GEOL 107 Lecture Notes - Lecture 16: Agnatha, Cambrian Explosion, Gnathostomata
18 Oct 2017
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Early vertebrates and the colonization of fresh water. 19,500 living species of fish living in virtually all oceanic environments, rivers and lakes. Nearly 95% are bony fish (osteichthyes), remainder are mainly sharks and rays (chondrichthyes), with only a handful of species of jawless fish (agnatha) Diversity of fishes probably higher now than at any point in the past, but the greatest disparity was undoubtedly back in the devonian, which has been called the age of fishes: vertebrate origins. Chordates closely related to echinoderms (similar embryology and molecular phylogeny) Two chordate-like phyla, urochordata (tunicates) and cephalochordata (amphioxus) on the way to fish (cowen, 2013, fig. Tunicate adults are colonial and sessile, but tunicate larva has a notochord and a muscular tail. Amphioxus has all of these features plus chevron-shaped muscle bundles and a fish-like anatomy. Pikaia from middle cambrian burgess shale (lecture 12) is strikingly similar to living amphioxus and is regarded as one of the oldest fossil chordates.
