THE AVIALAE
A. What is a bird? Shared, derived characteristics of birds
B. Archaeopteryx: significance, history, as bird, as dinosaur
C. Mesozoic birds
D. Feathered dinosaurs
E. Origin of birds
Hypothesis1: birds are not dinosaur descendents - evidence
Hypothesis 2: birds ARE dinosaur descendents - evidence
A. What is a bird?
Living birds are diverse (~9,000 species) and they possess some derived
characters that are analogous to significant derived characteristics in living
mammals (i.e., they evolved independently in birds and mammals). These
include upright posture, high metabolic rate/warm bloodedness and large
brain size.
All birds through time (beginning with the Late Jurassic Archaeopteryx)
belong to the clade Avialae; modern (Cenozoic) birds belong to the clade
Aves, also called Neornithes in some of the images from the lecture.
Shared derived traits that distinguish Aves from other living
vertebrates
Feathers used to be considered the essential defining character of birds, but
feathered dinosaur fossils found in China in the last decade show that that is
not the case; birds are now defined by the suite of physical, physiological
and skeletal specializations that apparently evolved to enhance their ability
to fly. They include (but are not limited to):
Feathers: planar feathers with asymmetrically-positioned shafts used for
flight (aka the ‘primaries’ on living birds). Feathers come in a variety of
forms, including down, contour feathers, semi-plumes, symmetrical
rectrices (tail feathers) and remiges (arm feathers). Feather have the
following components: quill, shaft, barbs and barbules, but not all
feathers contain all of these elements. Feather architecture varies with
function.
Endotherm (warm-bloodedness): necessary for prolonged flight
Four-chambered heart: necessary for endothermy (as in mammals)
Expanded skulls with huge orbits: large brains and eyes that can
sense ultraviolet light. Generally much better vision and color perception
than human eyes. Heads supported by long, mobile necks with very
complex muscle control.
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Large brains: facilitate cogitation and sensory perception, including
vision (see above), exceptional hearing abilities (especially for longwavelength sound), and atmospheric pressure detection.
Toothlessness: modern birds have no teeth
Carpometacarpus: reduced, fused wrist and hand, supports wingtips
and to reduces flexion
Alula: feather on tip of hand that breaks turbulence over wing.
Shoulder joint: birds have a unique ‘pulley’ arrangement in their
shoulders in which a tendon attached to the supracoracoideus muscle
passes through a hole formed by the intersection of the coracoid, furcula
and scapula to attach to the upper surface of the humerus. Used for the
upstroke during flight.
Enhanced skeletal rigidity: essential for flight control and
maneuverability; it is apparent in the synsacrum, tarsometatarsus, and
pygostyle (reduced tail).
Furcula (wishbone): Flexible wishbone stores energy from the
downstroke to assist with the upstroke.
Keeled sternum: attachment point for the large flight muscles
(pectoralis accomplishes the downstroke; the supracoracoideus the
upstroke).
Three-and-one feet: three toes in front, one turned to the rear (as in
most living birds) allows for perching, landing, hunting, hanging, etc.
Pneumatic skeleton: air sacs in bones take the place of marrow and
facilitate the unique, bellows-like avian respiratory system.
Bladderless urinary tract: nothing is retained that is unnecessary and
might inhibit flight
A key point of bird evolution. The traits above did not evolve all at
once. They accumulated over millions of years and in many taxa leading
from theropod
B. Archaeopteryx lithographica
Significance: the oldest, uncontested bird discovered to date and a real
‘missing link’, with a combination of bird-like and dinosaur-like features.
Perhaps the most famous single fossil known.
History: First Archaeopteryx was a single feather, found in 1860.
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The most famous Archaeopteryx is an almost complete skeleton that is
fully feathered. Thomas Huxley studied Archaeopteryx and used them to
argue the case for evolution; Darwin, oddly ignored Archaeopteryx in his
multiple new editions of The Origin of Species that post-dated discovery
of Archaeopteryx.
Only 7 skeletons/partial Archaeopteryx skeletons have been found, and
all come from a fine-grained limestone of Late Jurassic age, exposed near
the town of Solnhofen in Bavaria, Germany. Archaeopteryx was about the
size of a modern magpie or raven. This limestone was deposited in a
quiet coastal lagoon and contains well preserved remains of many
animals (crabs, pterosaurs, shrimp, etc.).
Archaeopteryx possesses many bird-like characters, including:
expanded braincase, asymmetric feathers, keeled sternum, pneumatic
bones?, three-and-one foot with sharp claws (suggesting that it flew
primarily and was not a ground-runner). Not clear if big-toe was in
opposition to hand.
Archaeopteryx possesses many dinosaur-like characters,
including:
teeth, three fingered hands with claws, long tail stiffened with
zygopophyses, a raptor-style saurischian pelvis, ascending process on
astragalus. To be clear, a furcula that is not flexible is actually a
theropod dinosaur trait.
C. Other Mesozoic birds
The fossil record for all birds is quite poor because their bones are
delicate. We have a record thanks to exceptional preservation a some
select localities.
The early evolution of birds shows a gradual accumulation of flightspecialization characters in some lineages during the Cretaceous.
Rahonavis: the most unspecialized Mesozoic bird excepting
Archaeopteryx, yet it is of Late Cretaceous age! This bird had a long tail
and an enlarged 2nd toe claw (like a dromaeosaur). It may also have a
pneumatic skeleton.
