt at
any time of the day (or night), or in any season, but it also means operating at maximum efficiency. All
creatures, whether warm- or cold-blooded, use the same basic biochemical processes to produce energy,
with the same enzymes and substrates. The chemical reactions involved generally have a particular optimal
temperature. For every drop of 10 degrees C, the process will be twice as slow - hence the sluggishness of
cold-blooded animals in cold environments or at night. Evolutionary theory thus demands that in any long
term competitive situation, warm-blooded animals will always win over their cold-blooded competitors, and
this is what the history of mammal development demonstrates. No large cold-blooded predators can develop
against mammalian competition - instead they remain small and occupy ecologically specialized positions
where they can hide for most of the time and only need to hunt for food occasionally when safe to do so.
There are, of course, attendant disadvantages to endothermy, not the least of which is the need for very
much larger expenditure of energy to maintain elevated metabolic rates, and a commensurate increase in
food requirements.

Given that mammals have such an enormous advantage, what are we to make of the Mesozoic era, when
for 140 million years dinosaurs reigned supreme and few mammals grew larger than a chicken. Mammals
and dinosaurs evolved together. Dinosaur ancestors ( thecodonts , particularly ornithosuchians ) and
mammal ancestors ( therapsids , particularly cynodonts) were in direct competition in the late Triassic , with
the therapsids initially appearing to have the upper hand. However, by the end of the Triassic the thecodonts
were on top, dinosaurs assumed the roles of top predator and large herbivore , and all other roles down to
the very small, which they left to the mammals and other reptiles. How did they manage to take over in the
first place, and then keep the mammals subservient for so long if mammals had such a potent evolutionary
advantage in being warm-blooded? The logical answer, of course, is that dinosaurs had to be warm-blooded
as well! For those who believe that dinosaurs are just large reptiles this is an unacceptable view.
Given that all we have left are lifeless bones and footprints, is it possible to produce evidence in support of
the warm-blooded dinosaur hypothesis? Surprisingly, perhaps, the answer is yes, although such evidence
must be largely inferential .

Some of the evidence has already been presented above, in the comparison of dinosaurs with present day
ecological structures. Other evidence comes from areas such as:

Bone structure and histology
Growth rates
Predator/prey ratios
Speed and agility
Rate of evolution
Similarities with birds
Parental Care
Bone Isotope Composition
Insulation
Arctic Faunas

All these lines of evidence and comparisons between dinosaurs, mammals and cold-blooded reptiles support
the idea that dinosaurs were warm-blooded, or at very least had a much higher metabolic rate than
conventional cold-blooded creatures. Not surprisingly, some counter arguments have been presented, such
as:

Gigantothermy
Rate of food supply
Respiratory turbinates
Lung structure

The question is yet to be decided, but on balance the likely outcome seems to be heavily weighted in favour
of at least partially warm-blooded dinosaurs. The tests for endothermy have also been applied to the
mammal ancestors (the therapsids - cynodonts , dicynodonts ) and thecodont dinosaur ancestors. In both
cases the results suggest that they were more like warm-blooded than cold-blooded animals, and that
endothermy may have developed in both lines at an early period (late Permian ?). In this scenario, dinosaurs
would simply be one group in a line of succession of warm-blooded animals.
What does seem clear is that dinosaur physiology was different, and may even have been different in
different types of dinosaurs. We can never know the answer by direct measurement, and as long as we are
forced to rely on analogy we will probably never reach a concensus on this controversial topic. Endothermy
is not an either/or proposition, as the large number of potential physiological mechanisms involved make a
wide range of alternatives possible. The large size range of dinosaurs alone probably means that they did not
all share a common physiology, nor use the same strategies and mechanisms to reach a particular
physiological state.

References

BONE STRUCTURE AND HISTOLOGY
(54)

