GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
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 later 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, and proportionately
larger brains than contemporary mammals (which are accepted as endotherms)
- 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
- 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
To Next Lecture.
To Previous Lecture.
Last modified: 14 July 2006