Reptile lineages: 3 skull types

• We discussed the history and biology of the anaspids (Testudines -turtles)
• We began discussing the history and biology of the diaspids (Lepidosauria - lizards and snakes)

Diapsid Reptiles: Superorder Archiosaura (crocodiles, dinosaurs)

Several improvements to the

basic reptile body plan:

• lighter, narrower skull
• socketed teeth
• tendency towards bipedal locomotion

Diapsid Reptiles: Order Crocodilia

• Arose in the late Paleozoic era
• Little morphological change since their beginnings
• long jaw for prey capture
• skull with massive jaw musculature provides a wide gape and rapid, powerful closure
• long body with laterally compressed tail for swimming
• secondarily evolved quadrupedal locomotion
• can reach large body sizes (Nile crocodile can reach 1000 kg, salt water crocodiles of AustralAsia can reach 20+ feet).

Order Crocodilia (cont.)

• Respiration

  • well developed lungs
  • second palate allows breathing when mouth is filled with water or food

• Reproduction

  • oviparous (eggs released by female, develop outside the body, though female often cares for and defends nest and young)
  • incubation temperature of the nest often determines sex of offspring
    

Diapsid Reptiles: Orders Saurischia & Ornithaurischia

Several additional innovations made these reptiles extremely successful:

• Erect, digitigrade (standing on digits) foot posture (faster, more efficient movement)
• lighter, stronger pelvic girdle (larger, stronger leg muscles)
• strengthened ankles (withstand strain of high speed locomotion)
• rigid tail (counterbalance during high speed maneuvering)

Orders Saurischia & Ornithaurischia (cont.)

• Saurichia ("lizard-hipped") had two independent lines of evolution

  • Theropods
  • mostly carnivorous
  • ranged between 3-40 ft.
  • included taxa like Tyrannosaurus and Allosaurus
  • Sauropodomorphs

    • mostly herbivorous
    • some were 80 ft. long!
    • secondarily evolved quadrupedalism
    • included taxa like Brontosaurus and Brachiosaurus

Orders Saurischia & Ornithaurischia (cont.)

• Ornithaurischians ("bird-hipped")

  • diverse group of ‘beaked’ herbivores
  • included taxa like Stegosaurus and Triceratops

End of the Age of Reptiles

• The C-T extinction (end of Mesozoic at the Cretaceous - Tertiary boundary)
  • Land-dwelling reptiles hit hard

    • 9 families of dinosaurs vanish
    • 3 families of crocodiles disappear
      

• BUT …. Are dinosaurs really extinct?
  

Vertebrate Flight and the Origin of Birds

Brief review: "Reptiles"

• Paraphyletic
• United by:
• Presence of amniotic egg
• Absence of various specializations found in birds and mammals

Birds and Mammals:
Convergent Specializations

• More efficient terrestrial locomotion
• Very high metabolic rate and associated changes
• Mammals and birds (and dinosaurs?)
• Complete division of heart (blood to lungs and body)
• Greatly improved respiratory systems
• Elaboration of nervous and sensory systems
• Very complex behavior: parental care, true social behavior

Birds and Mammals:
Convergent Specializations

• Flight
• In the ancestor of birds, and within mammals
• Also in pterosaurs
• We’ll focus on flight today
• Next time …
• Other features convergently shared by birds and mammals
• Mammalian synapomorphies and their significance

Vertebrate Gliders

• Gliding has arisen dozens of times
• In all major vertebrate groups except Agnathan fishes and Chondrichthyes
• Requires only an enlarged surface area
• Allows
• Escape
• Stability during leaping
• Low-energy locomotion

Vertebrate Powered Flight

• More difficult than gliding!
• Requires
• Strong, lightweight wing
• Light, streamlined body
• Powerful flight muscles
• High metabolic rate
• Precise control
• Three independent origins among vertebrates
• Pterosaurs (Diapsid reptiles, Archosaurians)
• Birds (Class Aves, monophyletic)
• Bats (Class Mammalia)

Vertebrate Powered Flight - Pterosaurs

• Two major groups
• Rhamphorhyncoids
  • Appear in Jurassic
  • Teeth
  • Tail
• Pterydactyloids
  • Lose teeth
  • Lose bony tail
  • Reduced hind limbs
  • Debate over Bipedal vs. Quadrupedal
  • Debate over wing attachment
  • Initial view
  • Could only soar
  • Needed to jump from cliffs
  • Clumsy on ground

Vertebrate Powered Flight:
Pteranodon Specializations

• Elongate wings
• Supported by 4th digit
• 27 foot wingspan
• Weighed only about 25 lbs.!
• Wing membrane supported by elastic fibers
• Shoulder (pectoral girdle) modified for powerful beat
• Enlarged sternum and pectoralis muscle
• Body small and rigid
• Hollow bones (like birds)
• Extensive fusion of bones
• Reduced hind limbs
• Modified brain
• Large optic lobes ->Vision
• Large cerebellum -> Balance
• Based on endocasts
• Head crest
• Balance

Pterosaur controversies

• Endothermic?
• Some pterosaurs had insulation (modified scales)
• Fast, bipedal on ground?
• Initially viewed as clumsy quadrupeds
• From late 70’s until relatively recently, viewed as agile bipeds
• Recently, resurgence of the quadruped view
  • Reinterpretation of membrane as attached to the hind limbs
  • Tracks

Pterosaur controversies

• Endothermic?
• Fast, bipedal on ground?
• Resurgence of quadruped view
  • Reinterpretation of membrane as attached to hind limb
  • Tracks suggest quadruped movement

Birds - Origins

• Descended from Saurischian dinosaurs
• Bipedal
• Ancestors had feathers
• Ancestors probably endothermic

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

• 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

Flight in Modern birds

• Group is very successful
• > 9,000 species
• Broad range of feeding habits, habitats
• Important advances relative to Archaeopteryx:
  • Enlarged sternum, pectoralis muscles
  • Extensive fusion of bones
  • Hollow bones
  • Tail reduced, feathers only
  • Flight surface of wings provided by feathers
    • feathers supported by a keratin axial rod, 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

Mammalian flight: Bats

• Fossils since about 50 mya
• Flight apparatus already formed
• Pretty conservative since then
• Wing outlines added to fossil at right
• Mammals, therefore synapsids
• More next lecture
• Probably arose from

• Bats are the sister-group of flying lemurs
• Wing supported by digits (compare to pterosaurs and birds)
• Compare the Structure of Wings in Pterosaurs, Birds, Bats

Bats
Disadvantages relative to birds

• Heat lost over membranous wing
• Highly vascularized
• Poorly insulated
• 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