Bone is made up of calcium phosphate mineral (in the form of hydroxyapatite)
deposited on collagen, a protein formed in long bundles or fibres. In slow
growing animals, the collagen fibres are laid down parallel in each layer,
producing bone that has a dense packing of mineral crystals all orientated in the
same direction. This 'lamellar' bone is in direct contrast to bone formed in
rapidly growing animals, where the collagen is laid down in a haphazard way to
form an irregular or 'woven' bone.
Woven bone (also termed fibro-lamellar) is typical of mammals and birds that
are warm-blooded and hence fast growing, whereas cold-blooded, slow growing
reptiles (eg crocodiles) typically have lamellar bone.
Bone from warm-blooded creatures is also typified by a much larger number of
vascular (blood vessel) canals and Haversian canals formed in rapidly forming
bone. Cold-blooded animals always have far fewer such canals.
Finally, most bone exhibits growth rings. These may be any of a number of
types
* Daily - found in teeth of dinosaurs, mammals and crocodiles.
* Seasonal or diet induced. During times of slow growth (eg winter), thinner,
denser layers of bone form. Cold-blooded animals are more responsive to the
environmental temperature and in warm climates without extremely dry seasons
(also slow growth times), tend to show more and better defined growth rings
than warm-blooded animals. In mammals these rings are typically much more
obvious in teeth than in bone.
* Lines of Arrested Growth (LAGs). These are formed wherever there is a
temporary halt to bone deposition. They may be caused by a number of factors
and do not necessarily indicate physiological or environmental effects. They are
found typically in mammalian jaws and in the long bones of most mammals,
where they usually are due to environmental changes. LAGs have been found in
many dinosaurs, including Massospondylus, Syntarsus, Brachiosaurus,
Hypacrosaurus and Maiasaura, and also in enantiornithine birds. In contrast to
mammals, where LAGs are normally found in slow-growing lamellar-zonal
bone, in dinosaurs they occur more often in fast-growing fibrolamellar bone.
They are assumed to reflect yearly bone deposition, but in dinosaurs there is no
direct evidence to confirm this assumption. Growth lines can also form under
other influences (unsymmetrical bone remodelling, changes in type of bone
deposited etc) and though these are generally distinguishable from true LAGs to
the trained eye, they can be a source of confusion. Growth lines vary
throughout a dinosaur skeleton, being generally less common in long bones, and
may reflect different rates of bone deposition in different parts of the skeleton.
Interestingly, although LAGs are found in the femora of virtually all dinosaurs
so far examined, one particular growth series of Dryosaurus shows no LAGs,
and they are also not found in polar hypsilophodontids from Australia even
though other dinosaur types from the same locality do show them.
Thus, although initially the finding of LAGs in dinosaurs was interpreted as
evidence for a warm-blooded physiology, today's consensus is that LAGs do
not necessarily correlate with physiology.

Hundreds of slices from bones of all types of dinosaurs have now been
examined and all show:
1) Woven bone typical of rapid bone growth
2) Vascular canals equivalent to those of birds
3) Generally poor growth rings but more obvious in teeth
ie in all regards they resemble warm-blooded rather than cold-blooded animals.
However, continuing studies have complicated the interpretation of these results.
Many small mammals and birds do not show either Haversian canals or
fibrolamellar bone, and at least one turtle has been found with a dense Haversian
bone. Dinosaurs do appear to differ from other reptiles in their ability to deposit
fibrolamellar bone continuously instead of periodically.
In 1993 a comparative study was made of the long bone growth plate of a
chicken (bird), dog (mammal) and monitor lizard (cold-blooded reptile) with
juvenile Maiasaura bone. In longitudinal section, the line between cartilage and
newly formed bone was straight in the dog and monitor, but undulating in the
bird and dinosaur. As well as reinforcing the close relationship between dinosaur
and bird, the authors conclude that the different bone formation processes that
cause the wavy line indicate rapid growth and are consistent with a high
metabolic rate .

References

GROWTH RATES

Warm-blooded creatures grow much faster (5 - 10 times) than cold-blooded
ones do, and this is reflected most strongly in their bones, as discussed elsewhere
under bone structure . Whereas a crocodile under normal circumstances will
only grow about 30 cm (1 ft) per year, a young Maiasaura (hadrosaur )
hatchling would reach its full size in about 4 years - a growth rate 5 times faster
than a crocodile.

