Vertebrate Zoo L6: Origin of Craniates & Vertebrates.....& the Evolution of Mineralised Skeletons

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  • Origin of Craniates & Vertebrates.....& the Evolution of Mineralised Skeletons
    • Diversity of Early Craniates
      • Craniata
        • Cyclostomi
          • Myxinoidea
            • Hagfish
          • Petromyzontiformes
            • Lamprey
        • Gnathostomata
          • Jawed Vertebrates
      • Ostracoderms(shell-skinned) jawless; fish-like organisms
        • Characteristics
          • Dermal skeletons
          • tail fin
          • anal fin (sometimes)
          • Paired pectoral fins (sometimes)
        • Gill apparatus = respiration
          • Seperate gill pouches opening to outside through separate gill pores
          • Used large/ muscular pharynx to create suction
        • Include earliest animals with dermatocranium (head shields w/  either naked or plate covered bodies)
    • Cyclostome hypothesis supported by analyses of fossil data shows that various extinct craniates are smeared over individual stems of three major craniate groups
    • Anaspida
      • Early branch of cyclostome stem
        • Resemblance to lampreys may be more than just superficial
      • Some scale-less; some tiny scales; some large scales on body and head;
        • Scales mineralised and highly fibrous lamellae (aspidin)
          • Overlain by enamel-like subsstance (enameloid)
          • Scales had no cell spaces, no enamel, no dentine basally
      • Lacked paired fins: single anal fin and hypocercal tail
    • Euconodonta (conodonts)
      • Minute, comb-shaped or claw-like denticles
        • Calcium phosphate
      • Elongated body; imprints of chevron-shaped muscles; trace of the notochord; large paired eyes; a caudal fin strenghtened by radials
      • Conodont organs are in the head; neart entrance to pharynx
    • Gnathostome stem group: Thelodonti
      • Bags of densely packed scales; flattened bodies' sturdy hypocercal tails
      • Some deep and compressed bodies with short broad tails; many with paired pectorall fin folds, a dorsal fin, and an anal fin
      • Most systematics is based on patterns of scale growth
        • Didn't overlap: acellular base, sometimes enamel, non-growing dentine and pulp cavity
    • Gnathostome stem group: Pteraspidomorphi
      • Varied body shapes
      • Massive dermal head armour (large ventral and dorsal shields on head too) some had spines
      • Acellular basal layer; intermediate spongy layer; upper layer of detine tubercles capped with enamel
    • Gnathostome stem group: Galeaspida
      • Helmet-like bony shield on head (varied; often with spikes and processes)
      • All have opening (external nostril) on top of head
        • allow communication with pharynx
      • Minute scales arranged in oblique rows and there is no other fin besides the tail fin
      • Two semi-circular canal
      • Skeelton: microspherulitic tubercles of acellular bone; acellular laminar layer w/ minerals arranged in cylindrical bundles and mineralised cartilage
    • Gnathostomes stem group: Osteostraci (many shared jawed verts. characteristics)
      • Paired pectoral appendages; internal skeletons and muscles
      • Heterocercal tail
      • 2 dorsal fins
      • Ring of ossicles (sclerotic rings) around the orbits
      • Slit-shaped branchial openings
      • Other
        • Horseshoe-shaped head (massive endoskeletal skull, w/ shield of dermal bone)
        • On dorsal head surface: closely-set eyes, pineal foramen, median, keyhole-shaped nasohypophyseal opening
        • Fields (depression of braincase with loose dermal bone platelets): sense organs or electric organs
        • Mouth and gill openings are on ventral side of head
        • Scales: upper level dentine tubercles w/ pulp cavity; a middle layer of cellular spongy bone with horizontal network of canals; thin layer of bone with radial vascular canals; basal laminar layer resting on endoskeleton


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