letc_2_Evolution of birds

The Evolution of Birds
& the Origin of Flight
Bird Origins –
2 hypotheses
Basal Diapsid Archosaurs
Birds ?? (~230 MYA)
Thecodont Hypothesis
Alan Feduccia
Pterosaurs – not birds
- no furcula
- wings supported by 4th digit
- patagium wing surface (bats)
But convergent
- large braincase
- ossified, keeled sternum
- hollow skeleton
- reduced fibula
- loss of teeth, tail
- fused vertebrae
Birds ?? (~180 MYA)
Theropod Hypothesis
Most everybody else
Basal Diapsid Archosaurs
Birds ?? (~230 MYA)
Thecodont Hypothesis
Alan Feduccia
Birds ?? (~180 MYA)
Theropod Hypothesis
Most everybody else
“This paper will stand as an exemplar of manipulation of information to conform
to preconceived ideas, but it is otherwise insufficiently credible or comprehensible
to constitute a lasting addition to knowledge”
Storrs Olson (quoted by Feduccia in the Auk 119:1188)
“By offering no testable alternative to the theropod origin of birds and maintaining
that the origin of birds is potentially unsolvable, Feduccia and other critics of the
theropod hypothesis of avian origins reject science itself. One-sided rejections of
the theropod origin reflect not on the hypothesis itself, but on intellectual weaknesses
of the critiques….My conclusion is that this is not science, but a rhetorical sham.
Richard Prum (Auk 120:559)
Who’s correct?
(1) Scientific concensus  theropod origin
(2) Evidence  theropod origin (All phylogenetic evidence supports the same
position for bird, a lineage derived from Dromaeosaurs
(3) A priori versus ad-hoc explanation  theropod origin
Why so controversial for years?
No clear single morphological feature that distinguishes the lineages of birds
NOT FEATHERS which are a primitive state within the lineage of Coelurosarian
theropods (velociraptors)…
Similarly, pygostyle found in oviraptors, wishbone (furcula) in theropods (T-rex)
Pneumatization of bones in theropods, feathers
Rather, it is the possession of a combination of characters that separate birds from
their closest relatives within Dinosauria…
…..possession of flight, contour and down feathers, and other skeletal features
Solnhofen quarry
Oldest known bird, but not
ancestral to modern birds
Inhabited coastal habitats where it
probably glided between conifers,
cycads, ginkos, etc. with its limited
flying ability
Paleozoic Era, Permian Period (>250 MYA)
Basal archosaurs – like this
Euparkeria flourishing
Only flying animals are dragonflies
w/ 1m WS
 Mesozoic Era, Late Triassic (220 MYA)
Small (< 10kg) basal theropods, Staurikosaurus
Primitive pro-avian body
Increased aerobic capacity
Long-fingered w/ sig. climbing skills
 Mesozoic Era, l. Triassic/e. Jurassic (200 MYA)
bird-like avepods (bird-footed dinosaurs); Syntarsus
short-armed, runners – tridactyl
nearly avian foot
pulmonary airsacs to increase aerobic capacity
feathers? (none found, but neither on any Mesozoic mammals)
 Mesozoic Era, mid Jurassic (180 MYA)
Averostra and Avetheropod (bird-snouted); Sinornithosaurus, Microraptor
simple feathers all over body – did not evolved to fly or glide
better climbing skills
elongated arms and hands with 3 fingers
All of these are
larger brains and eyes
for flight
better aerobic capacity
 Mesozoic Era, late Jurassic (150 MYA)
Avepectora (bird-shouldered) have arrived.