References

PREDATOR / PREY RATIOS

The large top predators in any system generally obtain most of their food intake
from large herbivores (it is not usually worth the amount of effort required to
hunt much smaller prey ). For warm-blooded predators , a large quantity of
food is required (about 10 times as much as for an equal-sized cold blooded
predator ), so that the larger the predator and the higher its metabolic rate , the
rarer it will be in any ecosystem . Predator/prey ratios are calculated by working
out the weight of the predator and its prey and counting the number of
predators and the total number of prey.
predator/prey ratio = predator weight x number
prey weight x prey number
For modern examples such as the lion on the African savannah game parks, this
ratio is about 1% or even less. For Permian cold-blooded predators such as
Dimetrodon, the ratio is much higher, at 20%, equivalent to today's crocodiles
and spiders. Prehistoric mammal predators such as sabre toothed tigers have a
ratio of about 3 - 5%. Tyrannosaurus ratios are almost exactly the same as for
sabre tooths, and large dinosaur predators average about 3.5%, with a range
from 5% for good habitats such as late Cretaceous Alberta down to less than
1% for the much more difficult environment of late Cretaceous Mongolia.
Dinosaurs clearly fall into the same group as the unquestionably warm-blooded
prehistoric mammals and are much lower than cold-blooded predators of today
or the Permian period. There are several possible reasons for the lower values
for the lion - the savannah is relatively open and provides little in the way of
hunting cover, there is considerable interference from man etc, so that the lion is
unable to operate at peak efficiency and so the herbivore population increases
beyond its expected level.
The studies cited above are not considered flawless by all experts, and no studies
have been done with bird predator/prey systems. Census counts based on
incomplete fossil assemblages may be unrepresentative, and the assumption that
predator density is always limited by prey density is largely untested.

References

SPEED AND AGILITY

The life style of the smaller, agile dinosaurs also supports the warm-blooded
hypothesis. Whereas modern, cold-blooded reptiles are 'sit and wait' hunters,
predatory dinosaurs were active in pursuing and attacking their prey. Such
activity requires a high metabolic rate . Some, such as Deinonychus, are
believed to have hunted at night - highly unlikely for a cold-blooded creature.
All theropods and many other dinosaur types were bipedal , an obligation which
requires more metabolic energy than a sprawling, four-legged posture. Some
commentators go so far as to say that bipedalism cannot be attained without
some form of endothermy.
Dinosaurs also put a lot of evolutionary energy into sexual display and territorial
intimidatory display (head butting, ornamental head crests, 'sails' on backs etc)
which is far more characteristic of warm-blooded animals than cold.
The average walking speed of today's mammals is much higher than that of
cold-blooded animals. Their speeds can actually be measured, and can also be
calculated from their footprints. Using the footprint calculations on prehistoric
mammals we get speeds the same as present day mammals, whereas the
cold-blooded reptiles and amphibians of the Coal Age are much slower (1 -2
mph/3 - 6 kph). Dinosaurs and thecodonts , on the other hand, appear to have
been just as fast as mammals, a conclusion supported by their fossil skeletons.
Their limbs were built for speed and prolonged exercise.
The ability to be fast and agile for an extended period requires a high metabolic
rate and thus a large heart and efficient lungs. Such organs do not, of course,
fossilize, but most dinosaur skeletons have a much wider body space in the chest
region when compared with cold-blooded reptiles and could easily have
accomodated large hearts and lungs. The hadrosaurs and horned dinosaurs do
not show such enlargement, but neither do some birds - they compensate by
having a series of air sacs throughout the vertebrae and in body spaces that are
connected to the lungs. Air flows continually in one direction instead of
breathing in and out, and the blood flows in the opposite direction at gas
exchange areas, leading to an extremely efficient system for providing oxygen
and removing waste gases. Most dinosaurs also show such air sacs in the
backbone, and may well have had body sacs also.
It has been suggested (as argued above), that sustained activity and obligatory
bipedalism , as exhibited by dinosaurs and birds, requires an endothermic
metabolism. Similarly, some writers refer to the wide range of relative brain size
of dinosaurs. At the top end of their range they approach birds and mammals,
and in theory this requires an abundant oxygen supply to the brain, in turn
implying a high respiratory rate and high metabolic rate.
Some authorities suggest that for animals of the size and apparent vigorous
life-style of dinosaurs, sufficient heat will be generated by the maintainence of
hich activity levels to make the animals effective endotherms, regardless of the
presence or absence of any specific mechanisms.

