Gastropod molluscs associated with low oxygen habitats have long been known. However, it wasn't until the discovery of the hot vent communities in the deep-sea in the mid1970s that diversity and abundances of gastropods associated with low oxygen conditions was truly appreciated.

Vents are geologically old structures. Arguments for the antiquity of taxa associated with vents are often based on fossil occurrences of taxonomically similar taxa. However this similarity has not been rigorously tested to determine whether or not it reflects relatedness. Other evidence marshaled in support of this antiquity is based on bizarre and supposedly archaic character combinations found in members of the hot vent faunas. Examples from the Gastropoda include taxa that are said to have both "archaeogastropod" and "mesogastropod" characters and therefore are likely intermediates between these two orders that diverged in the Paleozoic.

Morphological and developmental characters were used to reassess the phylogenetic relationships of the Gastropods. The analysis used 117 characters and includes 40 gastropod taxa. Five outgroup taxa were included, representing four conchiferan groups and Polyplacophora. The analysis resolved six major gastropod clades including the Patellogastropoda, Vetigastropoda (most of the groups previously included in the paraphyletic Archaeogastropoda), Neritopsina, Caenogastropoda, Neogastropoda and Heterobranchia. A major group of hot vent taxa (Peltospiridae and Neomphalidae) either form a paraphyletic or monophyletic group at or near the base of the vetigastropods or a clade with the Neritopsina. Mapping the first occurrence of dysoxiphilic taxa on this phylogeny indicates that many taxa with early Paleozoic origins are well represented in this habitat, while taxa with late Paleozoic and especially late Mesozoic origins are poorly represented, if at all, in these habitats.

75/125 YEARS