GEOL 104 Dinosaurs: A Natural History
Fall Semester 2009
Dinosaur Physiology
Hot- and Cold-Running Dinosaurs?
Among modern vertebrates, some gross generalizations:
Birds and
mammals are warm-blooded; that is, they are warmer than the environment around them in typical temperate and colder enivronments.
Crocodilians, lepidosaurs, turtles, amphibians,
most fish, and almost all invertebrates are
cold-blooded: their bodies are only about as warm as the general enviroment around them, so consequently they feel
cool to the touch outside of tropical situations.
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)
topics:
- 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
("slow metabolism")
- In "Warm-blooded" animals, rate of fuel usage is HIGH: called tachymetabolic
("fast metabolism")
- 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
dinosaurs?
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
batteries
- 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
ectotherms
- 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 below.
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:
- However, baby dinosaurs match adults in stable temperature; don't match poikilotherms
from same environment
- Growth rate: fantastic growth rate (see earlier lecture)
suggests tachymetabolism.
- Problem: Maybe due to very favorable conditions of Mesozoic: allow fast growing
ectotherms
- 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
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
the food
- Overheating:
- Because Mesozoic was warm, large dinosaurs would overheat if endothermic, so must have
been ectotherms
- 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
thermal physiology
- 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
radiators
Hearts, Lungs, and Faces: New Approaches to Dinosaur Physiology
Some thing to consider:
- Is this whole debate a false dichotomy?
- Is everything EITHER an endothermic tachymetabolic homeotherm OR an ectothermic
bradymetabolic poikilotherm?
Some additional possibilities:
Gigantothermy:
From geometry, as linear dimensions double, the surface area goes up by squares, and the
volume by cubes:
| Side Length | Surface Area (SA) | Volume (V) | SA/V
|
|---|
| 1 | 6 | 1 | 6/1 = 6
|
| 2 | 24 | 8 | 24/8 = 3
|
| 3 | 54 | 27 | 54/27 = 2
|
| 4 | 96 | 64 | 96/64 = 1.5
|
- As size increases, SA/V decreases.
- The mass of an animal, and the heat it produces, is based on its volume.
- The rate at which an animal can gain and lose heat is based on its surface area.
- Therefore, with bigger body size it takes longer and longer for heat to be lost or
gained:
- Become homeothermic without having the energy costs of endothermy!
- Problem: No good living models (elephants once thought to be partial gigantotherms,
but does not now seem true; marine leatherback turtles are, but are not in same type of
environment)
- Also, gigantothermy might apply to large dinosaurs, but would not apply to small species or
to babies.
Heterometabolism:
- Changeable metabolic rate: tachy- to bradymetabolic.
- Two main types: behavioral and ontogenetic:
- Behavioral heterometabolism:
- Normally operate as bradymetabolic, but shift into "high gear" in certain
circumstances
- Living examples: sharks in feeding frenzy; pythons while brooding
- Ruben suggests that specialized breathing structures may have let dinosaurs be
"turbo-charged", but have fully ectothermic physiologies
- Ontogenetic heterometabolism:
- In most animals, metabolic rates slow down as age (and thus size) increases
- Perhaps in dinosaurs was more extreme
- Problem: No good living examples
So, where do we stand on dinosaur metabolism?
- All living dinosaurs (Aves) are endothermic tachymetabolic homeotherms
- The living outgroups (crocodilians, lepidosaurs, turtles) are all ectothermic bradymetabolic
heterotherms
- Non-avian dinosaurs show many anatomical features suggesting levels of activity higher and/or
more continuous than that seen in modern "cold-blooded" animals
- Non-avian inosaurs show growth patterns comparable to those of modern endotherms, and unlike
those of modern and extinct ectotherms
What would be necessary to justify the above observations?
- Non-avian dinosaurs would need active ventilation (breathing) to power the
muscles and to fuel the growing tissue
- Non-avian dinosaurs would need strong, active heart to get the oxygen to the
muscles and tissues
- Non-avian dinosaurs would need structures to control heat
Is there evidence for these features in dinosaurs? YES!
