Meeting Abstract

S3-1.1  Saturday, Jan. 5  Vertebrate diversity and phylogeny across the fish-to-tetrapod transition COATES, M. I.; Univ. of Chicago, Chicago mcoates@uchicago.edu

The popular idea of the fish-to-tetrapod transition covers a series of changes to the gnathostome body plan: mid-line fins are lost; digited limbs replace paired fins; a sacrum links vertebrae to hips; gills are reduced; a distinct neck separates the head from shoulders. Such changes (and many more) occur within taxa traditionally designated as fish, deep within the tetrapod stem lineage. Moreover, if traditional, anatomical character-based definitions of taxa are used, then the broad shape of tetrapod evolution resembles an ice-cream cone: the classic spindle diagram. A few proto-tetrapods exhibiting a classic mosaic of fish- and tetrapod-like features emerge within the Devonian Period some 380 million years ago, and these earliest forms constitute a phylogenetic fuse preceding a dramatic evolutionary radiation within the Mississippian (around 340 million years ago) from which sprang the roots of modern amniotes and lissamphibians. However, if the tetrapods are defined on the basis of all taxa more closely related to living forms than to lungfishes (or coelacanths), then the picture of diversity flanking the fish-to-tetrapod transition changes. Diverse and abundant Devonian tetrapods are cut down by the end Devonian (360 million years ago) Hangenberg extinction, the causes and consequences of which are only now being investigated. Modern vertebrate diversity, dominated by tetrapods, teleosts and elasmobranchs, is contingent upon this event. The fish-to-tetrapod transition occurred within a very different and earlier faunal setting, and begs questions about survivorship versus extinction, recovery and replacement, and the extent to which the phylogenetic pattern apparent among early tetrapods is repeated within the other major vertebrate clades.