Chapter 16 – Dinosaurs, Birds and Extinctions
Why Study Birds and Extinctions
New dinosaur definition
An animal that is a member of the group
descended from the most recent common ancestor of Triceratops and birds
So Avian dinosaurs (birds) did not go extinct
Mass Extinctions
The K-T boundary is one of six mass extinctions
The other
5 occur at the end of the Ordovician, near
the end of the Devonian, at
the end of the Permian, at
the end of the Triassic and now
Opened up
ecological niches to birds and mammals, which diversified during the Cenozoic
Era
A mass extinction is defined an extinction event during which many species (and higher taxa, like genera, families and orders) go extinct in a geologically short period of time
Some
forms of planktonic algae, some
forms of land plants, some
forms of planktonic protozoans, ammonoids, pterosaurs, mosasaurs, ichthyosaurs and plesiosaurs, as well as nonavian dinosaurs went
extinct at the K-T boundary, among
others
Birds are the most diverse terrestrial
chordates with 10,000 species
Current
mass extinction most noticeable among birds
Birds
Definition of Bird (Clade Aves), Archaeopteryx, and Bird Lineages
See Figure 16.1 for relationship of Aves to
various dinosaur clades
Some Bird Characteristics (See Figure 16.2)
Tail with reduced number of vertebrae and
fusion of some into a Pygostyle
Forearm nearly as long as the humerus
Forelimb longer than hindlimb
Carpometacarpus - Fusion of wrist and hand
bones and digits I, II and III
Foot
Three
toes in front & one toe (hallux) in back
Digits II, III and IV in front & digit I in
back
Tarsometatarsus - Fusion of ankle and some foot
bones
*Fibula is small and does not articulate with
the ankle
Absence of Teeth and possession of a beak
Synsacrum - Fusion of sacral vertebrae &
hip bones
Flight and down (for insulation) feathers
Pneumatic Bones with thin walls supported by
splintlike buttresses internally
Pneumatic
foramina - openings in
the wall of the bone for air sacks to enter
Useful
for flight, but evolved
prior to flight (exaptation!)
Well-developed sternum
with a broad, deep keel (carina)
Sternum
connected to fused backbone by segmented ribs
Pillar like coracoid bones buttress against the
front of the sternum, the shoulder blades & wishbone (FURCULA = fused clavicles)
in the shoulder
Different muscles for downward stroke & for
recovery stroke
Large Brains & Advanced Sight
Developed
through possession of endothermy and for flight
Archaeopteryx lithographica
Importance of Archaeopteryx
Archaeopteryx is the oldest known bird, but not the common
ancestor of birds
History of Discovery
Found only in Late Jurassic Solnhofern
Limestone from Bavaria (SE Germany)
Very
fine-grained carbonate mud that can be etched & used for printing plates
First specimen was an isolated feather
impression found in 1860
Second specimen found in 1861 consisted of
feather impressions & an associated skeleton
Two years
after Darwin published “On
the Origin of Species”
Sold to
Sir Richard Owen & for the British Museum of Natural History
Third specimen found in 1877 also consisted of
feather impressions & an associated skeleton (see Figure 16.3)
Spectacular
specimen that was retained in Germany
(in Berlin)
An additional five specimens have been found
since
John
Ostrom recognized one specimen that had been mislabeled as a pterosaur
Another
was recognized in a similar manner – it had been mislabeled as Compsognathus
Anatomy of Archaeopteryx – a mix of bird and theropod
Tail is long, straight and stiffened by
zygopophyses
Modern
birds have a pygostyle
Tridactyl manus with fully movable, separate
fingers, each tipped with a recurved claw
Modern
birds have a carpometacarpus
Well-developed flight feathers (See Figure
16.4)
Barbs
form a planar structure called a vane in flight feathers
Downy
feather barbs radiate in seemingly random directions, forming ill defined or no vanes
Feathers
probably first developed for insulation and then for display
