MIDDLE ORDOVICIAN, ARENIG) of OMAN by STEPHEN K
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[Palaeontology, Vol. 54, Part 3, 2011, pp. 525–533] ALAURENTIANIOCRINUS HALL (CRINOIDEA, DISPARIDA) IN THE DAPINGIAN OR DARRIWILIAN (MIDDLE ORDOVICIAN, ARENIG) OF OMAN by STEPHEN K. DONOVAN*, C. GILES MILLER , IVAN J. SANSOMà, ALAN P. HEWARD§ and JAN SCHREURS– *Department of Geology, Netherlands Centre for Biodiversity – Naturalis, Postbus 9517, NL-2300 RA Leiden, The Netherlands; e-mail: [email protected] Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; e-mail: [email protected] àSchool of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK; e-mail: [email protected] §Petrogas, PO Box 353, PC112, Ruwi, Sultanate of Oman; e-mail: [email protected] –XGL, Petroleum Development Oman, PO Box 81, Muscat 1000, Sultanate of Oman; e-mail: [email protected] Typescript received 26 November 2009; accepted in revised form 4 June 2010 Abstract: Early and early Middle Ordovician crinoids are Cid et al., currently classified as a maennilicrinid, but mor- rare globally, and are best known from North America and phologically close to Iocrinus, in having arms that branch at the British Isles. The first Arenig crinoid from the Arabian least seven times instead of four and in having columnals Peninsula is Iocrinus sp. cf. I. subcrassus (Meek and Wor- typical of Iocrinus. Three species from Wales and the Welsh then), known from two near-complete individuals, and Borders, previously placed in Iocrinus, but differing in having numerous fragmentary specimens and ossicles. These are the smooth, conical dorsal cups, are reclassified as Margoiocrinus stratigraphically oldest Iocrinus specimens, and provide an Donovan gen. nov., type species Iocrinus shelvensis Rams- unexpected extension of the palaeogeographical range of a bottom. genus known otherwise from slightly younger deposits in Laurentia (North America) and Avalonia (Wales). The Oman Key words: Arabian Peninsula, Iocrinidae, Heviacrinus, specimens differ from Gondwanan Heviacrinus melendezi Gil systematics, palaeogeography, Margoiocrinus. E arly and early Middle Ordovician crinoids remain Wadi Daiqa, south of Muscat, Oman (23°5¢2.9¢¢N, rare. Since the review of Donovan (1988), only Guens- 58°49¢20.1¢¢E; Text-fig. 1). Isolated crinoid columnals burg and Sprinkle (2001, 2003, 2007, 2009) have added were recovered from calcareous nodules in shales that significantly to the diversity of taxa of this interval, other were processed for conodonts (23°5¢5.7¢¢N, 58°49¢17.5¢¢E). rare records describing single species (e.g. Donovan and A mixed Skolithos-Cruziana ichnofacies containing trace Cope 1989). Furthermore, these crinoids are known fossils of both suspension- and deposit-feeding tracemak- almost exclusively from North America and northern ers in association with the trilobites Neseuretus and Ogygi- Europe. A rare exception was provided by a group of cri- nus, the bivalved mollusc Redonia, crinoid remains (see noid arms from the Arenig of Morocco, tentatively below) and arandaspid agnathan fishes are indicative of assigned to Ramseyocrinus sp. (Donovan and Savill 1988), nearshore, shallow water conditions (Lovelock et al. 1981; but this remains the only Lower Ordovician crinoid Fortey and Morris 1982; Davies et al. 2007; Davies and recorded from the African continent. The specimens Sansom 2009; Sansom et al. 2009). recorded herein are from the early Middle Ordovician of The lithostratigraphy of the 3500 m thick Amdeh For- the Arabian Peninsula and are a significant addition to mation was initially established by Lovelock et al. (1981) the records of these sparse early crinoids. who described five successive members, Am1–Am5. This was subsequently modified following the regional geologi- cal mapping of Le Me´tour et al. (1986). Le Me´tour et al. LOCALITY AND HORIZON (1986) and Villey et al. (1986) placed the Wadi Daiqa expo- sures of the Amdeh Formation in the Am5 Member, yet the The latex casts described herein came from a decalcified sedimentary facies and lithologies are more typical of the shell bed in sandstone within a sequence of shell sandier Am4 (Sansom et al. 2009). Sansom et al. (2009) beds, sandstones and shales of the Amdeh Formation at suggested an uppermost Dapingian? to early Darriwilian ª The Palaeontological Association doi: 10.1111/j.1475-4983.2011.01042.x 525 526 PALAEONTOLOGY, VOLUME 54 TEXT-FIG. 1. Outcrop map of the Amdeh Formation in the Saih Hatat region of northern Oman, showing the crinoid locality at Wadi Daiqa (after Sansom et al. 2009, text-fig. 1, modified after Lovelock et al. 1981, fig. 1). age (Middle–Late Arenig) for the Wadi Daiqa exposures on Other species. Iocrinus crassus (Meek and Worthen, 1865) (Cin- the basis of palynological and trilobite studies. Conodonts cinnatian of Illinois and Ohio); Iocrinus pauli Donovan and from the samples which yielded the isolated crinoid colum- Gale, 1989 (Llanvirn of the Llandrindod Wells area, Powys, mid nals also indicate a Dapingian to Darriwilian age. Wales); Iocrinus similis (Billings, 1857) (Champlainian of