Zooid morphology and molecular phylogeny of the graptolite Rhabdopleura annulata (Hemichordata, Pterobranchia) from Heron Island, Australia

Abstract

Rhabdopleura Allman, 1869 is one of the longest surviving animal genera. The five-known species are the only living Graptolithina, a group well known from their diverse Paleozoic fossil record. Here we add information on the soft-bodied zooids and molecular phylogenetics of Rhabdopleura annulata Norman, 1921, which was previously only known from its tubes. Tubes and zooids were collected from Heron Island, Queensland, Australia. Zooids have a single pair of tentaculated arms. Dark pigment granules are found throughout the body, and particularly dense in the pair of arms and the anterior lip of the cephalic shield. Colonies grow encrusted in and on coral debris. The tubes are either creeping or erect, but no stolon has been found. Inside of the coral matrix lacunae, the tube cortex formed a parchment-like wallpaper. Phylogenetic analysis based on combined 18S+16S rRNA sequences placed R. annulata as sister to the remaining rhabdopleurids, albeit with weak support. The biogeographic range of R. annulata extends from Indonesia to Tasmania, and New Zealand. Its occurrence on Heron Island does not extend this range, but highlights that rhabdopleurids may be more common, and in shallower waters, than previously appreciated, permitting further studies that may shed light on graptolite paleobiology.