References

RATE OF EVOLUTION

The length of time that any one species , genus , family etc lasts varies greatly.
Cold-blooded animals are generally less susceptible to famine and drought and
come under much less evolutionary pressure. Hence their average species
lifetimes are relatively long - crocodiles and turtles have changed little from their
origins to today, with average species lifetimes of 30 million years. The late
Permian cold-blooded reptiles Dimetrodon ( carnivore ) and Edaphosaurus (
herbivore ) lasted for 20 million years. Cold-blooded families may be unchanged
for as long as 55 million years.
In contrast, warm-blooded animals are aggressive competitors that reproduce
rapidly and diversify to occupy as many ecological niches as possible. The
increased evolutionary pressure thus applied leads to much faster turnover
times. Mammals may generate 5 or 6 new genera every 10 million years, and
families may only survive for 25 million years (half as long as cold-blooded
animals). Dinosaur species changed as rapidly as every 5 -6 million years
(horned dinosaurs), and families lasted for the same length of time as mammal
families.
As well as shortened existence times, warm-blooded animals also produce more
species per genus and more genera per family than cold-blooded creatures.
Dinosaurs averaged 3 -4 species per genus and 12 genera per family, as did
mammals. The hadrosaurs and horned dinosaurs produced 7 and 5/6 new
genera respectively over 10 million years, comparable to mammals and in
contrast to, for example, the giant turtles, that have produced only 1 new genus
over the past 5 million years.

SIMILARITIES WITH BIRDS

As discussed in more detail in the topic ' Dinosaurs and Birds ', there are many
similarities between birds and mammals (erect posture, efficient hearts,
intelligence, bone structure, food requirements per size, feathers, and caring for
their young).
It is now agreed that birds are probably direct descendants of dinosaurs, and
may have evolved from them as early as the Jurassic period. As birds are
undoubtedly warm-blooded it is not unreasonable to suppose that their ancestors
were either already warm-blooded also, or developed the capability over the
next 100 million years.

PARENTAL CARE

In 1991 Lambert suggested that the question of dinosaur temperature regulation
might be addressed from the point of view of parenting behaviour. His
argument starts from the premise that post natal care of the young, such as
foraging for extra food, is an energy expenditure that cold-blooded animals, with
only short periods of sustained activity limiting the time and distance over which
foraging can occur, simply cannot afford. Using the Maiasaura nesting grounds
as an example, he cites the evidence that the young spent a considerable period
of time in the nest after hatching (badly fragmented eggshells indicative of
repeated trampling, juveniles of different ages within the nest, poorly developed
limb bone joints indicating limited locomotion of the young) to support the
theory that the parent hadrosaurs were involved in foraging to provide food for
their rapidly growing, nest-bound young, and that this behaviour was strong
evidence against them being cold-blooded. He also notes that evidence from the
second well documented north American nesting site of the hypsilophodont
Orodromeus (now identified instead as the theropod Troodon) is much less
clear, with eggshells relatively intact, no juveniles within the nests and well
developed locomotion in the young. They still remained in the vicinity of the
nest for a significant period, which may imply some dependence on the parents.
The evidence from this nesting site has since been challenged. It has been
claimed that the bones found in the nest were all from embryos , and that joint
development is far less critical for early movement than hip development which
appeared adequate in the Maiasaura chicks. Currently this argument lacks
support and most experts seem to concede parental involvement at this site at
least.

References

BONE ISOTOPES

The oxygen isotope composition of the phosphate part of vertebrate bone is
related to ingested water and to the body temperature at which the bone forms.
It is in equilibrium with the individual's body water, which in turn is in a uniform
state throughout the body. Therefore, by measuring the variation in the
phosphate oxygen isotope composition the average temperature variation both
within a bone and between bones can be calculated for fossil bones.
The results of such an experiment on well preserved Tyrannosaurus bone
suggest that its body temperature was maintained within 4 degrees C. Such a
degree of temperature uniformity is consistent with a relatively high metabolic
rate similar to known warm-blooded animals.
However, the basis for this type of calculation has been challenged. For the
calculation to be valid the assumption is made that the fossilized remains are
isotopically the same as the original bones. Kolodny and co-workers have listed
a number of reasons for questioning this assumption:
1) Living bones contain 1/3 of their weight of organic material which is replaced
during fossilization.
2) Living bones, in contrast to fossils, contain very little rare earth metals or
uranium.
3) In living fish the relationship between the change in oxygen isotopes in
phosphate and carbonate is poor, whereas there is a strong correlation in fossil
fish.
4) Similar changes in isotopes have been noted in fish, dinosaurs and other
reptiles from the same location.
5) Fish and mammals in contact with the same water source have overlapping
isotope values.
These workers caution that while isotope values of fossils may give us important
information about their burial environment, they may be of doubtful value in
interpreting physiology.

