Review: The Origin of Flight: 3 Hypotheses
- A familiar question - "What is the value of half a wing?"
- 1) Tree Down theory
- Wings arose in arboreal quadrupeds for gliding
- Enlarged for longer glides
- Developed increased control
- Became capable of powering flight
- Plausible, but …
- Birds evolved from bipedal cursorial (running) animals
- 2) Ground up theory
- Flight arose in running animals
- Wings initially used for prey capture and/or display
- Eventually wings became large enough for gliding, then powered
flight
- 3) Tree up theory
- A combination
- Flight arose in climbing bipeds
- Some bipeds are good climbers (e.g. hoatzin)
- Facilitated movement among trees
Archaeopteryx - an early
bird
- Oldest undisputed bird
- 150 mya
- Protoavis older, but controversial
- Many dinosaur-like features
- Solid bones
- Little fusion of bones
- Socketed teeth
- Long, bony tail
- Hand with three digits
- But close relationship with more modern birds, as evident from
many derived similarities
- Asymmetrical feathers (modified scales)
- Fused clavicles
- 3/1 toe pattern
Modern birds
- Very successful
- > 9,000 species
- Broad range of feeding habits, habitats
- Important advances relative to Archaeopteryx
- Enlarged sternum, pectoralis muscles
- Extensive fusion of bones — rigid skeleton for muscles
to pull against
- Hollow bones
- Tail reduced, replaced with feathers
- Flight surface of wings provided by feathers, not membrane
- Feather surface supported by a keratin rod ("shaft"),
not digits; compare to pterosaurs and bats
- Primary and secondary feathers on wing provide control
- Can be molted - repaired, replaced
- Toothless jaws, beak
- Larger brain
Bats
- Fossils since about 50 mya
- Flight apparatus already formed
when first fossilized
- Pretty conservative since then
- Mammals, therefore synapsids
- Probably arose from arboreal quadrupeds
- Bats are the sister-group of flying lemurs
- Membranous wing supported by digits
- Compare to pterosaurs and birds
Comparison: Pterosaurs, Birds, Bats
Bats
Disadvantages relative to birds
- Heat lost over membranous wing
- Highly vascularized
- Poorly insulated (hair — modified scales)
- Less precise control
- No primary or secondary feathers
- Harder to repair
- No molting, preening
- Solid bones
- Heavy skull, jaws
- Teeth
Bats
Advantages relative to birds
- Echolocation
- Can actively hunt at night
- Nocturnal habits may have helped them succeed as flying
vertebrates after the origin of birds
Mammalian Origins and Evolution
Birds and Mammals: A very high energy
lifestyle
- Very high metabolic rate, endothermy, and associated changes
- Mammals and birds (and some dinosaurs?)
- Complete division of heart
- Greatly improved respiratory system
- More efficient terrestrial locomotion
- Recent ancestors of birds and mammals were convergent
- Elaboration of nervous and sensory systems
- Very complex behavior
Origin of the "Reptiles": Diapsids vs.
Synapsids
Synapsida
- Arose about 300 mya (late Carboniferous of Paleozoic) within
the very early reptiles
- Major radiations
- "Pelycosaurs"
- "Therapsids" (mammal-like reptiles)
- Mammalia
"Pelycosaurs"
- Dominant on land in late Paleozoic (250 mya — lower
Permian)
- Large carnivores
- Large, narrow skull (to 4 m)
- Robust teeth
- Slight heterodonty
- Teeth in sockets
- Otherwise, retained primitive features
- Fairly sprawling gait
- Less efficient
- Requires extra bones and muscles ventrally
- Large post-dentary bones
- Dorsal sail in some
- Covered with skin
- Likely used for thermoregulation
- Could orient to
- Absorb heat in morning
- Avoid overheating midday
Early "Therapsids" (mammal-like reptiles)
- Arose about 260 mya (lower Permian — late Paleozoic)
- Became quite diverse
- Carnivores and herbivores
- More heterodont dentition
- Posture more erect
- Locomotion and support much more efficient
- Reduction in ventral muscles and bones
- Convergent with Archosaurs
- Beginnings of 2° palate
- Allows breathing while food is in the mouth
- Important for food processing
Advanced "Therapsids" — Cynodonts
- Sister-group of mammals
- Functionally complete 2° palate
- Can chew and breathe at the same time
- More solid skull
- Very heterodont
- Teeth complex
- Teeth specialized for different functions
- Cynodont means "dog-tooth"
- Enlarged dentary bone
- Temporal opening enlarged
- More space, attachment for jaw muscles
- More complex jaw musculature
- Can process food more effectively
- Precise control of jaws
- Atlas-Axis complex - greater head mobility
- Reduction in ribs
- Greater dorsoventral flexion in vertebral column
- Retained in modern mammals
- Body flexion — Fishes, ichthyosaurs, vs. whales, dolphins
- Reduction in tail
- No counterbalance, unlike archosaurs
- Inhibits bipedal movement
Primitive mammals
- Arose about 220 mya (upper Permian, late Paleozoic)
- Small and shrew-like
- Probably endothermic, with hair (modified scales)
- Nocturnal
- Rods > Cones
- Lived for 150 million years in the shadow of the dinosaurs
Mammalian synapomorphies — from early fossil
mammals
- Teeth replaced just once (milk teeth, permanent teeth) instead
of repeatedly, except for molars, which are not replaced at all
(only 1 permanent set, come in late)
- Molars with two roots
- Three cusps on molars
- Teeth functionally differentiated
- Incisors - nipping
- Canines - grasping, tearing
- Pre-molars, molars - slicing, grinding
- Precise occlusion
- More effective cutting, chewing
- Shearing action of teeth
- Requires rigid skull
- Facilitates high metabolism by more efficient processing of
food
- Dentary forms primary articulation with the skull
- Post-dentary bones very reduced
- Lactation
- Teeth not erupted in young of the early mammals, therefore
probably fed on milk
Mammalian synapomorphies - Modern
- Three middle ear bones - reduced from previously larger skull
and jaw bones at the junction of the lower jaw and cranium
- Hyomandibula -> stapes
- Articular -> malleus
- Quadrate -> Incus
- Dentary left alone as the lower jaw
- Divided circulation
- Probably arose earlier
- Advantages
- Allows high, aerobic metabolism
- Allows precise control of blood flow
Modern mammals - Two subclasses
- Prototheria
- Platypus, echidna
- Theria
- Metatheria -Marsupials
- Eutheria — Placental mammals
Prototheria
- Most primitive of the modern mammals
- Lay eggs! (leathery shell, laid in "nest" in burrow)
- Endotherms, but low body temperature (30° C)
- Reptile-like pectoral girdle
- Single opening for feces, urine, reproduction = "cloaca"
- Like other mammals, have:
- Hair (highly modified scales)
- Lactation (modified lymph glands)
- Three middle ear bones
- Only dentary forms the lower jaw
Theria
- More complicated teeth
- Live birth
- Metatheria - Marsupials
- Kangaroos, koalas, opossums, etc.
