GEOL 104 Dinosaurs: A Natural History
Fall Semester 2000
Hot- or Cold-Running Dinosaurs?
Among modern vertebrates, some gross generalizations:
Birds and mammals are warm-blooded; Crocodilians, lepidosaurs, turtles, amphibians,
and most fish are cold-blooded.
Old debate in dinosaur studies: were they warm-blooded or cold-blooded?
Owen in 1842 suggested dinosaurs might have been warm-blooded, or at least more
warm-blooded than typical modern reptiles.
Need to be precise as to definitions of terms.
"Warm-blooded" and "Cold-blooded" actually encompass several different (although related)
- Energy Source: whence comes the majority of the energy to "run" the animal?
- In "Cold-blooded" animals, the main energy source is the sun (and external
environments in general): called ectotherms ("outside heat")
- In "Warm-blooded" animals, the main energy source are specialized sub-cellular
structures whose main purpose is to convert food energy to heat energy: called
endotherms ("inside heat")
- Metabolic Rate: how much food energy ("fuel") is used up over time?
- In "Cold-blooded" animals, rate of fuel usage is low: called bradymetabolic
- In "Warm-blooded" animals, rate of fuel usage is HIGH: called tachymetabolic
- Temperature Variation over Time: how stable is the body temperature over time?
- In "Cold-blooded" animals, body temperature fluctuates with the external environment:
called poikilotherms ("fluctuating heat")
- In "Warm-blooded" animals, body temperature regulated by internal mechanisms and thus
more stable: called homeotherms ("same heat")
A typical cold-blooded animal is an ectothermic bradymetabolic poikilotherm: needs to get
its energy from the sun and fluctuates with external environment (but can moderate
fluctuations by moving from sunlight to shade and vice versa); however, needs very little
food (snakes can go weeks without feeding, for example). Cold blooded animals become
torpid at night and in colder weather.
A typical warm-blooded animal is an endothermic tachymetabolic homeotherm: its body
temperature is stable and activity levels can remain high for long periods of time, at
night, and in colder weather; however, needs a LOT of food or will die (imagine the
effects of not feeding a cat or dog for weeks…).
How can people determine the thermal physiology of extinct animals like non-avian
Owen suggested dinosaurs might have been warm-blooded because:
- Upright posture: today, all living animals with upright posture are warm-blooded
Many late 19th Century paleontologists considered dinosaurs to be more similar to modern
warm-blooded animals in terms of activity levels.
During early 20th Century, shift to lizard-like concept for dinosaurs.
Concept of warm-blooded dinosaurs revived in late 1960s by Ostrom because of a
number of lines of evidence:
- Upright posture: as in Owen
- Problem: No causal relationship ever established: just because all living animals
with upright stance are endotherms does not mean that upright stance requires endothermy
- Dental batteries of hadrosaurids and ceratopsids: useful for chopping up food
into very fine particles for fast digestion, but bradymetabolic animals don't have fast
digestion; suggests tachymetabolism in hadrosaurids and ceratopsids
- Problem: Most ornithischians and all sauropods lack sophisticated chewing or slicing teeth
- Does not negate observation that hadrosaurids and ceratopsids have dental
- Modern herbivorous birds make do without grinding teeth by using gastroliths
(gizzard stones), which are found in non-hadrosaurid, non-ceratopsid herbivorous dinosaurs
- However, gastroliths are also found in some ectotherms, so their presence is NOT
evidence of endothermy!
- Sickle claw and stiffened tail of dromaeosaurids: suggested a more dynamic mode of
attack for dromaeosaurids than in monitor lizards or crocs
- High blood pressure necessary to pump blood into brains of tall theropods,
ornithopods, and (most especially) sauropods: requires powerful, active heart
- Latitudinal distribution: dinosaurs (and therapsids) found in Mesozoic (and
Permian-Triassic) polar regions, although not as cold as today would still be cooler than
climates preferred by typical modern cold-blooded animals
- Problem: Earth's climate WAS warmer in Mesozoic
- However, some polar sites contain dinosaurs & mammals but not crocs, lepidosaurs,
turtles, etc., while other sites in Alberta of same age are chock-full of known
- Maybe the dinosaurs migrated out of the polar sites during cold winters?
- BUT energy requirements for large scale migration might arguably require endothermic
levels of metabolism!!
- Also, baby dinosaurs found in these sites: unlikely to have migrated
- Origin of birds from coelurosaurs: birds are known to be warm-blooded, so their
immediate relatives might have been, too
- Problem: Some argue that early birds themselves could have been ectothermic,
with endothermy evolving AFTER Archaeopteryx
- Complex social behaviors for at least some dinosaurs: no causal link, but more
typical of modern mammals and birds than crocs, lepidosaurs, and turtles
- Problem: As with upright stance, no causal link between endothermy and complex social behavior
- Also, no evidence for such behavior in most dinosaurs
Colleague from France: Armand de Ricqlès added additional line of evidence:
- Bone microstructure: lots of signs of reworking (bone being resorbed as
mineral source in metabolism, and redeposited), lots of Haversian canals.
