Shell structure and affinity of vermiform ‘gastropods’



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 stromatolites. SEM studies of the planorbiform or trochiform protoeonch reveal a shell wall comprising three calcite layers: an outer, initial acicular 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) calcitie 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 mollusean tubulation. Punetation may he a shell-strengthening response to uncoiling. Septa bear a centrai, anteriorly-projecting, probebly perforate protrusion. reminiscent of the siphunele of cephalopods and similar structures in tentaculitoida. The micro-lamellar layer in the protoconch, the micro-lamellar layer with distinctive ridge and groove structure and punetation in the teleoconch, and the structure of the septa point to a close affinity between vermiform ‘gastropods’ and the Tentaculitoidea. 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 molluses. ***Vermiform gastropods: microstructure, protoconch, teleoconch, acicular laver, prismatic laver, micro-lameller layer. cross-bladed structure. punetation, septation. Tentaculitoidea