The placement of Leptusa chiltoni is restored to its original status as sole member of the genus Baeostethus. Baeostethus chiltoni is a subantarctic intertidal species that is present on the shores of Auckland, Antipodes, and Campbell islands. Baeostethus is concluded to be a member of Liparocephalini based on the absence of seta v on the adult mentum, several setae distributed only on the mesal surface and apex of the galea, and contiguous mesocoxal cavities. The phylogenetic relationships of Liparocephalini are investigated in the context of a previous study and Baeostethus is shown to be a sister taxon to Liparocephalus based on 12 synapomorphies (nine are unique). The majority of Liparocephalini are distributed along the coasts of the northern Pacific rim, some 9000 km away from B. chiltoni. Most are apterous, and with Baeostethus as a relatively derived member of the group, it is difficult to reconcile the biogeographic pattern based on a dispersal event from the northern Pacific to the Campbell Plateau. We argue that the trans-Pacific pattern in Liparocephalini is due to an older contiguous distribution along the coastal margins of Pangea. The data are supported by an alternative area cladogram derived from a rerooted tree, congruence with a Pacific rim pattern seen in intertidal Aegalitinae (Salpingidae), and the distribution and phylogeny of terrestrial Gymnusini (Aleocharinae). |