Teeth that are conical and recurved, but not
serrated
Modern
birds are toothless
V-shaped furcula
Poorly
developed sternum
Semilunate carpal in the wrist
Ankle with unfused metatarsals
Astragalus has an ascending process
Tridactyl pes
Three
toes symmetrical about the middle digit in front & one toe behind
Saurischian pelvis
Pubis
points down, not back, and has a footplate
Six
unfused sacral vertebrae and a synsacrum is absent
Gastralia, or belly ribs, are present
*Femur is considerably shorter than tibia &
fibula and has a gentle S-shape
*Long Bones are hollow and pneumatic
Theropod origin of Birds
Relationships
Coelurosaurian dinosaurs and Birds share more
than 100 characteristics
Deinonychosauria and Aves are sister groups
When birds evolved
At least by the Late Jurassic
Absence of good maniraptoran fossils in the
Middle and Late Jurassic
Arboreal (trees down) vs. Cursorial
(running) Hypotheses
Arboreal - first birds glided down from trees
& developed powered flight later
Most arboreal gliders are quadrupedal with flaps of skin between
hind- and forelimbs
Cursorial - first birds were runners & developed powered flight to avoid obstacles
Birds are
bipedal & ultimately evolved from a cursorial ancestor
Favored
by paleontologist currently
T. H. Huxley
Recognized relationship between dinosaurs &
birds
Fell into disfavor during the early 20th
Century
Similarities
attributed to convergent evolution
Thecodont origin of Birds
Thecodonts
Paraphyletic group of archosaurs including both
ornithodirans & crurotarsans
Gerhard Heilmann
Rejected theropod origin because of lack of
furcula in theropods; Some primitive Triassic archosaurs had them
A structure lost is not REACQUIRED in an evolutionary lineage (Dollo’s rule)
Longisquama insignis
Late Triassic archosaur with featherlike
structures along its spine
Still under debate
Doesn’t really disprove theropod origin
of birds
The Early Evolution of Aves
Jurassic
Only Archaeopteryx
Early Cretaceous
Sinornis from China - only 15 million years younger than Archaeopteryx
More
modern than Archaeopteryx
pygostyle, Shortened body (reduced number of vertebrae), Moderately large sternum, Pillar like coracoids, Wrist allows wing to be folded tightly against body, Feet for perching
Retains
some primitive features
Teeth, Unfused fingers (no carpometacarpus),
Gastralia, Unfused pelvis
Several other forms from the Early Cretaceous,
including tracks of a shorebird in spain
Late Cretaceous
Flying & marine diving birds, as well as
some enigmatic forms
Hesperornis from western North America
Still
retains teeth
Specialized
features for diving
Mononykus from Gobi
Looks
like a typical theropod from the center of the back toward the tail
Digits
fused into short stout carpometacarpus,
with very short arms
Cenozoic
Only Neornithines survived the mass extinction
Toothed
birds of the Mesozoic – enantiornithines – went extinct
Carinates are flying birds; ratites are
flightless birds
Largest
birds are ratites – flightless
birds; carinates – flighted birds, include passerines, the largest bird group
Diatryma was a large, carnivorous
bird of the Eocene (see Figure 16.5)
Dinosaur and Bird Physiology: Different or the Same?
Thermoregulation and Food
Endothermy = “warm-bloodedness”
(Not restricted to birds & mammals)
Endotherms
regulate their temperatures internally
Ectothermy = “cold-bloodedness”
Ectotherms
regulate their temperatures using external sources of heat
Homeothermy = maintainance of a constant
internal temperature
Poikilothermy = internal temperature fluctuates
Endothermy & ectothermy are two
biochemically different methods of obtaining heat, while homeothermy &
poikilothermy are endpoints on a spectrum where many animals cluster at the
endpoints, but many do not
Humans
are endothermic homeotherms
Lizards
are ectothermic poikilotherms
There are
endothermic poikilotherms, however, like bats and hummingbirds, as well as hibernating species, like bears
Were dinosaurs endothermic or ectothermic?
