BS 120
Field Natural History
Spring
1
LECTURE 19
Reptiles and Birds – Kissing Cousins?
I. Careers in home heating and cooling – endotherms and ectotherms.
A. How animals heat their bodies.
1. Endotherms - animals that use metabolic heat to raise their body temperature. (“Warmblooded”)
2. Ectotherms - animals that obtain energy from their surroundings. (“Cold-blooded”)
a. Warmth from sunlight.
b. The ground in their burrows.
c. The water in which they live.
3. Homeothermic animals that have a constant body temperature.
4. Poikilothermic animals have a body temperature that fluctuates with that of the surrounding
environment.
5. Only birds and mammals are considered to be endothermic.
a. Endothermy is relatively rare because it is expensive metabolically.
b. The heat that they generate comes from the food that they eat.
6. Because endothermy is expensive, animals try to conserve the heat that they generate.
a. Heat is conserved by insulating layers- fat or fur.
7. However, endotherms cannot completely turn of the furnace.
a. Heat is lost by dilating the blood vessels near the surface of the skin so that it radiates.
b. Cooling of the body is also facilitated by evaporative cooling –sweat.
c. Panting is another way that some animals cool themselves.
8. In general, endothermic animals are homeothermic.
a. However, some ectothermic animals maybe homeothermic. A deep sea fish may be an
ectothermic homeotherm.
b. Its heat comes from the surrounding sea water, but because the temperature of the sea
water in its environment is constant, so is the body temperature of the fish.
9. All living reptiles are ectothermic and most of them are also poikilothermic.
a. They regulate their temperature by basking in the sun, or seek shade to avoid overheating.
II. The family tree – extinct dinosaur groups.
A. Four major extinct groups: Pelycosaurs, Therapsids, Thecodonts and Dinosaurs.
1. Pelycosaurs – early dinosaurs that were more successful than amphibians because of their
watertight eggs. Probably ectothermic
a. More powerful jaws than amphibians, with long, sharp teeth allowed them to be the first
land vertebrates to attack prey their own size.
2. Therapsids – were endothermic, and ate 10 times more than their pelycosaur predecessors.
a. As a result, they were more active, particularly under colder conditions or during the
winter. Before they went extinct, they gave rise to their descendants, the mammals.
3. Thecodonts – 230 million years ago, the world’s climate began to warm.
a. Endothermy was no longer advantageous in a warmer climate, therefore, thecodonts were
ectotherms.
b. They were the first land vertebrates to be bipedal – they stood and walked on two feet.
Major consequence -. They could run faster.
BS 120
Field Natural History
Spring
2
4. Dinosaurs -We used to think they were sluggish - we now believe that some, at least the
predators, could move fast.
a. Not necessarily stupid either. Evidence of sociality (herd prints with juveniles inside), and
nesting behavior.
5. Some scientists contend that dinosaurs did not go extinct, but that one group evolved into
Class Aves, the birds.
a. Why? Because; 1) They lay an amniotic egg (which is hard, rather than leathery) and 2)
reptilian scales are present on the feet and lower legs.
6. However, birds are classified separately because of the evolutionary novelties of birds:
feathers, hollow bones and more efficient lungs for flight.
III. Birds.
1. The success of flight in birds also relies on a unique morphological development – the feather.
a. Considered by some to be adapted from reptile scales.
b. Feathers also serve another function useful to an endotherm; they serve as insulators.
2. Feathers -Each feather is comprised of a shaft with lateral branches called barbs.
a. Projecting off of the barbs are barbules, which have microscopic hooks. These hooked
barbules cross-link, forming a light, but sturdy surface between the barbs.
b. Feathers are like scales. If lost, they can be replaced.
c. Feathers are considered unique. No other animal on earth, besides birds, has them.
3. Skeletal structure – the bones of birds are thin and hollow.
a. Many bones are fused together to make the skeleton even more rigid so larger, stronger
muscles may be attached. E.g. breast bone.
b. The flight muscles also connect to the fused collarbone, called the “wishbone”. No other
living vertebrates have a fused collarbone or a keeled breastbone.
IV.
Look! It’s a bird, no it’s a reptile, no it’s…..we’re not sure yet.
