The Classes Tergomya and Helcionelloida, and early molluscan evolution

The broad spectrum of shell morphologies assigned to the molluscan Class Mono­placophora, and widespread inconsistency in application, have motivated abandon­ment of the term in formal systematics. Two classes, Tergomya and Helcionelloida, are recognised to include two of the major groups of untorted molluscs formerly united within the Monoplacophora by some recent authors. The concept behind recognition of the Classes Tergomya and Helcionelloida is examined from the point of view of functional morphology. In the exogastric Tergo­mya, which includes the present day Neopilina Lemche, 1957 and its relatives, the apex of the generally low, cap-shaped shell is located anteriorly and the shell expands towards the posterior. More specialised tergomyans include tall hypseloconellaceans and strongly coiled cyrtonellaceans. Three orders are recognised: Tryblidiida, Cyrto­nellida and Hypseloconida (nov.). The Class Helcionelloida includes endogastrically coiled shells in which the apex lies at the posterior and the shell expands anteriorly. Helcionelloids are probably ancestral to the Classes Rostroconchia and Scaphopoda, and may have given rise to the first cephalopods. Recognition of Tergomya and Helcionelloida suggests that the Subphyla Diasoma and Cyrtosoma of recent authors may be polyphyletic; their use is discontinued. Models for the evolution of the molluscs are discussed from the point of view of the early fossil record.