References

INSULATION

The question of insulation arises with regards to body temperature control
because of the perceived problems insulation would cause for an animal whose
temperature control depends on the external environment. It would seem
counter-productive for a cold-blooded animal to be insulated, and there appear
to be no extant examples of such a situation, whereas all known warm-blooded
animals have some form of insulation (fur or hair for mammals, feathers or
down for birds, and fat layers in other cases).
Thus the discovery of an insulated dinosaur would provide strong evidence for
their status as warm-blooded animals. The problem with such a discovery is the
selectivity of the fossilization process, which tends not to leave evidence of skin
and associated features such as hair or feathers, even when they were present.
There is therefore enormous interest whenever such a discovery is a possibility.
The ornithomimid Pelecanimimus was initially thought to be such a case, but
further investigation revealed that the structures found with the skin were
actually internal structures.
The discovery of the Chinese coelurosaur Sinosauropteryx in 1997 again raised
the possibility. The initial reports were quite definite that this animal, an
undoubted theropod dinosaur, had feathers, and caused considerable excitement.
Subsequent investigations by a number of Chinese and Western experts have
not been quite so definite. Some detractors have gone so far as to suggest that
the hair-like structures have nothing at all to do with hair or feathers, while
others such as Currie consider the hair-like, apparently hollow structures as
good candidates for ' proto-feathers '. Further examination, including chemical
analysis, are underway, and a definitive answer may still be some way off.
Other Chinese discoveries, Protarchaeopteryx, Beipiaosaurus and
Caudipteryx, are all feathered and flightless. Cladistic analysis places them as
maniraptoran theropods, and although this is disputed by some who claim they
are simply flightless birds, their discovery appears to validate insulation in at
least some dinosaurs.

References

ARCTIC FAUNAS

Ectotherms , by prediction and by observation of extant examples, do not do
well in extreme cold. Late Cretaceous faunas are now known from a number of
locations in Alaska and other Arctic areas that were laid down when mean
annual temperatures ranged between 2 and 8 degrees C. These deposits are
notable for the almost complete absence of ectotherms such as the
crocodyliform champsosaurs that are normally abundant in comparable North
American faunas from warmer climates. However, these deposits contain large
numbers of dinosaurs - hadrosaurs , ceratopians and theropods . If these were
also ectotherms , the question becomes "how did they manage to survive so
well when other ectotherms did not?" One postulated explanation is that they
migrated during the coldest times, but the presence of neonates and many
juveniles suggests that this did not happen, and indeed, migration is not
generally considered a viable option for ectotherms due to metabolic restrictions
on the long-term activity levels needed for migration. A more likely explanation
for the presence of dinosaurs in this environment is that they were endotherms
and thus able to cope better with the extremes of temperature, possibly by
hibernating, or by some adaptation against freezing.

References

GIGANTOTHERMY

One argument against warm-blooded dinosaurs suggests that large dinosaurs
such as the sauropods would not need to be warm-blooded as their size would
prevent temperature fluctuations. Heat production is related to body mass, while
heat loss is related to body area. As an animal gets larger, its body area
decreases relative to its body mass, so heat loss decreases and it becomes more
efficient at maintaining body temperature. This theory, termed 'gigantothermy',
together with the possible aid of plates, spikes, frills or nasal cavities used as heat
exchangers , proposes that large dinosaurs living for the most part in a warm
environment could in fact have found the body temperature of a fully
warm-blooded animal thermally stressful and disadvantageous.
There are a number of counter-arguments. Although gigantothermy provides
for increased efficiency of temperature regulation, it is still far less efficient than
true warm-bloodedness (6 - 8 degrees C variation instead of 1 -2 degrees), and
warm-blooded animals would still be expected to win easily in any evolutionary
competition. Gigantothermy also does not address the problem that all dinosaurs
arose initially from relatively small ancestors, certainly too small for
gigantothermy to have any impact. If these small ancestors had developed
warm-bloodedness and thus competed effectively with the mammals for
domination, why would evolution produce giant descendants that had lost the
ability? The proposals regarding sails, plates and various other ' heat exchangers
' suffer from the observation that quite closely related species , both warm- and
cold-blooded, may or may not have had these additions. If they were important
and obligatory for heat regulation, all species should have had them. It seems at
present that these developments were probably evolved as display organs rather
than heat exchangers .

