Lecture 9
The Evolution of Birds and the Origin of Flight
Jan
30, 2002
Key
concepts:
Phyletic evolution
geological time scale
continental drift/plate tectonics
fossil record
Archaeopteryx
lithographica
I.
Where did birds come from?
Phyletic
evolution -
Species limits (totally subjective) speciation/extinction (probably
100,000 species of birds)—Gradual evolution or the punctuated equilibria of S.
J. Gould and colleague?
Modern
terrestrial vertebrates share common ancestors about 270 mya
mammals branched off early from therapsid reptiles, minor players on the
stage while dinosaurs ruled
thecodonts gave rise to dinosaurs, birds, crocodiles (early forms were
arboreal!), and dinosaurs. Where
did the birds branch off? Get to
that later.
II.
Set the stage
chronologically:
Cenozoic
Era: (age of birds and mammals)
Quaternary
Period:
Recent Epoch (0.01 mya = 10,000 ybp)
Pleistocene (1.5-3.5 mya) (cooler climates)
Tertiary
Period: (climate warm from pole to pole)
Pliocene
Miocene
Oligocene
Eocene (53 - 37 mya)
adeus Gondwanaland, hello Himalayas
Paleocene (65 - 53 mya)
Mesozoic
Era:
Cretaceous -- Atlantic opens up 100 mya-Laurasia & Gondwanaland
Jurassic (180-190 mya)
Triassic (230 mya) -- Pangea
Paleozoic
Era:
Permian (270 mya)
III.
The Characters:
A. L. Wegener - german meteorologist proposed continental drift -met with
resistance because the facts were convincing, but there was no mechanism that
could explain them - Not until plate tectonics offered the mechanism was the
idea accepted
Hutchinson’s
quote “never believe a theory unless there are facts to support it and never
believe a fact unless there is a theory underlying it.” c.f. “every fact is
useful but no fact is useful unless it is used” -
Likewise, Darwin’s theory lacked the mechanism (genetics)
Gregor Mendel Born Czech Republic in 1843 entered the Augustinian
monastery at Brunn (now Brno). He began studies of peas in 11856 - three years
before Darwin published his theory, but not until 20th century were the theories
wed in what is called the modern synthesis.
IV.
Clear from modern birds and
reptiles that they share common ancestors:
Nucleated
red blood cells
6
features of skull alone, including:
single occipital condyle
ear (single) and jaw bones
(many)
articulation of lower jaw onto quadrate
sclerotic ring supports eye
laterally expanded braincase
ankle location
scales
on legs (in fact feathers are just fancy scales)
similar
yolked egg
female
birds and some reptiles are heterogametic sex (ZW)
V.
The fossil record:
Solnhofen
limestone/shales (near Munich) - used by Romans for paving roads and roofing,
with the advent of printing - fine grain made them valuable for lithographic
plates (n.b. “Lithographic” comes from stone).
Quarrymen started paying attention to slabs for lithographic quality and
found fossils. If not for
fine-grained nature, the feathers would not have made impressions and all these
fossils would have gone as reptiles. Fossil
collection was in vogue, so quarrymen collected fossils often making more for
sale of fossils than they did from quarrying activities.
Lay collectors were amassing huge collections of fossils, often as
speculation.
Hermann
von Meyer - found 1st feather in 1861, then 1st Archaeopteryx
lithographica a couple of months later.
This was a crow-sized reptile with feathers, which by our modern
definition would be a bird, but everything about the skeleton said
“reptile.”
Here’s
where the plot thickens - Dr. Haberlein put the fossil up for sale after 3
months and a mad rush was on - The German court wanted it, but a Dr. Wagner of
the Munich Museum did not believe Darwinian theory and wanted nothing to do with
the fossil. He published a paper
exposing the fossil as a fraud, without ever having seen it!
In 1862 the Brits got the first specimen
6
specimens with skeletal material are known - 1st is in the BM, second (and best)
in Berlin. 3 of other 4 were
misid’d, one even by von Meyer as pterodactyl, another as Composgnathus,
a small Coelosaurian dinosaur common in the area (1 is just a feather).
