BISC 316 Lecture Notes - Lecture 6: Mesonephric Duct, Placodermi, Acanthodii
11 Dec 2016
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Document Summary
Fossil record gives little insight into evolution of jawed fishes. Probably due to duplication of hox gene complex. 4 clades of gnathostomes present: placoderms, acanthodians, chondrichthyes, osteichthyes. Benthic bodies dorsal ventrally flattened. Jaws (no true teeth: neural and hemal arches. Heavy armour: gas in bony plates allowed for head articulation. Jaws could not open wider and when animal was at rest. Bony immobile claspers, posture to the pelvic bones. Females pair of pelvic plates with ridges that articulated with the male claspers. External fertilization in osteichthyes and amphibia must have evolved from ancestor with. Internal fertilization is basal to all gnathostomes internal fertilization. Placoderm that had bones that were thought to have first originated in osteichthyes: these included the premaxilla, maxilla, and dentary. Results of phylogenetic analysis and palatoquadrate conditions among major gnathostome groups. Places placoderms as the stem of the gnathostome group. Placoderms are close to osteichthyes than to acanthodii and chondrichthyes.
