Shell structure and affinity of vermiform ‘gastropods’

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Abstract
Vermiform ''gastropods'' are reported from a variety of rocks ranging from Givetian to Lower Triassic age. Examples encrusting shells and plants have been identified in non-marine shales, in addition to previously recognized occurrences in shallow marine microbial bioherms and stromatolities. SEM studies of the planorbiform or trochiform protoconch reveal a shell wall comprising three calcite layers: an outer, initial acicula layer; a blocky prismatic layer; and an inner irregular micro-lamellar layer. Minor irregularities and microstructural details suggest a high original organic content, allowing flexibility for attachment. The sinistrally coiled (or hyperstrophic dextral) calcitic teleoconch is composed of an outer simple prismatic layer and inner micro-lamellar layer comprising sheets of irregular, platy, sometimes fused calcite tablets, displaying ridges and grooves similar to those of cross-bladed fabric. Repetition of layers may occur. Regular closely-spaced punctae, passing through and disrupting the micro-lamellar layer, are unlike any molluscan tubulation. Punctation may be a shell-strengthening response to uncoiling. Septa bear a central, anteriorly-projecting, probably perforate protrusion, reminiscent of the siphuncle of cephalopods and similar structures in tentaculitoids. The micro-lamellar layer in the protoconch, the micro-lamellar layer with distinctive ridge and groove structure and punctation in the teleoconch, and the structure of the septa point to a close affinity between vermiform ''gastropods'' and the Tentaculioidea. The three-layered microstructure of the protoconch and the coiled nature of the shell distinguish vermiform ''gastropods'' from tentaculitoids. A consideration of the shared characters indicates that the tentaculitoids and vermiform ''gastropods'' should be regarded as a sister group to the molluscs.