References

RATE OF FOOD SUPPLY

Another argument raised against warm-blooded dinosaurs and applied
particularly to the large herbivorous sauropods centres around the ability of the
animal to provide sufficient food to provide the extra energy required to
maintain body temperature. This argument tends to rely on observations of the
teeth and jaw. In some dinosaurs eg diplodocids , they had small heads and no
grinding teeth, and so (it is claimed), could not possibly consume enough food.
Some modern large flightless birds have exactly the same problems, but are
unquestionably not only alive but also warm-blooded. They cope by
modifications to food processing after it has been swallowed. Similar
modifications likely to have taken place in the dinosaurs are discussed in more
detail under ' Dinosaur Feeding '.
Another fact to be considered is that the enormous size of the sauropods gave
them a very small body area to body mass ratio. Heat loss was thus greatly
reduced and they would have required much less food on a weight basis than
smaller animals, just as an elephant only requires about one twentieth as much
relative food as a rabbit.

RESPIRATORY TURBINATES (70)

Respiratory turbinates (RT) are folded structures of bone or cartilage in the sinus
cavities, considered by some to be essential in conserving water and heat loss
and thus obligatory for warm-blooded creatures. John Ruben presented data in
late 1995 at the SVP meeting suggesting that dinosaurs did not have these
structures. His work was based on a reconstructed Nanotyrannus skull. Not
only did he not find turbinates but suggested that the nasal passage was too
small for any to exist. Others suggest that the skull is actually a juvenile T. rex
with a somewhat compressed nasal passage, and Horner presented counter
evidence that a Lambeosaurus skull CT scan shows RT-like structures, but the
scans were indistinct and the 'turbinates' incomplete at best and possibly
incorrectly orientated. Greg Paul has strongly opposed the conclusions of
Ruben, claiming that the only way to prove the lack of respiratory turbinates is
to show that there is not enough room in the dinosaur nasal passages and that
this has not been done; Ruben's conclusions being based on an illusion created
by not drawing both dinosaur and bird to the same scale. Most theropods have
very large heads, making the nasal passages appear small by comparison,
whereas their actual volume equals or exceeds that of birds. Ruben's evidence
has since been published in Science (Vol 273, pp 1204-7, August, 1996), and
includes both Horner and Currie as co-authors. The authors have used modern
medical imaging technology to examine the size and structure of the nasal
cavities of the theropod dinosaurs Nanotyrannus and Ornithomimus and the
hadrosaur Hypacrosaurus and compared these with 8 modern extant mammals,
3 birds and 4 reptiles. The results show that while all of the mammals and birds
have RTs, none of the reptiles or dinosaurs did, and for a given body weight,
reptiles (and dinosaurs) had smaller ( by a factor of 4) nasal cavity cross sections
than birds and mammals - too small, in Ruben's opinion, to have included RTs at
all, and also too small, again in his conclusion, to have allowed a high enough
lung ventilation rate to support the higher metabolic rate required for an
endothermic animal.
The paper concludes that a variety of Cretaceous dinosaurs possess crocodile or
lizard-like nasal passages, too small to have accomodated RTs and endothermic
lung ventilation rates, although conceding that their observations do not rule out
the possibility that they may still have had routine metabolic rates somewhat
greater than modern ectotherms .
Not surprisingly, the paper has come under attack from some sources for a
number of reasons including sample size and selection. These objections may or
may not be valid, but there are reasons for querying the conclusions drawn by
Ruben and his co-authors. Initially it appears odd that there is no attempt to
include any fossilized mammals or birds in the study, but apparently results
published elsewhere indicate that RTs have been found in fossilized bird skulls
back to 70 million years ago (but not earlier). This means, by the logic used to
exclude endothermy in dinosaurs without RTs, that Archaeopteryx and many
other early birds were also not endothermic - a very difficult option to accept
given that they were feathered and could most likely fly!
Greg Paul has noted that Ruben measured only the narrow, horizontal front
section of the nasal passage which is enclosed in bone and therefore fairly easy
to measure. He points out that birds have a rear section as well that is not
enclosed by bone and thus difficult to measure, but, importantly, also in a much
wider part of the skull so that the nasal passage in this area may be up to 3
times wider. In birds this region contains a larger RT, and it is possible that in
dinosaurs the same may be true. Paul is also critical of the use of a heavily
reconstructed Dromaeosaur skull to picture the nasal passage, when fossil
material from the nasal passage was not found. Closely related Velociraptor
complete skulls do indeed show the second, larger part of the nasal passage, and
it may have contained cartilaginous RTs that would be unlikely to fossilize.
Criticism can also be levelled at the conclusions regarding lung ventilation rates.
As the paper itself notes, nasal passages must be significantly enlarged to make
way for the presence of RTs. However, much of the increase in space is taken
up by the turbinates themselves. In the absence of detailed measurement of the
actual nasal passage free space it is impossible to determine whether the larger
nasal cavity size in endotherms is related to lung ventilation rates at all, or is
simply a reflection of the presence of RTs. The lung ventilation argument against
endothermy must at this stage be considered unproven, so that the
anti-endothermy evidence from the RT investigation rests solely on the absence
of such structures in dinosaurs and modern ectotherms , and is based on the
water conservation function of RTs. At least 2 modern groups of endotherms
are known that also lack RTs - the Pelecaniforme diving birds, and whales. In
both cases there are compensatory mechanisms to balance the additional water
loss and heat transfer ability of the missing RTs.
The argument then reduces to this; RTs appear to be necessary in modern
endotherms unless for some reason water loss during respiration can be
counterbalanced by some other mechanism. In order for this argument to infer
non-endothermy in dinosaurs it would be necessary to demonstrate that such
mechanisms were not present. The lack of a detailed model of dinosaur water
handling makes this impossible.
This subject has plainly not yet been decided one way or the other, and, like so
many other aspects of extinct animals where information on soft tissue remains
is unavailable, may never be definitively decided. The apparent lack of RTs in
dinosaurs is evidence for lack of endothermy , but hardly strong enough to
discard the body of evidence supporting it. The debate promises to continue for
some
considerable time.
NOTE: Respiratory turbinates should not be confused with olfactory turbinates
which some dinosaurs almost certainly did have but which were quite different
in function.

