Geol. Mag.: page 1 of 15 c  Cambridge University Press 2017 1 doi:10.1017/S0016756817000152 Laurentian origin of solutan echinoderms: new evidence from the Guzhangian (Cambrian Series 3) Weeks Formation of Utah, USA BERTRAND LEFEBVRE * † & RUDY LEROSEY-AUBRIL ‡ * UMR CNRS 5276, Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Campus de laDoua, Université Claude Bernard Lyon 1, 2 rue Raphaël Dubois, 69622 Villeurbanne, France ‡Palaeoscience Research Centre, School of Environmental & Rural Science, University of New England, Armidale, New South Wales 2351, Australia (Received 19 September 2016; accepted 8 February 2017) Abstract – A new solutan echinoderm, Pahvanticystis utahensis gen. et sp. nov. is described from the upper part of the Weeks Formation (Guzhangian). The Cambrian (Series 3) succession of the central House Range in western Utah documents the early diversification of the class Soluta, which is characterized by a major ecological transition from sessile, ‘pelmatozoan’ primitive taxa (Colei- carpus, Wheeler Formation), to more and more vagile, temporarily attached (Castericystis, Marjum Formation), to mostly unattached, ‘homalozoan’ derived forms (Pahvanticystis, Weeks Formation). The morphology of Pahvanticystis is remarkably intermediate between those of Castericystis and Minervaecystis. Its twisted, flattened dististele possibly represents an adaptation for a more efficient crawling atop soft substrates. This morphological feature also questions the phylogenetic relationships between syringocrinid and dendrocystitid solutans, and the possible evolution of the latter from basal, Pahvanticystis- or Minervaecystis-like syringocrinids by paedomorphosis. Keywords: Cambrian, Echinodermata, Guzhangian, House Range, Soluta, Utah 1. Introduction Echinoderms are one of the main components of Phanerozoic marine benthic communities. In spite of recent major discoveries (e.g. Smith, Zamora & Alvaro, 2013), their initial diversification in Cambrian times, and the phylogenetic relationships between major clades remain largely unresolved (Ubaghs, 1975; Paul & Smith, 1984; Sprinkle, 1992; David et al. 2000; Zamora et al. 2013a; Zamora & Rahman, 2014). Definitive echinoderm remains can be identi- fied as early as in Cambrian Epoch 2 (Age 3), based on the typical microstructure (stereom) of their high- magnesium calcitic skeletal elements (Kouchinsky et al. 2012; Zamora et al. 2013a). However, it is very likely that their first appearance in the fossil record only reflects the acquisition of a key innovation: the ability to produce a mineralized endoskeleton (Kouch- insky et al. 2012; Zamora & Rahman, 2014). This interpretation is supported by their initial relatively high diversity in Cambrian Epoch 2 (with repre- sentatives of, at least, three classes: Edrioasteroidea, Eocrinoidea and Helicoplacoidea) and their almost synchronous first occurrences on the shores of all main palaeocontinents (Gondwana, Laurentia and Siberia; Smith, Zamora & Alvaro, 2013; Zamora et al. 2013a). A putative yet unrecorded latest Precambrian – earliest Cambrian origin and initial diversification of echin- oderms would also be in good agreement with both †Author for correspondence: bertrand.lefebvre@univ-lyon1.fr molecular clock estimates (e.g. Peterson et al. 2008) and the interpretation of some Ediacaran fossils (e.g. Arkarua) as early, uncalcified echinoderms (Gehling, 1987; Smith & Jell, 1990; Sprinkle & Guensburg, 1997; Mooi & David, 1998). In middle Cambrian times, echinoderms under- went a major diversification, with the appearance of four new classes around the boundary between Cambrian Epochs 2 and 3 (Cincta, Ctenocystoidea, Stylophora) and, slightly later, during the Drumian (Soluta; Lefebvre & Fatka, 2003; Smith, Zamora & Alvaro, 2013; Zamora et al. 2013a). This second diversification pulse coincides with a major ecological change, characterized by the replacement of firm, Proterozoic-like sea floors by soft, more intensively bioturbated substrates (‘Cambrian Substrate Revolu- tion’; Dornbos, Bottjer & Chen, 2005; Dornbos, 2006; Kloss, Dornbos & Chen, 2015). All four echinoderm clades originating during the middle Cambrian are characterized by relatively similar, convergent, flatfish, fundamentally asymmetrical ‘homalozoan’ morph- ologies, strongly departing from the radial body plans of early Cambrian taxa (Sprinkle, 1992; Lefebvre & Fatka, 2003; Lefebvre, Nardin & Fatka, 2015). Most homalozoan taxa are generally interpreted as snow- shoe strategists, well-adapted for lying atop soft, soupy substrates (Jefferies & Prokop, 1972; Parsley, 1988; Sprinkle & Guensburg, 1997; Lefebvre, 2003; Lefeb- vre & Fatka, 2003; Noailles, 2016). In contrast, most early Cambrian echinoderms are considered either as shallow sediment stickers (e.g. helicoplacoids) or as https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756817000152 Downloaded from https:/www.cambridge.org/core. University of New England, on 15 Mar 2017 at 22:04:50, subject to the Cambridge Core terms of use, available at