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New Genus of Dimeropygid Trilobites from the Earliest Ordovician of Laurentia

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New Genus of Dimeropygid Trilobites from the Earliest Ordovician of Laurentia New genus of dimeropygid trilobites from the earliest Ordovician of Laurentia JONATHAN M. ADRAIN and STEPHEN R. WESTROP Adrain, J.M. and Westrop, S.R. 2006. New genus of dimeropygid trilobites from the earliest Ordovician of Laurentia. Acta Palaeontologica Polonica 51 (3): 541–550. The new genus Tulepyge includes a type species, T. tulensis nov., from the Barn Canyon Member of the House Formation, western Utah, USA, and T. paucituberculata from the Broom Point Member of the Green Point Formation, western New− foundland, Canada. Both species are earliest Ordovician in age, with occurrence immediately above the Cambrian–Ordo− vician boundary. Together with the hystricurid taxon Millardicurus, the new genus is likely to serve as an indicator fossil for the Cambrian–Ordovician boundary in Laurentia, as it is already known from different biofacies on opposite margins of the continent. Tulepyge is not closely comparable with contemporaneous taxa assigned to Hystricuridae but in its dor− sally convex and coarsely tuberculate exoskeleton it resembles taxa which have been referred to Dimeropygidae, most of which are considerably younger in age. Key words: Trilobita, Dimeropygidae, Hystricuridae, silicified fossils, Ordovician, Ibexian. Jonathan M. Adrain [jonathan−[email protected]], Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA; Stephen R. Westrop [[email protected]], Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, 2401 Chautauqua Avenue, Norman, Oklahoma 73072, USA. Introduction tulensis sp. nov., which is known from well preserved silici− fied material, and to discuss the systematic position of the ge− Systematics of Hystricuridae Hupé, 1953, was reviewed by nus. Adrain et al. (2003). The genus name Hystricurus Raymond, Institutional abbreviations.—SUI, Department of Geo− 1913, had been indiscriminately applied to a broad range of science, University of Iowa, Iowa City, USA; GSC, Geologi− Early Ordovician trilobite species. Adrain et al. (2003) re− cal Survey of Canada, Ottawa. stricted Hystricurus to a distinctive clade, which is mainly Stairsian in age, provided new diagnoses for the genus and for the subfamily Hystricurinae, and erected a new subfamily Hintzecurinae. As outlined by Adrain et al. (2003), there re− Localities and stratigraphy main many taxa which have previously been assigned to a wastebasket concept of Hystricurus and which fall outside Ibex area.—Trilobite faunas of the classic sections through the subfamilies Hystricurinae and Hintzecurinae. The sys− the Pogonip Group of western Utah were first described by tematics of these taxa require further work. Hintze (1953), following work by Ross (1951) on coeval fau− One such taxon is Hystricurus paucituberculatus Fortey, nas from the Garden City Formation of southeastern Idaho 1983, which was described from conglomerate boulders of and northern Utah. Although the latest Cambrian and earliest what is now the Green Point Formation, Cow Head Group, Ordovician House Formation contains the stratotype for the western Newfoundland (Fig. 1). This species occurs in the Laurentian Skullrockian Stage (Ross et al. 1997), its trilobite immediate vicinity of the Cambrian–Ordovician boundary. faunas had not been systematically revised in the half century Fieldwork for a comprehensive revision of the shallow water between Hintze’s work and that of Adrain et al. (2003). We faunas of the type Ibexian of western Utah and southern are engaged in a comprehensive, field−based revision of the Idaho (Adrain et al. 2001, 2003; Adrain and Westrop 2006) Ross/Hintze faunas. Large new collections have revealed has yielded a closely related species from immediately above species and genus diversity in the sections approaching an the Cambrian–Ordovician boundary in the House Formation order of magnitude greater than previously recognized of the Ibex area, Utah (Fig. 2). The species share several apo− (Adrain and Westrop 2003). Tulepyge tulensis is a case in morphies and are classified here in a new genus, Tulepyge, point. Despite the fact that it was sampled from the type hori− regarded as an early dimeropygid. The purpose of this paper zon of the species Symphysurina brevispicata Hintze, 1953 is to revise Tulepyge paucituberculata on the basis of new (a zonal name bearer in the standard trilobite biostratigraphic material, to describe the morphology of the type species, T. scheme of Ross et al. 1997; Loch et al. 1999; and Miller et al. Acta Palaeontol. Pol. 51 (3): 541–550, 2006 http://app.pan.pl/acta51/app51−541.pdf 542 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006 densis (Hintze, 1953; revised by Adrain and Westrop 2006), and an undescribed species of Clelandia Cossman, 1902. Western Newfoundland.