Scaphopod mollusks (Scaphopoda)

Abstract

Abstract The tusk shells (500 sp.) are grouped into 14 families and two orders within the molluscan Class Scaphopoda. Only two molecular studies have focused on phylogenetic relationships within scaphopods. Estimates of divergence times among families are estimated here. The initial divergence among scaphopods, separating Gadilida and Dentaliida, is estimated to have occurred near the Devonian–Carboniferous boundary, 359 million years ago (Ma), with the Fustiariidae, Rhabdidae, and Dentaliidae diverging in the Carboniferous (359–299 Ma). In contrast, the families included in the study from the Order Gadilida were estimated to have diverged from one another in the Cretaceous, 139–96 Ma. The scaphopods (Phylum Mollusca, Class Scaphopoda) are known as tusk shells because of their curved shape (resembling elephant tusks), open at both ends (Fig. 1). 7ey are relatively small, usually 3–6 cm in length. Scaphopods burrow into sediments with the wider (anterior) end of the shell oriented downward. Both the head and foot (used for burrowing) have an anterior location, whereas the viscera are posterior. 7ere are 500 valid species of recent scaphopods and about 800 valid fossil species. 7ere is some argument as to when the lineage originated. Scaphopod fossils have been described from the Ordovician, Silurian, and Devonian, but many of these specimens have been reclassiAed as belonging to other groups. Yochelson (1) and others have suggested that scaphopods most likely evolved in the early Carboniferous. Here we review the evolutionary relationships and divergence times of the members of the Class Scaphopoda.