Meeting Abstract

56.4  Tuesday, Jan. 6  Hemichordate evolution: Derived body plans and suspect families CANNON, JT*; RYCHEL, AL; SWALLA, BJ; HALANYCH, KM; Auburn University; University of Washington; University of Washington; Auburn University cannojt@auburn.edu

Hypotheses of deuterostome and early chordate evolution have commonly focused on hemichordates as typifying ancestral forms. Traditional taxonomic schemes divide Hemichordata into two classes, the solitary, free-living Enteropneusta, and the colonial, tube-dwelling Pterobranchia. There are two major hypotheses regarding hemichordate evolution: 1) pterobranchs are sister to a monophyletic Enteropnuesta; and 2) enteropneusts are paraphyletic, with pterobranchs originating from within the direct developing saccoglossid enteropneust lineage. Whether enteropneusts or pterobranchs are basal hemichordates has important consequences for reconstructing the last common ancestor of the deuterostomes. In the present study, we expand the number of hemichordate taxa used in phylogenetic analyses for 18S rDNA data and also employ more quickly evolving mitochondrial gene sequences. Two deep-sea hemichordate worms appear to be members of traditional enteropneust clades, not separate families. Pterobranchs fall within Enteropneusta as sister to Harrimaniidae, concordant with previous results based on 18S rDNA. These results suggest that colonial pterobranchs evolved from a solitary acorn worm-like hemichordate ancestor. Thus, pterobranchs are unlikely to represent the deuterostome ancestral form as has been suggested by many traditional theories of deuterostome evolution.