The phylogenetic position of the pterobranch hemichordates based on 18S rDNA sequence data.

Pterobranchs are a class of deuterostome metazoans that are sessile marine suspension feeders. Although this group has been poorly studied, understanding their phylogenetic affinities is central to understanding early metazoan evolution. Sequence data from the 5' end of the 18S rDNA gene was collected from a pterobranch, Rhabdopleura normani, and combined with other available 18S sequences. Using standard phylogenetic methods, the evolutionary relationships of deuterostome metazoans were reconstructed. The pterobranchs are most closely related to the enteropneust hemichordates. This was confirmed by bootstrap analyses and a topology-dependent cladistic permutation tail probability (T-PTP) test. My analysis agrees with Turbeville et al.'s (1994) and Wada and Satoh's (1994) finding that hemichordates are more closely related to echinoderms than to chordates, and it is proposed that Metschnikoff's (1881) name Ambulacraria be adopted for the clade defined by the last common ancestor of the hemichordates and echinoderms. These findings suggest that ciliated gill slits and the dorsal hollow nerve chord are pleisomorphic features of the Deuterostomia.