Terminology of the crinoid endoskeleton follows Moore Ontario); Iocrinus trentonensis Walcott, 1883 (Trentonian of New York) (Webster 2003). et al. (1978b), Webster (1974) and Ubaghs (1978). Higher classification of crinoids follows Simms & Sevastopulo Diagnosis. (Revised after Donovan and Gale 1989, p. 314) (1993) and Ausich (1998). Our philosophy of open Dorsal cup conical to slightly bowl-shaped, with promi- nomenclature follows Bengtson (1988). Specimens dis- nent radial ridges extending to angles of column, ridges cussed herein are deposited in the Department of Palaeon- separated by folds. Five prominent basal plates and five, tology, The Natural History Museum, London (BMNH). commonly larger radial ossicles. An anibrachial, sup- ported by the C ray radial, in turn supports the anal ser- ies on the left side and a free arm on the right. Anal sac SYSTEMATIC PALAEONTOLOGY complex. Arms with at least four isotomous branches. Column transversely pentastellate proximally, pentagonal Class CRINOIDEA J. S. Miller, 1821 in the mesistele and circular distally. Permanent attach- Subclass DISPARIDA Moore and Laudon, 1943 ment by distal, non-planar spiral coil. Order MYELODACTYLIDA Ausich, 1998 Family IOCRINIDAE Moore and Laudon, 1943 Remarks. Donovan and Gale (1989, text-fig. 6) recognized two morphologically distinct lineages within Iocrinus Hall. Genus IOCRINUS Hall, 1866 The typical North American species have radial ribs in the dorsal cup corresponding to the angles of the column. Type species. Heterocrinus (Iocrinus) polyxo Hall, 1866, p. 5, pl. 1, figs 1–4 (=Actinocrinus subcrassus Meek and Worthen, 1865, The proximal column is pentastellate, with the folded p. 148), by monotypy, from the Ordovician (Champlainian to areas extending onto the cup. This includes the Welsh Cincinnatian) of Ohio, New York and Ontario (Moore et al. Iocrinus pauli and the species from Oman described 1978a, p. T522; Webster 2003). herein. In contrast, other Iocrinus from Wales and the DONOVAN ET AL.: IOCRINUS HALL FROM OMAN 527 Welsh Borders have a conical dorsal cup without ridges Remarks. Donovan and Gale (1989, p. 317) discussed and grooves, and a column that is pentagonal in section how the known species of Iocrinus Hall could be differen- throughout. The latter clade is named Margoicocrinus tiated. Iocrinus pauli and I. subcrassus are broadly similar. Donovan gen. nov. below. Basals and radials of I. subcrassus are wider than high; ba- sals are about as wide as high in I. pauli and radials are Range. Ordovician: Champlainian to Cincinnatian of North slightly higher than wide. Iocrinus trentonensis is similar America; Llanvirn of Wales; Arenig of Oman (revised after Web- to I. subcrassus. Iocrinus crassus has a large dorsal cup ster 2003). with a radial:basal height ration of 3:2, in contrast to 2:1 in I. pauli. Iocrinus similis is aberrant with an extra plate between the C radial and the anibrachial; it is only known Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865) from a single specimen. The Oman specimens are classi- Plate 1; Text-figures 2, 3A,B fied tentatively as Iocrinus sp. cf. I. subcrassus; Kelly (1978, p. 57) considered that Iocrinus subcrassus showed Material. Five slabs of sandstone preserving incomplete speci- much variability in overall morphology. mens as external moulds, BMNH EE 13822–EE 13824 (Text- The curved and open coiled fragments of the more dis- fig. 2), EE 13829, EE 13830; disarticulated ossicles derived from tal mesistele (Pl. 1, fig. 1; Text-fig. 2B) occurred after the rock broken down for conodont analysis, BMNH EE 13825 and EE 13826 (Text-fig. 3A,B); and large latex casts taken from the breakage of the crinoid from the dististele, presumed to field from particularly complete specimens that, unfortunately, have been a distal non-planar spiral coiled attachment could not be collected, BMNH EE 13820 to EE 13821(1–4) (sensu Brett 1981, table 1, fig. 4E,F) as in other Iocrinus (Pl. 1). species (Kelly 1978, pl. 1, figs 7–8). Such an attachment would have been a much tighter spiral than is seen in the Locality and horizon. Wadi Daiqa, south of Muscat, Oman; Oman specimens. It is presumed that individuals were Dapingian or Darriwilian, Middle Ordovician, upper Arenig (see broken off and transported by the same current that above). buried them in sand. Well-preserved Iocrinus commonly have a more or less straight or gently curved proxistele Description. Attachment structure not seen. Column xenomor- and more proximal part of the mesistele (Pl. 1, figs 2–5; phic, pentastellate in section proximally and pentagonal more Kelly 1978, pl. 1, figs 4, 9–10; Donovan and Gale 1989, distally. The short proxistele gently tapers away from the base text-figs 2–3). The curved specimens of the Oman species of dorsal cup, grading imperceptibly into the untapered and are more distal and indicate the flexibility of the column longer mesistele (Pl. 1, fig. 2). Mesistele at least five times longer than proxistele, more rounded pentagonal in section immediately above the attachment, although the latter, of with less inflated nodal angles.