- Typical bradymetabolic animals have little reworking and few Haversian canals;
typical tachymetabolic animals have lots. Dinosaurs resembled tachymetabolics.
- Problem: Suggestion that very old bradymetabolic animals might develop tissue similar to
younger tachymetabolic animal
- However, even baby dinosaurs show endothermic-style bone tissue
Ostrom's undergrad student Robert T. Bakker: main advocate for the "hot-blooded"
dinosaurs model. Added his own observations:
- Ecological replacement: many paleontologists argued that therapsids were at
least partly warm-blooded, but were replaced by archosaurs.
- Problem: We do not know for certain that an ectothermic group would necessarily
be out competed by endotherms
- Also, other possible selective factors (i.e., water retention)
- Predator-Prey ratios: we'll discuss these in more detail in a later lecture.
Additional lines of evidence (primarily from 1980s and 1990s):
- Oxygen isotopes: can determine body temperature and (importantly) variation of
body temperature over time: dinosaurs show stable temperatures, while contemporary
non-dinosaurian reptiles show larger variation.
- Problem: Large bodied animals expected to have stable temperatures:
- See gigantothermy next lecture
- However, baby dinosaurs match adults in stable temperature; don't match poikilotherms
from same environment
- Growth rate: fantastic growth rate (see earlier lecture)
- Problem: Maybe due to very favorable conditions of Mesozoic: allow fast growing
- However, known contemporary ectotherms (like giant crocs) show typical slow-growing
rate comparable to modern ectotherms
- Presence of feathers on non-avian coelurosaurs: since not flight features,
might have been for insulation (which small endotherms need or they lose too much heat).
- Problem: Not all dinosaur clades show feathers
Not everyone convinced that dinosaurs were fully endothermic tachymetabolic homeotherms.
Two main types of evidence to the contrary:
- Evidence suggested to counter claims of dinosaur endothermy (shown as Problems
in the text above)
- Evidence suggest to support dinosaurian ectothermy
Lines of evidence supporting dinosaurian ectothermy:
- Small brain size:
- Most dinosaurs characterized by brain sizes expected in crocs or lizards of that size;
modern endotherms all have much larger brains!
- Problem: However, no causal link established between brain size and metabolism
- Also, coelurosaurs at least have larger brains than typical dinosaurs
- Small head size in herbivores:
- Lack the big maws of large herbivorous mammals: how could they get enough food?
- Problem: However, many large flightless birds have tiny heads, yet they are endotherms
- Lack of specialized teeth in most herbivorous dinosaurs:
- Non-hadrosaurid, non-ceratopsid ornithischians and sauropods lack sophisticated
chewing or shearing teeth
- Problem: These other dinosaurs are known to have gizzards, which could process
- Because Mesozoic was warm, large dinosaurs would overheat if endothermic, so must have
- Problem: Very large mammals (known endotherms) are found in comparably warm
periods of Cenozoic
- Some dinosaurs may have thermal "radiators" to dump heat (see below)
- Growth lines:
- Dinosaur bones show "growth rings" (Lines of Arrested Growth, or LAGs),
typical of reptiles and (once thought to be) lacking in mammals
- Problem: Now known in perfectly good endothermic mammals
- Are a symplesiomorphic feature of vertebrate bone growth; may not signal any aspect of
- Conspicuous potential solar collectors &/or radiators:
- Stegosaur plates, neoceratopsian frills, sails in mid-K equatorial dinosaurs might be
good radiators to dump heat or collectors to get heat
- Problem: However, might be for display instead
- Additionally, some endotherms (like elephants and their ears) have large solar
- Respiratory turbinates:
- A favorite of John Ruben and colleagues
- RTs are bony (in mammals) or cartilage (in birds) structures in the snouts to trap
- Because of need to breath A LOT, endotherms need RTs to collect moisture or would
- Although RTs of dinosaurs would presumably be cartilage (and thus not preserved), need
sufficient space to house them, which could be recovered
- Initial studies of Ruben & colleagues suggested not enough space in dinosaur snouts
- Problem: However, a handful of endotherms do not have RTs (including humans!)
- Also, some errors in initial studies in at least some dinosaur calculations
- New calculations show many large derived dinosaurs (ceratopsids, hadrosauriforms,
ankylosaurs, sauropods, large prosauropods, neoceratosaurs, carnosaurs, and tyrannosaurids)
DO have enlarged nares or large nasal chambers suggesting some form of RT was
possible (see also more below)
- Still, small ceratopsians, small ornithopods, stegosaurs and basal thyreophorans,
small prosauropods, coelophysoids, basal tetanurines, and smaller coelurosaurs seem to
lack space for these structures: a puzzlement…
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