References

LUNG STRUCTURE

The debate about dinosaur lung structure
and its application to the question of
thermoregulation stems from an analysis of
Sinosauropteryx published by Ruben,
Jones, Geist and Hillenius (the group also
responsible for papers on respiratory
turbinates and hatchling bone maturation
critical of the endothermic dinosaur
postulate).
They begin their arguement with a detailed
morphological comparison of lung
structure in reptiles and birds, suggesting
that the simpler, bellows-like
reptilian/crocodilian lung is probably
incapable of supporting the increased
metabolic rates necessary for active
endotherms . While birds have a similar
septate lung, modifications leading to a
series of connected air sacs throughout the
thorax and abdomen and a uni-directional
airflow increase oxygen transport efficiency
sufficiently to allow a very high metabolic
rate . Ruben et al also maintain that the
processes by which the lungs are powered
differ significantly in reptiles and birds.
They then turn their attention to dinosaurs,
observing that although these probably had
the same septate lungs as reptiles and
birds, they did not have the necessary
skeletal and muscular mechanisms
necessary to provide a bird-like, high
efficiency air circulation and
oxygen-extraction system. In particular,
they focus on photographs of the first
Sinosauropteryx specimen, claiming that it
shows a clear division of the thoracic and
abdominal cavities by a crocodile-like
vertical partition. Further arguements on
the shape and function of the pelvis of
primitive birds such as Archaeopteryx and
dinosaurs leads them to conclude not only
that with reptilian style lungs dinosaurs
could not have been endotherms , but also
that it makes it much less likely that birds
are descended from dinosaurs.
This paper has come under considerable
criticism, as yet unpublished, by various
experts including Greg Paul, Guy Leahy
and, reputedly, Luis Chiappe. In particular
they point out that the crocodilian system
requires a mobile pelvis , and that the
pelves of theropods are simply not able to
substitute. Look out for papers in Science
refuting many of the Ruben claims,
particularly the presence of a vertical
partition in dinosaurs and the Ruben
analysis of pelvic structure and other
skeletal modifications and air- sacs
reputedly absent in dinosaurs.

A second, related paper by the same group
was subsequently published in Science, this
time dealing with the juvenile theropod
Scipionyx. This small dinosaur from Italy
is superbly preserved, to the extent that
remains of internal organs are evident.
Ruben et al interpret these internal organ
structures as supporting their idea of an
hepatic-piston respiratory system in
theropods, and suggest that such a lung
system would have enabled dinosaurs to
rapidly increase oxygen intake to maintain
high activity levels for extended periods
without an endothermic basal metabolism;
a sort of turbo-charged dinosaur. Again,
their interpretation and conclusions have
received considerable criticism.

References