Dinosaur Breathing:
- Mammal-style diaphragm breathing is an advanced therapsid feature; most tetrapods
breath by gulping air
and by rib breathing
- Specialized lizards developed
neck breathing separate from rib breathing
- Crocodilians have their own specialized breathing:
- Pubis is mobile, and rocks back and forth pushing & pulling the liver
- Functions like the mammalian diaphragm, to have additional active breathing
- Living dinosaurs (birds) have extremely specialized breathing:
- Speculation: belly breathing is an archosaurian synapomorphy:
- In primitive archosaurs, primitive crurotarsans, and most dinosaurs
other than birds, muscles from the pelvis would pull gastralia down, which would inflate the lungs
- This would give these animals extra oxygen for their metabolism
- Becomes modified in crocodilians (liver pump with mobile pubis), pterosaurs (a mobile "prepubis"; another liver pump?), birds, and
ornithischians (mobile pubes or other parts of the pelvis in some ornithischian groups)
- Furthermore, strong evidence that theropods and sauropods (at least) had
air sacs like those of birds:
- Chambers in vertebrae are very similar to those of birds
- Air sacs may have been present in other dinosaurs, but apparently did not
enter the vertebrae
For more on vertebrate breathing, check out the website
of experimental work on modern amphibians and non-avian reptiles.
Dinosaur Hearts:
- Turtles and lepidosaurs have three chambered hearts
- Birds and mammals have four chambered hearts:
- A "double pump" system, so the heart acts as a control between lungs and body
- Shunts blood to lungs before going out to body, so all the blood getting to the tissues
are fully oxygenated
- Also, can allow these animals to be taller, since the heart pressure control separates
lungs and body, and therefore pressure on lung blood vessels won't get too high
- Crocodilians actually have specialized (NOT primitive) four-chambered hearts:
- Operate as four-chambered heart on land, shifts to
two chambered underwater since doesn't need to get blood to lungs
- Since both birds and crocodilians have four-chambered hearts, assumption is that all
extinct archosaurs, including non-avian dinosaurs, did too
Dinosaur Temperature Regulators:
- Some dinosaurs have conspicuous large sails or plates or frills or long necks or long
tails that might have been used to dump waste heat
- However, other structures may have also been used to regulate temperature:
- The antorbital fenestra (also the promaxillary and maxillary fenestrae
of various theropods) housed soft tissue air sacs
- These air sacs may have been useful to transport waste heat
- Also, many larger dinosaurs have enlarged and/or elaborate nares
- These may have been useful in dumping waste heat
The enlarged narial regions may support tissues for a different function: recovery of moisture. In
living endotherms, rapid rate of respiration would dry out lungs if not for some specialized
tissues called nasal turbinates:
- Rare or small in modern ectotherms
- Click on the "Dynamic Cutaway: Coronal" animation on this
website to see the lack of turbinates in a CAT scan of the modern Chinese crocodile lizard
- Extremely large in mammals, where they are scroll
work of bone in the snout, supporting thin tissues which trap moisture going out, and
rewets on way back in
- Click on the "Dynamic Cutaway: Coronal" animation on this
website to see the turbinates in a CAT scan of a house mouse
- Fairly large in many birds, but are cartilage
rather than bony
- Also, some birds seem to rely on air sac system for this purpose
- Most non-avian dinosaurs do not show much evidence for internal nasal turbinates, but
the air sac system and/or tissues in the enlarged narial regions of bigger dinosaurs may
have served this function
Still much work to be done in interepreting the physiology of extinct dinosaurs
Eat or be Eaten: Dinosaur Paleoecology
Ecology is NOT what most people think it is!
It is not environmental activism. Instead:
- Ecology is the scientific study of the factors which control the abundance and
distribution of organisms.
Therefore, ecology isn't about saving the whales, but it WILL tell us something about HOW to
save the whales...
Paleoecology is attempting to reconstruct the ecology of extinct forms.
Some aspects about paleoecology:
- Ecological niche: the "way of life" of a particular taxon
- Species ranges: how large an area did a particular species occupy at a given
point in time (very difficult for most dinosaurs, as they are known from very few specimens)
- Trophic relationships: aka "who eats who". Often restored as a food web
- Can get some good idea about carnivores vs. herbivores, but difficult to establish
exactly which carnivores ate which herbivores, and which herbivores ate which plants.
Bakker used his interpretations of trophic relationships to try and determine the
thermophysiology of dinosaurs and other extinct forms. His technique:
Predator-Prey ratios:
- In modern endothermic communities very few predators compared to many herbivores
(tachymetabolic predators require a lot of food, so only a few can survive in a given
region).
