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
- Crocodilians have 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:
- Pump their lungs by rocking their hips up and down
- Special air sacs in the torso, vertebrae, and limbs help keep the air flowing in
one direction, rather than a simple in-and-out
- Speculation: belly breathing is an archosaurian synapomorphy:
- In primitive archosaurs, primitive pseudosuchians, 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, birds, and ornithischians
- 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
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 three 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
- Possible (but problematic) "fossil dinosaur heart" found in hypsilophodont
Thescelosaurus: see website on this specimen
- However, even if this specimen isn't really a heart, we would recognize that dinosaurs
had four-chambered heart by their phylogenetic position
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
Still much work to be done in interepreting the physiology of extinct dinosaurs
To Next Lecture.
To Previous Lecture.
To Syllabus.