History of L ife 32

VIII . Archosaurs.A. Diapsid reptiles – Descended from

Thecodonts.1. 1st appear in late Triassic.2. Primitively bipedal.3. Tail functioned as counterbal-

ance to head and body.4. Built for speed – light, hollow

bones.5. In birds, hollow long bones asso-

ciated with air-sacs, unidirectional airflow and highly efficientO2 utilization.

B. Two major dinosaur groups distin-guished on the basis of pelvic mor-phology.1. Saurischia

a. Pubis directed forwards as inthecodonts.

b. Sauropods, theropods, birds.2. Ornithischia

a. Pubis redirected posteriorlyalongside ischium.

b. Stegosaurs, duckbill s, ceratopsians.

C. What were they like?1. Traditional view – stupid, lumbering giants.2. Revisionist view – active, intelli gent, bird-like.

Dinosaur Pelvis. Saurischian (left)and ornithischian (right) .

Tr iassic thecodont, Heterosuchus.

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D. Discussions of the active dinosaur hypotheses focus on endo-thermy because mammals are1. active2. smart3. endothermic.

E. Recall .1. When muscles contract, in situ metabolism initially aerobic.2. When oxygen exhausted, metabolism becomes anaerobic =>

lactic acid production.

Dinosaur Family Tree. Note the division into “bird-hipped” (ornithischian) and “ li zard-hipped” (saurischian) dinosaurs and the derivation of birds from the latter.

History of L ife 34

Anaerobic and aerobic metabolism. The former gen-erates 2 molecules of ATP per molecule of glucosecrunched; the latter 36 ATP equivalents.

History of L ife 35

F. Endotherms engage in aerobic respiration to far greater extent thanectoderms.1. Allows endoderms to sustain greater rates of energy output over

longer periods of time.2. Both ectoderms and endoderms can increase MR 5-10 X above

BMR.3. But BMR in endoderms much higher.4. Endoderms consume 10-30 X as much energy; consume more

O2; generate more ATP.

G. High BMR Î1. High body temperature;2. Need to cool the animal.

a. Especially in large animals after extended periods of high ac-tivity.

b. Hence, sweating / panting.c. Fur / feathers /adipose tissue can also keep animals cool –

protect from radiative heating – and further serve to keepthem warm when inactive.

Relative endurance in a mammal (endotherm) and a crocodile (ectotherm).

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G. Regulation of core temperature.1. Evolution of constant body temperature, Tc, in higher tetrapods

driven by the fact that eff icientmuscle activity requires hightemperature.

2. One way or another, musclesmust be warmed.

3. Ectotherms regulate Tc behavior-ally – basking, seeking out shade– but principal regulation envi-ronmental.

4. Endotherms regulate Tc viaa. High rates heat production –

consequence of elevated BMR.b. Heat retention – fur, feathers,

etc.c. Heat loss / production mecha-

nism – sweating, panting /shivering.

d. Adjusting• BMR• rates of peripheral blood flow.

e. Behavior – “mad dogs and Englishmen.”2. Thermoregulation not limited to birds / mammals – some insects

regulate Tthorax to maximize eff iciency of flight muscles.

G. Scaling Issues.1. BMR size dependent.2. Roughly,

Temperature Regulation in ecto-therms and endotherms.

History of L ife 37

3. On a per gram basis, BMR scales with the –(1/4) power of bodymass Î Shrewslive faster thanelephants.

4/3kMBMR =

History of L ife 38

IX. “ Hot Blooded” Dinosaurs?A. Dinosaurs perfected upright stance

– suggests aerobic li fe style.1. Reflects changes in pelvis mor-

phology2. As in synapsids, Î

a. more eff icient lung function –not compressed by side to sideflexing of trunk muscles.

b. larger lungs.3. Shorter femur/calf length ratio,

elongate toes (animal “up” ontoes) Î greater speed.

4. Footprints and anatomy suggestspeeds up to 60 km/h in somespecies.

B. Mechanisms for processing largeamounts of food.1. Secondary palette – but tur tles

have them too.2. Ceratopsian tooth rows.

