Evolution and systematics

Scaphopoda is the most recent class of mollusks to appear in the fossil record, dating from at least the Mississip-pian Carboniferous period about 360 million years ago (mya). Several earlier fossils, however, have been assigned to the Scaphopoda, but not without some debate as to their relationship to extant scaphopods. These earlier specimens date from the Devonian into the Ordovician period (about 450 mya). The earliest putative representatives cited are Plagioglypta iowensis and Rhytiodentalium kentuckyensis. The most recent classification system places scaphopods into two orders, Dentaliida and Gadilida. Within these orders, there are 14 families and 60 genera, 46 of which are extant.

There is little agreement on the relationship of scapho-pods to other mollusks. While scaphopods certainly belong among the uni- or bivalved mollusks in the subphylum Conchifera, studies in the past several years have identified many different taxa as the closest scaphopod relative, including cephalopods, bivalves, gastropods, and the extinct class of bivalved mollusks known as Rostroconchia. Historically, this unclear picture has likely been the result of difficulties in comparing the morphology (form or structure) of scaphopods to that of other mollusks. Recent molecular studies, however, have brought new evidence to bear on the problem. Expression of the engrailed gene in the early development of the scaphopod shell indicates an affinity with univalved cephalopods or gastropods rather than with bivalves. Moreover, 18S RNA sequence data indicate that scaphopods are most closely related to cephalopods among living mollusks.

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