EARLY ONTOGENY OF THE SHELLS OF PELAGIELLID MOLLUSCS AND HYOLITHS FROM THE CAMBRIAN (DYERAN) PIOCHE FORMATION, SOUTHEASTERN NEVADA

The accretionary growth characteristic of mollusc shells can preserve a record of early ontogeny into mature shells, allowing insights into the life history and larval ecology of extinct taxa. Relatively little is known of the protoconchs of Cambrian molluscs and hyoliths (lophotrochozoan problematica) because of the loss of detail during recrystallization of their aragonitic shells: only a few definite examples are known, and much of the literature is based on indirect inferences from internal molds. We report on material from the latest Dyeran (traditional “Lower” Cambrian) Combined Metals Member of the Pioche Formation from several sections in the Pioche–Caliente region, southeastern Nevada. While many samples from this unit produced phosphatic internal molds following acid maceration, a few also yielded specimens with a delicate phosphatic layer, likely a replaced periostracum, preserving the outer surface of the shell, including details of ornamentation and growth lines. Such fossils include the remains of two forms of pelagiellid, a group of dextrally coiled molluscs widespread in the Cambrian. The first of these bears a series of prominent costae and can be assigned to Costipelagiella nevadense. Such specimens consistently show a smooth semiellipsoidal cap at the apex of the shell, ca. 85 µm tall, set off from the remaining shell by a ridge; we interpret this as the embryonic shell. The succeeding portion of the shell is smooth until the appearance of the first costa about 50 µm past the edge of the protoconch; the first few costae successively increase in width until they reach the adult shape. The second pelagiellid is characterized by much more subdued ornamentation; at a shell diameter of ca. 350 µm, this shifts from comarginal growth lines to a series of low antimarginal grooves, presumably recording some ontogenetic transition. The protoconchs of co-occurring hyoliths, probably belonging to Parkula esmeraldina, show a much larger protoconch, 200–250 µm tall, in the form of a smooth ellipsoidal apical region; the remaining portion of the shell preserves comarginal growth increments, showing that aperture is initially straight and gradually develops a broad ventral shelf (ligula) and a mid-dorsal sinus.