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 Copyright © 1990, Wiley Blackwell. All rights reserved