A. Origin of birds.
1. The origin of birds is actually greatly debated. Part of the problem is the preservation of
fossilized remains.
2. Archaeopteryx – a fossilized bird is discovered. The skeletal features of this fossil have many
similarities to that of a theropod dinosaur (such as velocoraptors).
a. Archaeopteryx is about the size of a crow.
b. Its skull has teeth, and very few of its bones are fused.
c. Its bones are solid, not hollow like a birds.
d. It also had a long reptilian tail.
e. It had no large breast bone for the attachment of strong flight muscles.
f. Finally, it had the forelimbs of a dinosaur.
g. These are dinosaurian features – not those of a bird.
h. Originally classified as another small dinosaur of similar size – a coelurosaur called
Compsognathus -until feathers were discovered on the fossils.
3. What makes Archaeopteryx distinctly avian is that it has feathers on its wings and tail.
a. It also has some other bird like features, most notably a wishbone. Dinosaurs did not have
wishbones. However, the predecessors of dinosaurs, the thecodonts, did have wishbones.
BS 120
Field Natural History
Spring
3
4. Biologists still classify birds separately from dinosaurs because of their unique features:
feathers, hollow bones and such physiological mechanisms as super efficient lungs that could
permit sustained, powered flight.
B. How and Why did wings evolve?
1. Before we can answer how and why flight evolved, we must understand how and why
wings evolved; without wings there can be no flight. How did wings evolve? Scientists
generally agree that wings must have been exaptations; they were used by the ancestor for
one function, and became useful for flight among the descendants (if they weren't
exaptations, then they were adaptations, which would mean that they were wings already
used for powered flight; a circular argument).
Various hypotheses proposed include:
(a) Wings evolved from arms used to capture small prey in; the large wing area acted as a
net.
(b) Wings evolved because bipedal animals were leaping into the air; large wings assisted
leaping.
(c) Wings were used as sexual display structures; bigger wings were preferred by
potential mates.
(d) Wings evolved from gliding ancestors who began to flap their gliding structures in
order to produce thrust.
2. Some scientists see that hypotheses #2 and #4 are the best possibilities. Each one works
from a specific flight origin; a) ground up or b) tree down.
C. The evolution of flight - two main hypotheses.
1. Ground Up Scenario: Given a bipedal cursorial (running) ancestor of a flying lineage, flight
must have proceeded from the ground into the air, assuming that the ancestor did not normally
live in trees, or if there were no trees around.
a. Predaceous dinosaurs would also be quick runners, pursuing prey with outstretched, winged
forelimbs.
b. The use of the wings as a cage-like trap can be seen in some modern predatory birds.
c. The wings in this position may have acted like airfoils, from which true flapping flight
could have evolved.
d. A point of note: A quadrupedal ancestor would have problems evolving flight from the
ground up; it would have to be able to be bipedal in order to use its wings at all!
2. Trees Down Scenario: Given an arboreal ancestor of a flying lineage, flight must have
proceeded from the trees into the air. A semi-bipedal leaping and gliding animal could have
evolved flight -- leaping off of trees would provide the acceleration
and speed necessary for flight.
a. Ancestors of the birds were tree-dwelling reptiles and that flight evolved from gliding.
b. The claws on the end of Archaeopteryx’s forewings would have enabled it to climb trees.
BS 120
Field Natural History
Spring
3. Why did flight evolve? -- Several Main Hypotheses:
a) To help escaping predators
b) To help catching flying or speedy prey
c) To help moving from place to place (leaping or gliding)
d) To free the hindlegs for use as weapons.
e) To gain access to new food sources or an unoccupied niche
D. Endothermy in birds and dinosaurs.
1. Birds are endothermic. They maintain their body temperatures significantly higher than most
mammals, 104-107 ° F compared to 98.6 for humans.
2. Only animals that are endothermic require insulation to retain the body heat that they
generate.
3. Some biologists and paleontologists contend that some dinosaurs must also have been
endothermic to be fast-running predators.
a. That Archaeopteryx had feathers is evidence to some that perhaps some of the dinosaurs
were endothermic. In fact, some would argue that feathers had greater adaptive value for
insulation than flight, which may have been a secondary adaptation.
4. It has been suggested that endothermy arose twice – once in dinosaurs and later in mammals.
4