- Bradymetabolic predators require a lot less food, so same amount of potential food
can support many bradymetabolic predators.
- In order to calculate P/P ratios, Bakker had to consider the different sizes of the
various populations. Used biomass (# kgs or tons of flesh) rather than number of
individuals
- Found that modern populations had P/P ratios of 0.5-4 %
- Looking at fossil record, found:
- 1st Radiation (Basal synapsids): 25-30%, much higher than modern populations. Most
paleontologists have accepted this as a cold-blooded community
- 2nd Radiation (Therapsids): 10-20%, seemingly between endo- and ectothermic
populations
- 3rd Radiation (Pseudosuchians): 10-20%, as in 2nd
- 4th Radiation (Dinosaurs): 0.5-3.5%, as in modern endotherms!
- 5th Radiation (Mammals): 0.5-4.5%, known endotherms
- Problems:
- How do you know the dinosaur mass estimates are correct?
- How do you know that the numbers accurately sample fossil populations?
- At least 1st and 5th Radiation populations seem to match expectations (but new finds
from Germany may show almost mammal-like levels for upland 1st Radiation communities!)
- How do you know which dinosaurs ate which?
- At best can only give the thermal physiology of predators!
- For herbivores would need a Herbivore Biomass – Plant Productivity ratio
- Some preliminary evidence suggests that MORE herbivores per acreage in dinosaur
populations than in modern or fossil mammal populations
So, P/P ratios are problematic, at best.
Some ways in which dinosaurs are distinctly different from modern mammalian communities:
- Much higher rate or production: dozen eggs per year, independant of size vs. litter size
and gestation period scaled to body size
- From this, dinosaur populations could absorb many more fatalities and survive than
equivalent-sized mammals
- Also, dinosaurs occupied many more niches in their lifetime than a mammal, because all
dinosaurs begin very small
So, what is the answer to dinosaur physiology & ecology? We still don't know.
Current status, and some scenarios:
We know that:
- Living dinosaurs (birds) are all endothermic tachymetabolic homeotherms
- The living sister group to dinosaurs, crocs, are all ectothermic bradymetabolic
heterotherms
- All groups of dinosaurs show upright stance and other adaptations suggesting active
lifestyles
- All large dinosaurs, and many small ones, show signs of having high breathing rates
- If P/P ratios are real, dinosaur ecologies were more similar to mammals than to basal
synapsids
- Many dinosaurs (particularly Late Cretaceous forms) show very sophisticated feeding,
locomotory, and social adaptations
Scenario I: Bakker or "Hot-Blooded Dinosaurs" model
Dinosauria (and probably Ornithodira) were endothermic tachymetabolic homeotherms;
therapsids and crurotarsans had intermediate rates (crocs would thus be a reversal).
Scenario II: Ruben or "Good Reptile" model
No dinosaur was warm-blooded, but at least some had means of rapidly oxygenating their
blood to be "turbo-charged" and thus function temporarily as highly active animals. True
endothermic tachymetabolic homeothermy doesn't appear until after Archaeopteryx.
Scenario III: an intermediate model ("Damn Good Reptile" model)
All dinosaurs had some degree of endothermic tachymetabolic homeothermy while young; small
dinosaurs retained this into adulthood. Large dinosaurs experienced a slow down in
metabolic rate, but still higher than any cold-blooded animal (~ 2/3 the rate of mammals
of same size). Efficient oxygenation of blood and gigantothermy allowed these dinosaurs
to be as active as mammals without the same energy costs.
- Benefit of model:
- Explains large amount of "meat on the hoof" in Mesozoic: a 4 ton hadrosaur with 2/3
mammal metabolism would only need as much food as a 800 kg bison.
- Thus, the higher capacity for dinosaur population growth would be realized
- Variations:
- Development of insulation in coelurosaurs suggest that they were fully endothermic
- Birds retain primitive condition of warm-blooded juveniles into adulthood
- Progressive variation: as Mesozoic continued, different groups of dinosaurs
(hadrosaurids, ceratopsids, coelurosaurs, maybe even titanosaurs and ankylosaurids)
independently developed full endothermy from the original Scenario III condition
Still much work to be done.
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Last modified: 17 August 2009
