Tetrapods Conquer the Lands...almost

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  • Tetrapods Conquer the Lands...almost
    • Amphibians
    • Amniotes
    • Deepest Ancestry
      • Essentialist, character-based definitions create problems
        • Because research aims to provide sequences of intermediates to show assembly of anatomical characters
          • Also, presupposition that any defining character has evolved only once
      • Phylogenetic definitions
        • imply total-group tetrapods include all verts that are more closely related to modern tetrapods than any other group
        • Crown-group tetrapods include latest common ancestor of modern tetrapods plus all its descendants
      • Ichthyostega
        • Stapes
          • Weird: broad, flat and thin (unique function? Sort of like an eardrum?)
            • Rested against air-filled chamber in spiracle passage? Picking up air-borne vibrations and transferring them to otic capsule via fenestra vestibuli
      • Acanthostega
        • Skull (Grace specimen)
      • Post-Devonian world of Tetrapods
        • Romer's gap= hiatis in early fossil record
          • Forms discontinuity in our knowledge of fossil faunas and floras, with primitive forests and high diversity of fishes during end of Devonian and more modern aquatic and terrestrial assemblages of early Carboniferous
          • Gap is slowly being filled
    • Roster of transformations
      • Respiratory: out of water, gills disiccate and must be reduced
      • Skeletal: on land, body supported against gravity and still able to move
        • Requires robust paired limbs with digits, strong vertebral ossifications and articulations, and consolidation girdles (pectoral girdle becomes detached from back of skull, leading to neck formation)
        • Skeleton becomes more massive, further articulation develop and digits emerge, ancestral rays lost; extensive distal ossifications evolve to form wrists/ankles
      • Feeding: a fish has no neck, to pick food up off the ground; so head must bend with respect to the trunk
      • Sensory: all senses need retooling to functiion in air
      • Olfaction: olfactory epithelium must be kept moist
      • Vision: accommodation needs improving because refractive indices differ in air and water
        • Better visual range obtaind with eye sockets moved to skull top (invaded shallow waters, sunlight significant)
        • Surfacing and selection for lens and cornea changes to enable focused image... began to develop full power of long-range vision facilitated by larger eye size
      • Hearing: air is much less dense than water, so structures must be re-purposed to concentrate sound
        • Acoustic impedance: resistance of medium to the flow of sound through it (is a function of medium density)
          • Body & water density similar = sound passes through both into otic capsule at full intensity
          • Air is less dense = for otic capsule to register, sound must be collected and then amplified by impedance matching ears
        • Sound collected by tympanium or through jaw
          • Land verts: stapes or columella transmits tympanum vibrations to small opening in otic capsule
            • Sound is concentrated into small area, so amplified
            • Stapes: part of ancestral hyomandibular arch modified. W/ Eustachian tube that connects middle ear to pharynx
              • Homologous to spiracle
              • Fenestra ovalis is ancestral ventral extremity of otico-occipital neurocranial fissure of the ancestral sarcopterygian neurocranium
      • Lateral line: while useless out of water, this ancestral feature is retained by adults that spend significant amount of time in water
      • Nostrils: choana, posterior nasal aperture or internal nostril is one of 2 openings at back of nasal passage between nasal cavity and throat where a secondary plate occurs (no secondary plate?= mostly olfaction use)
        • Land verts: air through nasal capsule through internal nostril (choana) into oral chamber
        • Excurrent nostril of other aquatic osteichthyans has been shifted across upper lip then down onto palate


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