- Marsupials
- Eutheria
- Placentals
- Wide array of habitats and ecological niches
Metatheria (marsupials)
- Successful and diverse
- Only 277 species today
- Formerly much more diverse
- Diversified in the New World during the late Paleozoic and
early Mesozoic
- Expanded into Eurasia, Antarctica, Australia when these
continents were connected during the late Paleozoic — early
Mesozoic
- Went extinct in North America, Eurasia, and Antarctica during
late Mesozoic and Tertiary
- Were replaced by Placentals in North America and Eurasia
(which were connected during the Mesozoic; placentals underwent
very extensive radiation in early Tertiary); Antarctic became
glaciated with global cooling at end of Mesozoic and early
Tertiary and remains so today
- Remained and dominated the ecology of the "island continents"
of Australia and South America
- Highly diverse, occupied all of the niches that placentals now
do in South America (and that marsupials still do in Australia)
- Persisted in Australia and South America until:
- Central American land bridge between North and South
America rose - 3 mya (Pliocene of Tertiary)
- Many placentals inhabiting North America migrated south
over land bridge
- Mass extinction of large South American marsupials
(Miocene, Pliocene) — usually considered to have resulted
from influx of placentals (though this is debated)
- Opossums (the only marsupial remaining in the Americas
today) bucked the trend, migrated north
- Many examples of ecological convergence between marsupials
(i.e., in Australia today, in South American fossils) and
placentals (see slides)
- Very short gestation
- Young born tiny and immature
- Young crawl to marsupium (pouch) and attach themselves to a
nipple
- Maternal investment reduced
- Can "abandon" offspring when stressed
- Can gear up for reproduction quickly and reproduce rapidly
when conditions are good — high rate of population increase
- But …
- Growth of offspring slower than in placentals
- No well-developed placenta to facilitate embryonic nutrition
Eutheria — Origins
- Small and inconspicuous until 65 mya (late Cretaceous, end of
Mesozoic, which coincided with extinction of dinosaurs)
- Explosive radiation in Tertiary
- Most orders appear within 10 million years after beginning of
Tertiary
- Relationships among placental orders obscure
- Much recent progress based on
- Molecular characters
- New fossils - e.g., origin of whales (see slides)
Eutherian Radiation
- How can this be explained?
- Sudden opening of many niches
- Perhaps built upon previous diversification
Eutherians — Reproduction
- Long gestation period — born at extremely advanced stage
- Fetus well protected
- Fetus grows quickly
- Well developed placenta — exchange gases, food, waste
with maternal circulation
- Maintained at high body temperatures
- But slow response to changing conditions (in contrast to
marsupials)
Animal Evolution - Looking back
- What is the diversity of animal life?
- How are the major groups of animals related?
- What is the history of the "great Tree of Life"?
- Why were some branches "vigorous" and others "feeble"?
- Given an animal, you should have a good idea of:
- Where it fits on the Tree of Life
- Peculiar or unique features it (and its relatives) have
- How it solves the major challenges of animal life:
- Exchange of O2, CO2
- Acquisition of nutrients
- Disposal of metabolic wastes
- Reproduction
- Locomotion
What you have learned - Patterns
- ! Changes in life through time
- Origins of animals
- Changes in animal life over time
- Rates of evolution highly variable
- Patterns of relationship
- The branches on Darwin’s great tree of life
- Crucial for understanding how evolution works
What you have learned — Processes
- Origin of major taxa
- E.g., neoteny and the origin of Bilateria
- E.g., neoteny and the origin of Chordates
- Form, function, and adaptation - Convergence!
- Sessile lifestyle, protective armor
- Corals, Hydrocorals, Bivalves, Brachiopods
- Aquatic to terrestrial
- Chelicerates, Isopods (Crustacea), Insects, Vertebrates
- High speed aquatic locomotion, streamlining
- Cephalopods, fish, ichthyosaurs, cetaceans
- Extremely high metabolic rate
- Natural selection is the underlying process
- Constraints lead to similar solutions
- Diversification
- Key Innovations
- Bilaterality
- Mesoderm
- Coelom
- Segmentation/metamerism
- Wings
- Amniotic egg
- Opportunities
- Extinction shuffles the deck
- Cambrian Explosion
- Diversification of Mammals
What you have learned - Tree Thinking
- An historical view of Life
- Very powerful for
- Organizing information
- Reading literature
- Thinking critically
- Formulating questions
- This will stick with you!
Additional slides
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