Last discovered in 1987 on display in Germany, but it has since been
removed from public display. Fortunately
John Ostrom studied it - most important for its preservation of one
of the toe claws - which is sharply curved, rather than flat - suggesting
arboreal habits, rather than cursorial.
VI.
Was Archaeopteryx
a theropod or did it evolve from a basal thecodont? - more simply, did birds
evolve from dinosaurs or their ancestors?
Answer
depends in large part on hypothesized origin of flight – ground-up or gliding
arborealist?
The
design of a flying machine
Key Concepts:
evolution as tinkering - not engineering
evolutionary redundancy
adaptive radiation
ecological niche - nature abhors a vacuum
how does any new trait evolve -
redundancy, other purpose
No master plan
gradualistic, each small step confers an advantage
Richard Dawkins said, "...An animal can never be strictly adapted to
its present environment. It is
always adapted to a sum of past environments in which its ancestors survived.
More strictly still, the sum is a weighted sum, with the weights
diminishing as we go back in time." Nat.
Hist 9/95 - e.g. infants foot curl response.
Lecture 10
1 February 2002
Chapter
2 in Gill
Modern
biodiversity (define) results from phyletic evolution, speciation, &
extinction
Feathers
from scales. Feathers = keratin in a protein matrix. Not the same as in modern
reptilian scales, so some biochemical switch long ago.
What’s
the adaptive value of proto-feathers?
Thermoregulation? Locomotion? Displays? (overhead)
VIII.
Evolution of flight:
Could Archaeopteryx fly?
Assymetrical vanes on
feathers says yes!
Lack of keeled sternum,
but strong furcula
but pigeons with cut supracoracoideus tendon can fly, just can’t take
off.
Arboreal or Cursorial?
did flight evolve from
animals that were gliding in trees?
escape from predators
prey capture
reduce injury
virtually all animals
showing precursor to flight do so from trees:
Flying squirrels
snakes
lemurs
marsupials
frogs
Ostrom
and almost all extant paleontologists argue that it was a theropod, cursorial
origin of flight. Ostrom revived an old hypothesis (of Huxley) that the
protobirds were cursorial predators.
feathers developed to help
catch prey
then aided in leaping up,
maneuvering, etc. (overhead)
what of sharply curved
halux claw? (overhead)
most argue that it implies arboreal habits
Ostrom
points to large (turkey-sized) bipedal predators as birds’ ancestors
Feduccia
argues they came from reptilian ancestors “basal thecodonts” and
similarities between Archaeopteryx and
Coelosaurs is a case of convergence.
Archaeopteryx lithographica
predates Coelosaurs by 70 MM years
Paleontologists
counter these lines just didn’t evolve much
Dinos too big for feathers to have given aerodynamic advantage
forelimbs already reduced - would require reversing evolutionary trend
rear-directed halux never found in cursorial forms
sharply decurved claw on halux is that of an arboreal form
wings of Archaeopteryx lithographica like those of modern woodland
birds
birds
became secondarily bipedal when forelimbs were shifted to flight--those that
took to the ground resemble dinos because of biophysical constraints inherent in
bipedalism
Dino
advocates are “strapped” with the cursorial origin of flight because the
potential avian ancestors were large (turkey sized or bigger) and already had
short forelimbs (not good for climbing)
basal
archosaurs (thecodonts--not yet dinosaurs) were small, often arboreal, with
longer forelimbs.
Feduccia
says that cladistics has been adopted to the exclusion of any other approach.
The bird-dinosaur link is the “crown jewel of paleontology.”
Review
cladistic method (overhead)
To
most paleontologists, birds are living dinosaurs.
Feduccia
warns of the “inherrent dangers of strict adherence to phylogenetic dogma,
where, regardless of the evidence, anatomical and functional explanations must
fit unerringly into a rigid cladistic framework.”
The
3-digit hand is an important link between the theropods and birds, BUT in birds
it’s digits 1 and 5 that are lost and in dinosaurs it’s 4 and 5! So the
bird-dino camp has to invoke a bizarre developmental argument.
Pterosaurs
were shown by cladistic analysis to be a sister group of dinosaurs and hence had
to be evolved from bipedal forms, but later analyses showed them to be
quadripedal.
Feathered
dinosaurs? Feduccia discounts all examples.