Amniotes

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  • Origin of Amniotes
    • Land-dwelling vertebrates: mammals, turtles, lizards, crocs, birds
    • General characteristics
      • Mostly four-limbed or reduced or lost in some groups
      • Water-resistant epidermis with various annexes
        • Impermeable skin w/ thick keratinous layer
        • Annexes: reptile scales, mammalian hairs, feathers, quills, horns
      • 3-4 chambered heart
        • 3-chambered (reptiles)
          • Lizards
            • O2 (left atrium) & CO2 (right atrium) contracted into left chamber of ventricle
              • Pressure increases, CO2 forced through gap in partition into right chamber of ventricle
                • Ventricle contraction: CO2 into pulmonary artery to lungs & O2 into systemic arteries (aortas) to bodies
          • Turtles
            • Ventricle is not perfectly divided; some slight mixing of CO2 blood can occur
          • Snakes
            • All 3 arterial trunks comes out of the chamber of the ventricle that receives CO2 blood from R atrium
              • Ventricular contraction: muscular ridge forms a partition that guides the CO2 blood to pulmonary artery
                • O2 received by other chamber of ventricle is forced through the opening in the ventricular septum and out to the aortas
        • 4-chambered (birds, mammals, partially so in crocs)
          • Crocs
            • Ventricular septum is complete; two aortas come out at different ventricular chambers
              • Semilunar valve at entrance to left aorta prevents CO2 blodds in R ventricle from flowing into aorta
                • Part of O2 blood from L ventricular chamber pumped into the R aorta flows into L by opening (Foramen of Panizza)
      • Internal fertilisation: ovi/vivi/ovoviviparous
      • Egg w/ porous leathery or hard shell; embryo surrounded by extra-embryonic membranes; direct development
        • Amnion
          • Innermost membrane; forms sac where embryo is suspended; filled with fluid; protects embryo from shock and carries out hydration
        • Allantois
          • Stores nitrogenous wastes produced by embryo, and facilitates resp.
        • Chorion
          • Permits exchanges of O2 and Co2;
          • Formed together w/ placenta in mammals- placenta attached to embryo via umbilical cord
        • Mesoderm and ectoderm extend beyond embryo to form chorion and amnion
          • Layers fuse below yolk so chorion lines the shell; tissues form allantois
      • Robust musculo-skeletal system; complex girdles; differentiated backbones
        • Two or more sacral ribs
    • NO OTIC NOTCH!!
    • Bone in the ankle joint complex (astragalus)- deriving from fusion of originally separate bones
    • Elongate and curved ribs (often barrel-shaped)
    • Enlarged mid-ventral sternum (may be lost altogether in e.g. turtles and snakes)
    • Trachea w/ cartilaginous rings
    • Cranial features
      • Ventricle
        • Pterygoid w/ distinct palatal surface, transverse flange and quadrate ramus
        • Caniniform tooth on maxilla
      • Dorsal
        • Premaxilla w/ palatal maxillary & nasal processes
        • Various patterns of fenestration
        • Squamosal contributes to margin of postemporal fenestra
        • Frontal contacts orbit
        • Hemispherical & ossified occipital condyle
      • Anapsid Pattern
        • Universal absence of skull fenestration
          • Modern turtle
      • Synapsid pattern
        • Lower temporal fenestra bordered minimally by the jugal, postorbital and squamosal
          • dog
      • Diapsid condition
        • Lower temporal fenestra generally bordered by jugal, postorbital, squamosal, and occassionally quadratojugal and an upper temporal fenestra bordered by postorbital, squamosal, parietal and often postfrontal
          • Modern lizard
    • Complex, in-folded lungs
    • Spool-shaped vertebral centra, long neck, separate scapula and coracoid ossifications

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