—Stratigraphy of the Cow Head Group has been comprehensively documented by James and Stevens (1986). The group ranges in age from Middle Cam− brian to earliest Middle Ordovician, and contains a unique re− cord of Laurentian shelf margin shelly faunas, preserved in boulders in a complex series of allochthonous megaconglo− merates interbedded with autochthonous ribbon limestones and shales. The rich trilobite record of the group has been treated by Young and Ludvigsen (1989; Middle Cambrian), Ludvigsen et al. (1989; Sunwaptan Stage, Upper Cambrian), and Whittington (1963; early Whiterockian Stage, Middle Or− dovician). Work on trilobite faunas from other parts of the suc− cession is in progress, and the only other interval to have re− ceived detailed treatment comprises the strata surrounding the Cambrian–Ordovician boundary (Fortey et al. 1982; Fortey 1983). Fortey’s (1983) type material of T. paucituberculata was collected from a single boulder from the “B1" conglomerate (Fortey et al. 1982) at the Broom Point South section, Broom Point, western Newfoundland. This bed is Unit 57 of James and Stevens (1986; see Fig. 2), assigned to the Broom Point Member of the Green Point Formation. Material of T. pauci− tuberculata illustrated herein is all from a second boulder, BPS 496, in the C. H. Kindle collection of the Geological Survey of Canada, which was also collected from Unit 57. This unit contains the earliest Ordovician trilobites to appear in the sequence. Tulepyge occurs together in Unit 57 with Fig. 1. Location of Section B (only the basal part of the section sampled Millardicurus (Fortey 1983: table 1; Adrain and Westrop herein is shown—Hintze’s (1951, 1953) section continues up the ravine to 2006) and material identified by Fortey (in Fortey et al. 1982; the northeast). Inset: position of Ibex area in Utah, and position of main de− Fortey 1983) as Symphysurina brevispicata. Hence, it is tail map in the Tule Valley and southern House Range, south of US High− way 6 in Millard County. likely of very similar age to horizon B 1.1 m in Utah, occupy− ing a position immediately above the base of the Ordovician. 2003) and Millardicurus millardensis (Hintze, 1953), its presence has not previously been reported, and it does not ap− pear in Hintze’s classic monograph nor in his faunal list Significance and ecology (Hintze 1953: 26) for the horizon. The horizon from which Tulepyge tulensis was sampled is Although known only from two species, Tulepyge occurs with Hintze’s (1953) B−1, positioned seven feet above the base of the genus Millardicurus Adrain and Westrop, 2006, in strata his Section B in the Barn Canyon Member, House Formation, of almost exactly the same age on opposite margins of the southern House Range (Fig. 1). In our new measurements Laurentian paleocontinent. Further, the species are derived (Adrain and Westrop 2006), this horizon is Section B 1.1 m. from substantially different environments. The Barn Canyon The base of the Ordovician (FAD of the conodont Iapeto− Member (Miller et al. 2001) of the House Formation is com− gnathus fluctivagus Nicoll, Miller, Nowlan, Repetski, and posed of cherty lime mudstone, calcisiltite, occasional grain− Ethington, 1999) is not exposed at Section B, but B 1.1 m cor− and packstone, and minor intrclastic rudstone. The environ− relates locally and regionally to a position within the Barn Can− ment of deposition was a relatively high energy shallow sub− yon Member of about 13 m above the Cambrian–Ordovician tidal setting, above storm wave base. The boulders containing boundary (Adrain and Westrop 2006), and within what Loch et T. paucituberculata from the Broom Point Member in New− al. (in Miller et al. 2003) term the “Symphysurina bulbosa foundland are of the classic “pure white limestone” type (Lane Subzone” of the earliest Ordovician (see Adrain and Westrop 1972) characteristic of the shelf−margin algal buildups which 2006 for discussion of the criteria for recognition of this sub− typify many of the trilobite faunas of the Cow Head Group. zone). Tulepyge tulensis occurs at horizon B 1.1 m together Given the radically different geographic and environmental with Symphysurina brevispicata Hintze, 1953, two additional occurrences of the known species, it may be predicted that ad− undescribed species of Symphysurina, Millardicurus millar− ditional species of Tulepyge will be recovered in other regions ADRAIN AND WESTROP—NEW DIMEROPYGID TRILOBITE 543 Fig. 2. Geographic and stratigraphic position of conglomerate bed in unit 57 of Broom Point South section (James and Stevens 1986), from which earliest Ordovician boulder BPS 496, containing the illustrated material of Tulepyge paucituberculata, was collected. NFLD = Newfoundland, Canada; T.C.M. = Tuckers Cove Member. and environments. The close similarity in age of the known Hystricuridae has often been considered a paraphyletic taxon taxa, and their association with the more widely known genus which “gave rise” to (i.e., contains the sister taxa of) other ele− Millardicurus Adrain and Westrop, 2006, which has a simi− ments of the order Proetida Fortey and Owens, 1975. As ar− larly narrow stratigraphic range, indicate that Tulepyge may gued by Adrain et al. (2003), the hystricurid group contains serve as a potentially important trilobite indicator fossil for the major monophyletic units, including the taxa Hystricurinae position of the base of the Ordovician in Laurentia.
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