Post-Archaeopteryx bird evolution Stage 1: Pygostylia: acquisition
of a pygostyle (Confuciusornis)
Post-Archaeopteryx bird evolution Stage 2: Ornithothoraces
carpometacarpus, flexible furcula, backstroke pulley in shoulder, reduced
number of trunk vertebrae
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Two Key Groups:
Enantiornithes: dominant birds of Mesozoic (e.g., Iberomesornis), a few
shared derived characters we won’t worry about
Post-Archaeopteryx bird evolution Stage 3: Ornithurines: even
shorter trunk, full caudal rotation of pubis, synsacrum, tarsometatarsus
(e.g., Late Cretaceous Hesperornis (diver) and Ichthyornis (gull-like)).
Alvarezesauridae: Odd flightless birds of the Late Cretaceous, including
Shuuvia and Mononychus. We have no idea what to make of them.
Post-Archaeopteryx bird evolution Stage 4: Aves (Neornithes):
loss of teeth
D. Feathered dinosaurs
First discovered in the Liaoning province of China in the mid-1990’s, in
lake bed sedimentary rocks approximately 124 million years old (Early
Cretaceous)
Include:
Sinosauropteryx (1997) - a little bipedal theropod with down along its
back and many sharp teeth; one specimen was found with mammal
remains in its ribcage
Sinornithosaurus – a deinonychosaur with vaned feathers
Caudipteryx – a little sharp-toothed oviraptosaurian with symmetrically
vaned feathers on its arms and tail, short arms, and GASTROLITHS
Beipiasaurus – a large (ostrich-sized) therizinosaurus with primitive
feathers
Protarchaeopteryx – a small theropod with feathers covering most of its
body, including its arms and tail and fang-like teeth
Microraptor (2003) – small deinonychosaur with asymmetrically vaned
feathers on its arms and legs
How do we know the feathered dinosaurs (except maybe
Microraptor) didn’t fly?
On most, feathers are symmetrically-vaned
Relatively short arms (not long like birds)
Relatively few feathers on arms and tail
So, what are feathers for if not flight?
Insulation
Display/species recognition
Insect-catching
Increased speed/lift
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E. Origin of birds
Objections to birds as derived theropod dinosaurs:
1. Dinosaurs with feathers (Caudipteryx, Protarchaeopteryx) are really
flightless birds that evolved dinosaurian characteristics
2. The first true feathers really appear in the Late Triassic, on the nondinosaurian archosauromorph Longisquama.
3. The first true bird (Protoavis) appeared in the Late Triassic.
4. Archaoepteryx (a bird) appears in the Late Jurassic, whereas
dromaeosaurids (the supposed closest theropod relatives of birds)
don’t appear in the fossil record until the Cretaceous.
Current consensus: point 1 is wrong; Longisquama doesn’t have true
feathers (point 2), Protoavis is too poorly known to be compelling
evidence of anything (point 3), point 4 is true and a little troubling, but
certainly doesn’t prove false a dinosaur ancestry for birds. After all, the
oldest monotreme is also Cretaceous, but we believe its last shared
ancestor with other mammals was early Jurassic.
Many shared derived characters of birds resemble those of
dinosaurs:
1. Perforate acetabulum: SDC of Dinosauria
2. Opisthopubic pelvis: SDC of (some Dromaeosauridae; moreover,
embryonic birds have a normal saurischian pelvis that modifies
during development into a synsacrum
3. Thin-walled, hollow bones: SDC of Theropoda
4. Ornithoid-style eggshell: SDC of Theropoda
lizards, turtles, crocs, sauropods and ornithischians all have a
distinctive eggshell microstructure that is different from the type
found in theropod and bird eggshells
5. Expanded braincases and kinetic skulls: SDC of Theropoda
6. Furcula/Wishbone: SDC of many Theropoda
Many non-avian theropods have been discovered with furcula,
including Gorgosaurus, Albertosaurus, Tyrannosaurus, Velociraptor,
Oviraptor, Segisaurus, Allosaurus, Protarchaeopteryx, Coelophysis,
etc.
7. Three-fingered hand: SDC of Theropoda: the chicks of the living
Hoatzin have three-fingered hands that develop into a
carpometacarpus as the chicks mature
8. Rigid tails: SDC of Tetanurae
9. Semilunate carpal, ascending process on astragalus: SDC of
Coelurosauria
Semilunate carpals in birds allows for unique flexing of the wrist
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10. Feathers: appears in Coelurosauria
A wide variety of feather morphologies is found in coelurosaurs, but
at present the proposed evolution of feathers is not reflected in the
fossil record
11. Enlarged second toe claw (fossil Rahonavis): SDC of
Dromaeosauridae
12. Hand-arm structure: SDC of Dromaeosauridae
Archaeopteryx had arms as long as its legs; this may be the
culmination of a trend in the more derived coelurosaur theropods
In terms of evolutionary steps, it is much simpler to derive birds from
dinosaurs than from any other group of organisms. Thus, most
paleontologists consider Aves to be part of the Dinosauria and the tiny
hummingbird Mellisuga hellenae to be the smallest adult dinosaur.
F. Origin of Flight
Bottom up: running and flapping, perhaps catching insects with wings.
This later conjecture seems pretty random. Wrong motion.
Climbing trees and gliding down: hard to get from there to flapping.
Recent work suggests that flapping helps birds run up inclines. A plausible
route to flapping flight.
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