Abstract
AbstractExceptionally 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’, c. 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.
Highlights
DEXTRALLY coiled pelagiellids, globally distributed in early to mid-Cambrian assemblages of small shelly fossils, are among the best known of the early conchiferans
We show that Pelagiella was both an innovator in the THOMAS ET AL.: CAMBRIAN STEM GASTROPOD WITH CHAETAE 603
Our evidence from the fossil record provides an affirmative test of this hypothesis, which until now has been based on phylogenetic analyses of molecular and morphological data drawn from living organisms (Peterson & Eernisse 2001; Giribet 2008)
Summary
DEXTRALLY coiled pelagiellids, globally distributed in early to mid-Cambrian assemblages of small shelly fossils, are among the best known of the early conchiferans. Their tiny shells (mostly 1–2 mm across, but Pelagiella atlantoides (Matthew, 1895a) reached 9 mm) expand asymmetrically away from the plane of the first whorl, down the spiral axis, back upward to establish an adult form that is almost planispiral, with a depressed apex (Fig. 1) They have been treated implicitly or explicitly as stem group conchiferans (Peel 1991; Wagner 2002; Fryda et al 2008; Zhao et al 2017), as untorted helcionelloids (Runnegar 1981; regarded at that time as monoplacophorans) and explicitly as gastropods presumed to have undergone torsion during their development (Pojeta & Runnegar 1976; Parkhaev 2001a, 2008; Landing et al 2002). Together with the aldanellids, they have been linked erroneously to the Hyolitha (Dzik & Mazurek 2013)
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