Pleurotomarioidean gastropods.

Pleurotomarioidean gastropods are continuously present in the fossil record since the Upper Cambrian and survive into the Recent fauna, thus providing rare insights into the evolutionary history of the class Gastropoda. Pleurotomarioidea achieved greatest numerical and morphological diversity during the Paleozoic, and dominated global shallow water marine gastropod faunas during the Paleozoic and Mesozoic. Only a single family, the Pleurotomariidae, survived the end-Cretaceous Extinction, but was restricted to deep water through most of the Cenozoic. The first living pleurotomariid was discovered during the mid-nineteenth century, along the bathyal zone of the western Atlantic. Subsequently collected specimens of these "living fossils" revealed that these animals comprise a mosaic of primitive and highly derived characters that originally inspired a new model of gastropod evolution, but more recently defy the orderly inclusion of this group within a cladistic framework of gastropod phylogeny. Molecular studies have likewise shown that the 18S rDNA gene of pleurotomariids contains novel insertions, and evolves more rapidly than in related taxa. A number of studies confirm that the Pleurotomarioidea may be included in the clade Vetigastropoda together with the Trochoidea, Fissurelloidea, Haliotoidea, and Scissurelloidea. However, neither the position of the superfamily within Vetigastropoda, nor the position of Vetigastropoda within Gastropoda is yet robustly resolved. The anatomical and molecular data are reviewed; the latter used to produce a well-resolved phylogeny of the genera within the family, and to justify the naming of the long-used informal grouping "Perotrochus Group B" as the genus Bayerotrochus. A review of the geographic and bathymetric distributions of pleurotomariids reveals that the higher taxa segregate bathymetrically, while the species within each genus generally segregate geographically, so that most species exist in allopartry. The diet of pleurotomariids is reviewed, based on direct observations as well as analyses of gut contents, and is shown to consist primarily of sponges, but may include stalked crinoids, octocorals and, under aquarium conditions, fish and clam tissue. Despite their thin and relatively fragile shells, Pleurotomariidae survive an extraordinary number of attacks by predators, primarily crustaceans and fish. Their hypobranchial glands are capable of rapidly secreting a white fluid that probably serves as a chemical defense to repel predators. Circumstantial evidence suggests that this defensive mechanism may have originated during the Paleozoic. Despite the great antiquity of this lineage, pleurotomariids possess a substantial number of morphological, molecular and ecological novelties.