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A New Aglaspidid Euarthropod with a Six-Segmented Trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco

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A New Aglaspidid Euarthropod with a Six-Segmented Trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco Geol. Mag. 153 (3), 2016, pp. 524–536. c Cambridge University Press 2015 524 doi:10.1017/S0016756815000710 A new aglaspidid euarthropod with a six-segmented trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco ∗ JAVIER ORTEGA-HERNÁNDEZ †, PETER VAN ROY‡† & RUDY LEROSEY-AUBRIL§ ∗ Department of Earth Sciences, Downing Site, University of Cambridge, Cambridge, CB2 3EQ, UK ‡Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520, USA §Division of Earth Sciences, School of Environmental & Rural Science, University of New England, Armidale, NSW 2351, Australia (Received 13 July 2015; accepted 27 August 2015; first published online 30 October 2015) Abstract – A new euarthropod with an uncommon morphology, Brachyaglaspis singularis gen. et sp. nov., is described from the Early Ordovician (middle Floian) Fezouata biota of Morocco. The presence of a pair of postventral plates, widely attached to each other and located under the posterior-most trunk tergite and the base of the tailspine, indicates a phylogenetic relationship with the enigmatic group Aglaspidida. The overall morphology of Brachyaglaspis most closely resembles that of the ‘Ordovician-type’ aglaspidids, more specifically the late Cambrian – Early Ordovician genus Tremaglaspis. However, the presence of a prominent cephalon and only six trunk tergites in the new genus deviates from the organization of all other known aglaspidid species, notably extending the known range of morphological disparity of the group. A taxonomic revision of this euarthropod group indicates that the most accurate name and authorship combination correspond to Aglaspidida Walcott, 1912. Keywords: Aglaspidida, Floian, postventral plates, Ordovician-type aglaspidid, Tremaglaspis. 1. Introduction (Van Roy, 2006; P. Van Roy, unpub. thesis, Ghent Uni- versity, 2006). (2) Aglaspidida is monophyletic and Aglaspidida Walcott, 1912 (= Aglaspidida sensu subdivided into two groups: a ‘Cambrian-type’ clade stricto cf. Van Roy, 2006; P. Van Roy, unpub. thesis, whose overall morphology reflects the ‘traditional’ Ghent University, 2006; Lerosey-Aubril, Ortega- aglaspidid diagnosis (e.g. 11 trunk tergites, subtrian- Hernández & Zhu, 2013; Ortega-Hernández, Legg gular glabella, genal and pleural spines, long tailspine) & Braddy, 2013) represents a major and diverse – and an ‘Ordovician-type’ clade that includes taxa with yet historically problematic – group of early Palaeo- derived features (e.g. effaced cephalon, reduction/loss zoic euarthropods typified by a biomineralized phos- of dorsal eyes, rounded genal angles, reduced ter- phatic exoskeleton (Briggs & Fortey, 1982). Although gite count, short tailspine) (Ortega-Hernández, Legg aglaspidids are relatively poorly understood, recent & Braddy, 2013). (3) Most of the evolutionary his- studies have produced significant insights into their tory of aglaspidids is obscured due to their poor fossil morphology (e.g. Van Roy, 2006; P. Van Roy, unpub. record, most likely as a consequence of their general thesis, Ghent University, 2006), biostratigraphic range preference for relatively shallow-water environments (e.g. Fortey & Rushton, 2003, 2009; Lerosey-Aubril (Lerosey-Aubril et al. 2013). et al. 2013), palaeobiogeographic distribution (e.g. Van Aglaspidid fossils are remarkably rare, and the Roy, 2006; P.Van Roy, unpub. thesis, Ghent University, paucity of well-preserved material often makes it dif- 2006; Ortega-Hernández et al. 2010; Lerosey-Aubril, ficult to recognize new species that may provide ad- Ortega-Hernández & Zhu, 2013) and phylogenetic pos- ditional information on the evolutionary history of ition within the evolutionary context of Artiopoda Hou these enigmatic euarthropods. Here we describe a & Bergström, 1997, and even the entire phylum Eu- new aglaspidid from the Lower Ordovician Fezouata arthropoda Lankester, 1904 (Ortega-Hernández, Legg Konservat-Lagerstätte in Morocco (see Van Roy et al. & Braddy, 2013; see also Legg, Sutton & Edgecombe, 2010; Martin et al. 2015; Van Roy, Briggs & Gaines, 2013). 