YES
Predator:Prey Ratios (See Figure 16.6)
Endotherms are capable of higer levels of
activity sustained over longer periods of time than are ectotherms, but require
larger (10-30 times) amounts of energy
An acre
of land is capable of supporting fewer endotherms than ectotherms
Endothermic predators must eat more prey than
ectothermic predators
Less than
1% of predator-prey biomass as endothermic predators vs about 25% of
predator-prey biomass as ectothermic predators in modern ecosystems
Dinosaur
body fossils - 3 to 5 %; Dinosaur tracks - about 15%; Taphonomic biases
Related only to whether PREDATORS were
endothermic or ectothermic; says nothing about whether the PREY were
endothermic or ectothermic
Additional Means of Evaluating Dinosaur
Thermoregulation
Neurophysiology
Brain size as reflected by EQ may reflect the total level of activity of an animal and therefore its metabolism
Small
theropods had EQs equivalent to living mammals & birds
Large
theropods & ornithopods had EQs less than living mammals & birds, but
significantly higher than living reptiles
All other
dinosaurs had EQs equivalent to living reptiles
LAGs & Growth Rates
Densely vascularized Haversian bone forms from
fast growth rates & suggests that many dinosaurs grew at rates comparable
to living birds & mammals
LAGs (lines of arrested growth) form in modern
ectotherms when bone stops growing due to seasonal temperature fluctuations;
they are poorly-developed in living endotherms
Many dinosaurs also have LAGs, suggesting that
they had a metabolism intermediate between living ectotherms & endotherms
Some
Mesozoic birds also exhibit LAGs
Bone Geochemistry
Oxygen isotopes vary with temperature
Ectotherms should have large variations in oxygen isotopes, while endotherms should have smaller variations
Dinosaurs generally have relatively small
variations
Social Behavior
Brooding, parental care and herding are
characteristic of endotherms
Noses
Respiratory turbinates extract moisture from
outgoing breaths of living endotherms
Dinosaurs do not seem to have repiratory
turbinates
Posture
Dinosaurs maintained a fully erect stance, which among living vertebrates occurs only in mammals & birds, both of which are endothermic
Also
long-leggedness is characteristic of living endotherms
Also the
only living tetrapods that are bipeds are endotherms
A fully erect stance allows lungs to fill to capacity because the torso isn’t flexed
Body Size
Large dinosaurs would have been homeothermic, even if ectotherms
Problem
is actually in dissipating heat
Also the
only living tetrapods that are bipeds are endotherms
Blood pressures are consistent with endothermy
Soft parts
Feathers for insolation suggest endothermy
Suggestion of heart like endotherms in Thescelosaurus
Paleobiogeography
High latitudes
Living
ectotherms do not live at high latitudes
Dinosaurs
did live at high latitudes in the Late Cretaceous
Migration
Dinosaurs
lived at high latitudes in Late Cretaceous & probably migrated seasonally
Leatherback turtles, large ectotherms, also
migrate large distances
Phylogeny
Crocodiles are ectothermic; Birds are
endothermic
Bottom Line
Dinosaurs had their own unique style –
they were around for 165 million years and were diverse
Dinosaur Extinctions
Definitions and Causes of Mass Extinctions
Definitions
Local extinction – a population of a
species dies out in one part of the world but is still present elsewhere
Global extinction – a species is gone
everywhere
Most
species that have ever lived are extinct
Average
fossil species lasted 1 to 2 million years
Living fossil – according to your text a
species that is thought to have been extinct for millions of years and is
discovered no to be (a more general definition is any species that has remained
morphologically unchanged for many millions of years)
Coelacanths
have a 100 million year gap in its fossil record
Opossums
are also living fossils because they are similar to Mesozoic mammal and there
is no gap in their fossil records
Lazarus species – a fossil species that
was thought to be extinct that is found later in younger strata
Causes of mass extinctions
Major mass extinctions
The mass
extinction at the end of the Paleozoic (Permian) is the most severe – 95%
of all species appear to have gone extinct
The mass
extinction at the end of the Mesozoic (Cretaceous) is the next most severe
Paleontologists
recognized the tremendous species turnover represented by these mass
extinctions - established
these eras on the basis of the change in fossil assemblages
Proposed Causes of Mass Extinctions
Climate
change
Changing
positions of the continents
Impact of
a bolide, an
extraterrestrial body
Proposed Causes of the Terminal Permian Mass
Extinction
Formation
of Pangea resulted in interior deserts,
which have low biodiversity
Lowered
sea level restricted the amount of shallow marine continental shelf
environments, which have
high biodiversity
Pangea
straddled the equator and blocked ocean circulation patterns
The
voluminous Siberian flood basalts were erupted and contributed a great deal of
CO2 to the atmosphere, triggering
global warming
Bolide
impact had been proposed, and
although peculiar carbon compounds thought to have resulted from impact have
been found, an impact
crater hasn’t been
Terminal Triassic Mass Extinctions Led to
Dinosaur Success
Two
