FIRST MIDDLE ORDOVICIAN OSTRACODS FROM WESTERN AVALONIA: PALEOGEOGRAPHICAL AND PALEOENVIRONMENTAL SIGNIFICANCE ED LANDING, 1 MOHIBULLAH MOHIBULLAH, 2,3 AND MARK WILLIAMS 2 1 New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA, ,elanding@mail.nysed.gov.; 2 Department of Geology, University of Leicester, Leicester LE1 7RH, UK; and 3 Department of Geology, University of Balochistan, Quetta, Pakistan ABSTRACT—Two new species of ostracods, Conchoprimitia cassidula n. sp. and Sorornanopsis avalonensis n. gen. n. sp., represent the first described Middle Ordovician ostracods from western Avalonia. They were recovered as phosphatized carapaces dissolved out of a late early Darriwilian (ca. 467 Ma) limestone boulder from the Triassic Lepreau Formation of New Brunswick, Canada. The ostracods form a low-diversity component of a higher energy, near-shore, shelf marine fauna dominated by the trilobites Neseuretus and Stapleyella and by the conodonts Drepanoistodus and Baltoniodus. The low diversity of this Avalonian ostracod fauna contrasts with more diverse (tens of species), coeval ostracod faunas from Laurentia and Baltica. The association of Darriwilian ostracods and trilobites from New Brunswick demonstrates continuing exchange of open marine, cool water biota between Avalonia, Baltica, and West and North Gondwana that began in the late early Cambrian. INTRODUCTION O STRACODS HAVE a stratigraphic record that extends back to the Cambrian, where they are first conclusively distin- guished from coeval bivalved arthropods (bradoriids and phosphatocopids) by the anatomy of their appendages (Harvey et al., 2012). The record of ostracods from the Early Ordovician is confined to carapaces (Williams et al., 2008). The earliest Ordovician ostracod assemblages were generally low-diversity, not exceeding 10 species (e.g., Tinn and Meidla, 2004; Salas et al., 2007; Williams et al., 2008; Ghobadi Pour et al., 2011; Salas and Vaccari, 2012), and had a distribution that centered on the paleocontinents of Gondwana and Baltica (Williams et al., 2008; Salas, 2011). Ostracod generic and species diversity increased slowly during the later Early Ordovician (Floian) and earliest Middle Ordovician (Dapingian) (e.g., Salas, 2011), but later Middle Ordovician (Darriwilian) faunas show greater diversity, with faunas comprising twenty or more species documented from Laurentia (e.g., the Oil Creek Formation of Oklahoma; Harris, 1957), Poland (the Malopolska Terrane; Olempska, 1994), and the Baltic craton (e.g., Tinn and Meidla, 2004). Despite over 100 years of ostracod study beginning with T. R. Jones in the 1850s (see bibliography in Siveter, 2009), the record of Ordovician ostracods from the earlier Ordovician part of the succession on the Avalonia paleocontinent is, by contrast, sparse. In part, this reflects the often poorly preserved moldic nature of the Avalonian ostracod fossils, which are dominantly known from mud rocks. Poorly preserved remains considered possible ostracods are documented from the earliest Ordovician (early and middle Tremadocian) of eastern Avalonia (Siveter et al., 1995; Williams and Siveter, 1998). Five species are listed in open nomenclature from later Early Ordovician (Floian) rocks (Botting, 2002; Siveter, 2009). The Floian of Avalonia includes unnamed species of Parapyxion Jaanusson, 1957, and Con- choprimitia ¨ Opik, 1935. In this report, we provide the first detailed, systematic description of Middle Ordovician (early Darriwilian) ostracods from the western part of the Avalonia microcontinent. This report is based on isolated, formic acid-disaggregated material from a limestone boulder within the Triassic Lepreau Formation near Saint John, New Brunswick (Fig. 1). The ostracod material is low diversity, comprising just two species, Conchoprimitia cassidula n. sp. and Sorornanopsis avalonensis n. gen. n. sp. GEOLOGICAL SETTING Tectonic setting.—Terminal Ediacaran–Ordovician rocks occur in fault-bounded and down-folded inliers in southern New Brunswick. They comprise a shallow-water, siliciclastic-domi- nated cover sequence that is unconformable on a late Cryoge- nian–early Ediacaran basement dominated by a volcanic arc succession. This volcanic arc succession underwent dramatic, late Ediacaran block faulting and erosion prior to deposition of the cover succession (e.g., Keppie et al., 2003; Landing, 2004). Most of southern New Brunswick, as well as the Doctor’s Brook allochthon in mainland Nova Scotia (Fig. 1); the Burin Peninsula, eastern Newfoundland; and North Wales are referred to the marginal platform of the early Paleozoic Avalonia paleocontinent (Landing, 1996a). Cover successions of the New Brunswick marginal platform extend from Beaver Harbour to Hanford Brook East (Fig. 1, localities BHr and HBE; e.g., Landing et al., 2008). The Saint John, New Brunswick, region, as most other marginal platform areas, has a thick, terminal Ediacaran–lowest Cambrian (Fortunian Stage) succession with an upper, coarse- grained quartz arenite (Fig. 2). This thick terminal Ediacaran– Fortunian succession is absent on uplifted blocks in the marginal platform (Malignant Cove and Bourinot blocks; Fig. 1) that formed with early–middle Cambrian transtensional faulting and show significant early Cambrian volcanism (Landing, 1996a). The Fortunian Age quartz arenite of the marginal platform prograded south and southeast where it forms the oldest cover sequence unit on the Avalonian inner platform (i.e., eastern Massachusetts; Avalon and Bonavista peninsulas, eastern New- foundland; Welsh Borderland; South Wales; England) (Landing, 1996a). In New Brunswick, an inner platform inlier lies east of Saint John at Cradle Brook (Fig. 1, locality CBr; Landing, 1996b). Red Head.—Red Head is a point on the northeast margin of Saint John harbor (Fig. 1). It has 4.0 m of fault-bounded, north- dipping, subaerial fanglomerates of the Triassic Lepreau Formation (Currie, 1983) that crop out 200 m east of the tip of the point and 200 m west of Red Head Road. The clasts include cobbles and boulders that are lithologically comparable to 269 Journal of Paleontology, 87(2), 2013, p. 269–276 Copyright Ó 2013, The Paleontological Society 0022-3360/13/0087-269$03.00