A New Archaeogastropod (Rhipidoglossa, Trochacea) from Hydrothermal Vents on the East Pacific Rise

Melanodrymia aurantiaca gen. et sp.n., a conispiral archaeogastropod of trochacean affinities, occurs on chimneys and stacks of polymetallic sulphide deposits at hydrothermal vents on the abyssal ocean floor at 21°N Latitude off Baja California, Mexico. The shell is unique in its combination of reflexed growth lines, strongly prosocline aperture, discordant sculpture, discoidal geometry and pattern of apertural angulations. The similarity in form with Paleozoic euomphalaceans and pleurotomariceans is considered superficial and the unusual shell form and simplification of shell microstructure are interpreted as evolutionary correlates of small size. The radula is unusual, but it shares many features with the rhipidoglossan radulae of four undescribed limpet-shaped species from hydrothermal vents on the East Pacific Rise. It is hypothesized that the adaptive radiation occurred in situ , late in the Tertiary or Quaternary, and from a coiled ancestor.