Computer reconstruction and analysis of the vermiform mollusc Acaenoplax hayae from the Herefordshire Lagerstätte (Silurian, England), and implications for molluscan phylogeny

Abstract

ABSTRACT. Acaenoplax hayae is a spinose worm-like animal from the Silurian Herefordshire Lagerstätte of England, a deposit that preserves high-fidelity soft-part morphology of invertebrates in three dimensions. Specimens have been serially ground and reconstructed by computer to their three-dimensional form. Acaenoplax bears serially repeated transverse ridges dorsally, each with an array of probably aragonitic spines inclined posteriorly. Oblique ventral loberows correspond in position to the dorsal ridges. Aragonitic dorsal valves overlie every third ridge; the penultimate valve is absent leaving a total of seven. The anterior termination is disk-like, surrounding an antero-medial mouth. At the posterior termination there is a cavity between the posteriormost dorsal and a posteroventral valve, from which soft-part projections extend. Acaenoplax is interpreted as a creeping epibenthic animal that fed on sessile prey. It is an 'aculiferan' mollusc, combining unique autapomorphic structures with characters typical of the Polyplacophora and Aplacophora. Its morphology can be accommodated within both of the competing schemes of molluscan phylogeny, and its closest living relatives in both cases are aplacophorans. The highly serialized morphology of Acaenoplax is unique within the Mollusca, but the pattern it highlights is an attractive candidate for the primitive state of serial repetition within the phylum.

Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons

“…Numerous Cambrian forms such as Wiwaxia and Halkieria or the Silurian Acaenoplax have been more or less ambiguously assigned to this animal phylum (26)(27)(28). Kimberella is another putative mollusc extending the age of the group back to the Neoproterozoic (29).…”

The origins of molluscs

“…D) with seven dorsal shell plates and a posterior ventral plate (Sutton et al . , ), fitted the scenario in which aplacophorans evolved from a chiton‐like ancestor through secondary reduction and loss of the foot and dorsal shell plates (Vinther et al . ).…”

A Silurian armoured aplacophoran and implications for molluscan phylogeny

“…The arrangement and morphology of the valves of Acaenoplax differ from those in living polyplacophorans and their fossil representatives, and some features of the body have no direct equivalent in living aplacophorans 13,14,20,21 . The valves of Phthipodochiton in contrast, are typical of palaeoloricate 'polyplacophorans'; this taxon also has a ventrally complete (or near-complete) girdle, although this feature is not sufficiently well preserved for satisfactory characterization.…”