C. Insulation as an aid to maintainingconstant body temperature.1. Some theropods (raptors) had

feathers;2. some pterodactyls, fur.

Dental battery (cheek teeth) ofTriceratops.

Theropod leg bones. Femur fore-shortened with inturned head. Noteelongate tarsals.

History of L ife 39

D. But, dinosaurs didn’ t have tur-binates!1. Neither do birds.2. Forward placement of a fleshy

nostril would achieve same re-sult.

E. But, dinosaurs didn’ t have dia-phragms.

1. Crocodiles use “pelvic assistedrespiration” to increase volumeof air moved.

2. Legacy of a more active past?3. If yes, consistent with 4 cham-

bered crocodil ian heart.4. More recently, unidirectional air-

flow (UAF) observed [Farmerand Sanders, 2010 (Nature)] inalli gators.

F. UAF and Air-Sacs.1. In birds, each aliquot of inhaled

air requires two breathing cyclesto pass through the animal.a. 1st inhalation: to the posterior

air-sacs;b. 1st exhalation: to the lungs;c. 2nd inhalation: to the anterior

air-sacs;d. 2nd exhalation: back out to the

environment.

Fleshy Nosed Dinosaurs? FromWitmer, J. M. 2001. Nostril posi-tion in dinosaurs and other verte-brates and its significance for nasalfunction. Science. 293: 850-853.

History of L ife 40

2. Unidirectional airflow increases respiratory efficiency,a. With each breath (in or out), fresh air brought to the lungs;b. Eagle over Everest.

3. Air-sacs penetrate into the long bones, which are hollow.a. Like thecodonts.b. Long speculated that air-sacs an early archosaurian trait.c. More recently, O’Connor and Claessens [2005 (Nature)] pub-

lished evidence for air-sacs in non-theropod dinosaurs.4. Thus,

a. UAF without air-sacs may have evolved in response to low[O2]atm in the late Permian / early Triassic.

b. Air-sacs evolved a bit later as an adjunct to increased meta-bolic rate.

5. Or – absence of air-sacs in alligators derived.

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G. H.

History of L ife 42

G. Feathered dinosaurs pre-dating Archaeopteryx stronglysupport Avian descent fromsmall theropods.1. Retained ancestral retractile

claw.2. Feathers believed to have

evolved from dinosaur“hair.”

3. Earliest forms, e.g., Micro-raptor, Anchiornis, had fourwings – dinosaurian flying squirrels.

4. However, “birds from dinosaurs” hypothesis not universally ac-cepted. Alternative is that some (many?) theropods were “neo-flightless” birds.

Anchiornis huxleyi. Colors inferred fromthe shape and densities of preservedmelanosomes.

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G. Brain size.1. Endocasts suggest EQ’s intermediate between ectoderms and

endoderms with small theropods having the largest brains andapproaching the condition of observed in birds and mammals.

2. Endocasts may underestimate dino intelli gence becausea. Enlarged sacral complex relieved brain of certain functions.b. Evidence of advanced social behavior – herds, maternal care,

extended family groups – reminiscent of what we see in birds.c. Small brained parrots evidence primate-level intelli gence –

see Pepperberg, I. 1999. The Alex Studies: Cognitive andCommunicative Abilities of Grey Parrots. Harvard Press.Cambridge, MA.

Brain Size vs. Body Size in reptiles, birds, mammals and dinosaurs.

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Dinosaur Encephalization Quotients. Calculated with reference to theregression, 3/2005. PE = . Crocodiles at Q = 1.00. From Carroll (1988).

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H. Circulatory Physiology.1. Crocodiles have 4-chambered

heart.2. If sauropods held their heads

high, they must have had fullyseparate pulmonary / systemiccirculation.

3. Fossil evidence equivocal.

I. Histology.

1. A mixed bag.2. Principal sources of argument –

Haversian bone and LAGs.

B. Distribution – Arctic dinos.

C. Ecology – higher herbivore / carnivore ratios in Dino / Mammal-ian faunas consistent with higher energy requirements of endo-derms.

Bakker’s Predator -Prey Ratios.