Pelagiella exigua, an early Cambrian stem gastropod with chaetae: lophotrochozoan heritage and conchiferan novelty

Exceptionally well-preserved impressions of two bundles of bristles
protrude from the apertures of small, spiral shells of Pelagiella exigua,
recovered from the Kinzers Formation (Cambrian, Stage 4, “Olenellus Zone”,
~ 512 Ma) of Pennsylvania. These impressions are inferred to represent
clusters of chitinous chaetae, comparable to those borne by annelid
parapodia and some larval brachiopods. They provide an affirmative test in
the early metazoan fossil record of the inference, from phylogenetic
analyses of living taxa, that chitinous chaetae are a shared early
attribute of the Lophotrochozoa. Shells of Pelagiella exhibit logarithmic
spiral growth, microstructural fabrics, distinctive external sculptures,
and muscle scars characteristic of molluscs. Hence, Pelagiella has been
regarded as a stem mollusc, a helcionelloid expressing partial torsion, an
untorted paragastropod, or a fully torted basal member of the gastropod
crown group. The inference that its chaeta-bearing appendages were
anterior-lateral, based on their probable functions, prompts a new
reconstruction of the anatomy of Pelagiella, with a mainly anterior mantle
cavity. Under this hypothesis, two lateral-dorsal grooves, uniquely
preserved in Pelagiella atlantoides, are interpreted as sites of
attachment for a long left ctenidium and a short one, anteriorly on the
right. The orientation of Pelagiella and the asymmetry of its gills,
comparable to features of several living vetigastropods, nominate it as
the earliest fossil mollusc known to exhibit evidence of the developmental
torsion characteristic of gastropods. This key adaptation facilitated an
evolutionary radiation, slow at first and rapid during the Ordovician that
gave rise to the remarkable diversification of the Gastropoda.