stages of extinction near the end of the Triassic separated by ~ 10 million
years
Climatic
and bolide impact appear to be the cause
Dinosaur Fossil Record and the K-T Boundary
Neocatastrophism
Revived view that the geologic and fossil
record records global catastrophes
Contrasts with original early 19th
Century Catastrophism of Cuvier in not being tied in with religious beliefs
Just over 20 years old; based on a
peer-reviewed Science paper
by Luis & Walter Alvarez & colleagues in which they proposed the earth
was hit by a bolide at the K-T boundary that caused the terminal Cretaceous
mass extinction
The Asteroid Impact Itself is Well
Documented
Luis and Walter Alvarez and colleagues proposed
that a 10-km wide bolide hit the earth 65 million years ago based on an
anomously high concentration of iridium in a 2.5-cm thick clay layer at the K-T
boundary in Gubbio, Italy
Meteorites
contain 10,000 times more Ir then crustal rocks (3,000 ppb vs. 0.3 ppb)
The
Gubbio clay layer had an Ir concentration of 9 ppb, 30 times greater than average crustal
concentrations
Well over
100 sites worldwide on land & in the ocean have been found to have Ir
anomalies, with the
relatively highest concentrations in North American sites
The 10-km
diameter of the bolide was based on how much Ir was distributed around the
world
The Ir
anomaly is a great stratigraphic marker
Bolide was traveling 20 to 25 km per second
Impact
released LOTS of energy
Effects of impact
Produced
the Chicxulub crater that is about 180 km in diameter, now buried in the subsurface of the
Yucatan Peninsula and immediate offshore area
Vaporized
rocks and organisms in the direct impact area
Jettisoned
microtektites, small
glass spheres of material melted during impact, found in North American, Gulf, Caribbean
and western Atlantic K-T boundary sites
Produced
shocked quartz, a form of
quartz produced by extreme pressures, at
K-T boundary sites in North America, the
Gulf, the Caribbean and
western Atlantic
Triggered
major earthquakes out to a distance of several hundred kilometers from the impact
site, causing collapse of
the continental margins around Campeche Bank – the resulting sedimentary deposits have been
drilled by the DSDP
Ignited
vast forest fires in surrounding forested ecosystems from the heat wave
associated with the energy release, producing
soot-rich horizons found at several K-T boundary sites
Knocked
down trees hundred to thousands of km away from the impact site
Triggered
tsunamis that devastated shelf and coastal areas in the Gulf and Caribbean, producing tsunamites in these areas
Carbonate
and sulfate-rich rocks underlying the Yucatan were combusted and combined with
water to produce long periods of acid rain
How the Impact May Have Caused a Mass
Extinction
Blockage of sunlight for ~3 months from
material blasted into the atmosphere
Short-term (~1 month) global warming from the
heat energy generated by impact
Long-term (several years) global cooling from
material blasted into the atmosphere
Long-term (several years) global warming from
shutdown of organic productivity & release of carbon dioxide into the
atmosphere
Acid rain by reaction of nitrous oxide produced by the impact with water to form nitric acid & production of sulfuric acid from the evaporites at Chicxulub
That the Impact Caused the Mass Extinction
is Still Under Debate
Impact certainly had an effect
Other causes acting in concert with the impact
may have caused the MASS extinction
Rapid
seafloor spreading & subduction in the Pacific Ocean
Extensive
flood basalts in India - Deccan
Traps
Continental
configuration was similar to today, although
Asia & North America were connected,
South America, Antarctica
& Australia were connected and Africa & India were isolated continents
Seafloor
spreading rates actually decreased from a maximum about 85 million years ago to
the end of the Cretaceous, causing
a sea level drop, with
progressive shrinkage of inland seaways & regression, exposing more
of the continents
The
climate was generally cooling, although still warm
Biological Trends Near the K-T Boundary
Ambiguous
Some workers see gradual decreases in species
prior to the K-T bounadry; others see no decrease and an abrupt extinction
Hell Creek Formation – straddles the K-T boundary – has been interpreted to show either gradual or sudden extinction of dinosaurs
Some
ambiguity - impact-generated
acid rain dissolved dinosaur bones immediately below the boundary
Differential
survivorship - ~90% of
aquatic species but only ~10% of terrestrial species survived
Placental
mammals fared better than marsupial mammals
Pollen record of Montana, North Dakota & Wyoming shows Cretaceous species go extinct precisely at the Ir anomaly, followed by a spike of fern pollen and then a return of flowering seed plants characteristic of the Paleocene; that of southern & central Canada indicates a series of extinction events before, during & after the K-T boundary
Megafloral record in the U.S. indicates a major & significant extinction precisely at the K-T boundary, while that of New Zealand indicates little change
Some workers see planktonic foraminifera having gone extinct abruptly & ocean productivity suffering a rapid and complete breakdown at the K-T boundary
Generally, larger animals fared poorly, smaller animals fared better
Attribution of the Mass Extinction to Deccan
Trap volcanism has been abandoned