[Palaeontology, Vol. 63, Part 4, 2020, pp. 601–627] PELAGIELLA EXIGUA,ANEARLYCAMBRIAN STEM GASTROPOD WITH CHAETAE: LOPHOTROCHOZOAN HERITAGE AND CONCHIFERAN NOVELTY by ROGER D. K. THOMAS1 , BRUCE RUNNEGAR2 and KERRY MATT3 1Department of Earth & Environment, Franklin & Marshall College, PO Box 3003, Lancaster, PA 17604-3003, USA;
[email protected] 2Department of Earth, Planetary, & Space Sciences & Molecular Biology Institute, University of California, Los Angeles, CA 90095-1567, USA 3391 Redwood Drive, Lancaster, PA 17603-4232, USA Typescript received 8 October 2018; accepted in revised form 4 December 2019 Abstract: Exceptionally well-preserved impressions of two appendages were anterior–lateral, based on their probable bundles of bristles protrude from the apertures of small, functions, prompts a new reconstruction of the anatomy of spiral shells of Pelagiella exigua, recovered from the Kinzers Pelagiella, with a mainly anterior mantle cavity. Under this Formation (Cambrian, Stage 4, ‘Olenellus Zone’, c. 512 Ma) hypothesis, two lateral–dorsal grooves, uniquely preserved of Pennsylvania. These impressions are inferred to represent in Pelagiella atlantoides, are interpreted as sites of attach- clusters of chitinous chaetae, comparable to those borne by ment for a long left ctenidium and a short one, anteriorly annelid parapodia and some larval brachiopods. They pro- on the right. The orientation of Pelagiella and the asymme- vide an affirmative test in the early metazoan fossil record try of its gills, comparable to features of several living veti- of the inference, from phylogenetic analyses of living taxa, gastropods, nominate it as the earliest fossil mollusc known that chitinous chaetae are a shared early attribute of the to exhibit evidence of the developmental torsion character- Lophotrochozoa.