2015). Despite having an unusual morphology for the It is possible to draw three major conclusions from group, the new taxon is recognized as an Ordovician- these findings. (1) The presence of postventral plates type aglaspidid, and indicates that these problematic (paired sclerotized structures located underneath the euarthropods possessed a greater degree of morpholo- posterior trunk tergites and covering the base of the gical disparity than previously considered. tailspine ventrally) and anterior tergal processes rep- resent the only autapomorphic characters for the clade 2. Geological setting and preservation †Authors for correspondence: [email protected], peter.vanroy@ The new taxon is described based on a single almost- yale.edu complete individual collected from Bou Chrebeb, an Downloaded from https:/www.cambridge.org/core. Open University Library, on 25 Jan 2017 at 05:33:39, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756815000710 Fezouata aglaspidid with short trunk 525 06° 00' 05° 45' Bou Chrebeb 30° 30' 1000 km 36° 04° 12° 08° Tanger. Mediterranean Sea Sandbian First Bani ATLANTIC Oujda. Upper 458.4 Group Rabat Fès . Darriwilian Tachilla Fm. Casablanca . Meknès 467.3 Middle Dapingian 470.0 32° Zini . Fm. Upper OCEAN Marrakech Floian Fezouata ORDOVICIAN Formation . 477.7 . ZAGORA Agadir Lower Lower Tremadocian Group Outer Feijas Fezouata ZAGORA Formation 485.4 28° 100 km Ordovician outcrops CAMBRIAN 10 km Figure 1. Ordovician outcrop map of the area north of Zagora, southeastern Morocco. Crosshairs indicate the position of the Bou Chrebeb locality where Brachyaglaspis singularis gen. et sp. nov. was found. Insets show the position of the map below in Africa, the study area within Morocco and the stratigraphic context with the position of the Fezouata biota (arrow). outcrop ca 35 km NE of Zagora in south-eastern Mo- preserved essentially in situ and is considered to have rocco (Fig. 1). This locality was originally considered lived in fairly shallow water, near storm-wave base to belong to the Upper Fezouata Formation, having a (Martin et al. 2015); the biomineralised taxa are typ- Floian age (Vidal 1998a, b; Destombes 2006;P.Van ical for an Early Ordovician fauna living in a normal, Roy, unpub. thesis, Ghent University, 2006; Van Roy open-marine environment (Sepkoski, 1979, 1984). et al. 2010). However, in a recent study, Martin et al. The holotype (YPM 226552) represents a dorsovent- (2015) situated this locality at the top of the Lower rally flattened individual exposed from its dorsal side, Fezouata Formation, claiming a latest Tremadocian age as evidenced by the pattern of trunk tergite overlap (Hunnegraptus copiosus biozone) for the site. This at- and degree of convexity (Fig. 2). The specimen exhib- tribution, however, has now been shown to be incorrect, its the typical vivid colouration usually associated with based on an erroneous stratigraphic correlation to an- Fezouata fossils. The yellowish and reddish colours are other site in the wider area; Bou Chrebeb in fact does related to the presence of iron oxides; these minerals are fall within the Floian of the Upper Fezouata Formation, the result of pyrite oxidation, the precipitation of which and has a middle Floian age, as considered previously permitted the preservation of soft tissues (Vinther, Van (J.C. Gutiérrez Marco, pers. comm.). The Lower and Roy & Briggs, 2008;VanRoyet al. 2010; Gaines et al. Upper Fezouata Formations represent a globally trans- 2012; Van Roy, Briggs & Gaines, 2015; Van Roy, Daley gressive sequence of mudstone and siltstone, except for & Briggs, 2015). The more restrained brownish colour the upper part of the Upper Fezouata Formation, which of parts of the trunk is typical of originally biomineral- records a substantial shallowing of the depositional en- ized structures that have been replaced by clay miner- vironment. The Fezouata Formations have yielded rich als (Vinther, Van Roy & Briggs, 2008;VanRoyet al. assemblages of non-biomineralised organisms includ- 2010), suggesting that YPM 226552 originally had a ing a wide range of iconic Burgess Shale-type elements lightly biomineralized exoskeleton. The light-coloured co-occurring with forms typical for the post-Cambrian mineralization in the cephalic region probably results Palaeozoic. These non-biomineralised biotas are ac- from the presence of further authigenic clays. companied by diverse classical ‘shelly’ faunas, includ- ing abundant trilobites and echinoderms (VanRoy et al. 2010; Van Roy & Briggs 2011; Van Roy, Briggs & 3. Materials and methods Gaines, 2015; Van Roy, Daley & Briggs, 2015; Vin- The specimen was mechanically prepared using Pa- ther et al. 2008; Martin et al. 2015; Fortey 2009, leoTools MicroJack1 and 5 air scribes, needles and 2011, 2012; Lefebvre & Fatka 2003; Sumrall & Zamora scalpels. It was glued with Paraloid B-72 dissolved 2011; Kröger & Lefebvre 2012). The Fezouata biota is in acetone, after which it received a protective coat Downloaded from https:/www.cambridge.org/core. Open University Library, on 25 Jan 2017 at 05:33:39, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756815000710 526 J. ORTEGA-HERNÁNDEZ, P. VAN ROY & R. LEROSEY-AUBRIL (a) (b) 10 mm 10 mm (c) (d) 10 mm 10 mm Figure 2. Brachyaglaspis singularis gen. et sp. nov., holotype and only specimen YPM 226552, Upper Fezouata
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