Birds and avian flight
have arrived
and w/ flight were changes to pectoral
girdle and increased in sternum
Yixian formation in China - 1995
e. Cretaceous ~125 MYA)
Early birds abundant and almost always
clothed in feathers
Thousands of specimens of Confuciusornis
(earliest toothless bird) uncovered and suggest
they lived in large flocks
Possibly the first evidence of
sexual dimorphism in birds
Modern Ornithurines
Cenozoic Era
Mesozoic Era
Enantiornithines: once
abundant, these “Opposite”
birds die out with the dinosaurs
at the end of the Mesozoic
Primitive Ornithurines
Birds quickly diverged into different lineages
Transitional shorebirds
Thick knee; Tinamou
Early Cenozoic (50 MYA)
“Terror Birds” such as this
Diatryma were huge, flightless
top carnivores
Were relatives of
cranes (Gruiiformes)
Titanis walleri
What about Chatterjee’s Protoavis?
…would predate Archaeopteryx by >100 MY…
What is a bird?
Dromaeosaurs – In the last 6 years no fewer than 8 non-avian theropods with
filamentous appendages homologous to avian feathers have been found in
early Cretaceous deposits in China and Mongolia
Sinornithosaurus and Microraptors have pennaceous (interlocking) feathers
with vanes and barbs indistinguishable from birds
“Could anyone imagine better evidence that birds evolved from a lineage of
theropod dinosaurs than the presence of modern pennaceous feathers in a
theropod dinosaur lineage that was independently hypothesized to be most
closely related to birds?”
Prum 2003
Sinornithosaurus –
A feathered dinosaur
or early bird??
The Origin of Flight
(1) Cursorial (ground-up) Theory
(2) Arboreal (tree-down) Theory
(3) Combination – e.g., protofliers ran into strong winds or down steep slopes,
using gravity as the initial power source
The Origin of Flight
(1) Cursorial (ground-up) Theory
Primitive feathered wings/airfoils arose for:
- for capturing food, including Ostrum’s insect net hypothesis
protofliers were aerial insectivores that ran, hopped, or leaped after prey
- for display; increasing height and duration of sexual displays
- for safety
Food: But, leaping after flying insect seems inefficient, always fighting gravity, no
contemporary examples (bats and pterosaurs arose from non-leapers), there are no
running gliders or leaping aerial insect specialists
Display: ??
Safety: Wing-assisted running in Chukar chicks
50° incline after hatching, 60° incline at 4 days, vertical ascent at 20 days
(2) Arboreal (tree-down) Theory
Primitive feathered wings arose for controlled leaps/gliding, faster descent, and
increased travel between high places
- closest pre-birds small and adapted to climbing (e.g., reverse hallux for perching)
- drag induced airfoils distally placed
- wings enlarged to transform falls into horizontal glides
The major obstacle is getting
through the transition from gliding
to flapping flight
e.g., flying squirrel
Species like the Kakapo does use small, slow wing beats to extend glides
How about Archaeopteryx:
Glider or flyer?
 Power-Flyer – perhaps better than
the Kakapo, by still crude by
modern standards
- Wing span and wing loading within
the general avian range
- Feathers possess aerodynamic properties
- Furcula, ossified keeled sternum
- No quill nodes to anchor feathers
- Outer arm not rigid
- Lacked full flapping extension
- Lacked rigid ribcage
- Partially develop supracoracoides
Bird origins: Theropod origin, specifically a lineage derived from Dromaeosaurs
When: ~180 MYA
Evolution of feathers: First present in Dromaeosaurs (180 MYA) as simple filaments
all over the body; likely had an initial thermoregulatory function
Origin of flight: Cursorial (or Ground-up Hypothesis) versus Arboreal (or
Tree-down Hypothesis)
Primitive feathered wings likely evolved first for controlled leaping, distal airfoils
expanded leading to longer distance glides, flapping airfoils increased the length
of glides and as the pectoral girdle developed eventually powered, flapping flight
However, alternative hypotheses are possible: airfoils evolved for more impressive
and sustained intersexual displays or wing-assisted running to escape predators.
In other words – flight evolved out of selection pressures to efficiently and effectively:
travel (arb), capture prey (arb), attract mates (cur), or avoid predators (cur)