New Archaeogastropod Limpets from Hydrothermal Vents; Superfamily Lepetodrilacea I. Systematic Descriptions

Nine new species, six in the new genus Lepetodrilus and three in the new genus Gorgoleptis, are proposed in two new families, which together compose the new archaeogastropod superfamily Lepetodrilacea, as yet known only from the deep-sea hydrothermal-vent habitat in the eastern Pacific. Shells are limpet-shaped, of non-nacreous aragonite, with tough periostracum enveloping the shell edge. The apex is posterior, in some species projecting posteriorly, and deflected to the right. Sculpture is lacking or of beads or imbricate radial ribs. The muscle scar is horseshoe-shaped and narrowed posteriorly. The radula is rhipidoglossate and unique in forming a V-alignment of lateral teeth descending toward the rachidian. The families differ in morphology of the first lateral tooth, morphology of the ctenidium, and in placement of the penis: on the right ventral side of the neck in Lepetodrilidae and an outgrowth of the left oral region in Gorgoleptidae. Gorgoleptidae further differ in retaining the operculum and in having a posterior periostracal band shielding the posterior viscera and extending adjacent to the operculum. Anatomy is treated in part II by Fretter (Phil. Trans. R. Soc. Lond. B 318, 33 (1988)). Three species (L. pustulosus, the type species of Lepetodrilus, L. elevatus and L. cristatus) are known from the Galapagos Rift and two sites on the East Pacific Rise, near 21 $^\circ$ N and 13 $^\circ$ N. One species, L. ovalis, is known from the two sites along the East Pacific Rise. The remaining species are as yet known only from single sites: L. guaymasensis from the Guaymas Basin, L. fucensis from the Juan de Fuca and Explorer Ridges, G. emarginatus from 21 $^\circ$ N, G. spiralis from 13 $^\circ$ N, and G. patulus from the Galapagos Rift. Only one of the broadly distributed species, L. elevatus, exhibits sufficient geographical variation to warrant the recognition of a subspecies, L. elevatus galriftensis, n. subsp., at the Galapagos Rift. These species are known only from sites exposed to warm hydrothermal effluent, not from the hotter environments of the black smokers or from cold sulphide seeps. Shell characters are most similar to the `tapersnout' superfamily, yet to be described, from which these species differ in having pitted sculpture on the protoconch. The Jurassic to early Cretaceous Symmetrocapulidae had similar shell proportions but were much larger; the Symmetrocapulidae are best considered an archaeogastropod sister group. The hydrothermal-vent habitat has been available throughout geological time; hydrogen sulphide toxicity should prevent invasions of new kinds of predators, thus promoting stability and longevity of species established in this community. Differences from other archaeogastropods at the superfamily level suggest that the origin of the Lepetodrilacea took place in the late Palaeozoic to early Mesozoic, the time at which other living archaeogastropod superfamilies appeared. The rift-vent habitat was most likely entered via shallow to successively deeper sites along ridge crests. Unique anatomies and radular characters are considered remnants of early archaeogastropod diversity from the period in which archaeogastropods were the dominant gastropods in shallow seas.