MIDDLE ORDOVICIAN, ARENIG) of OMAN by STEPHEN K

[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 (235¢2.9¢¢N, rare. Since the review of Donovan (1988), only Guens- 5849¢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 (235¢5.7¢¢N, 5849¢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 geological 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 exposures 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, supported by the C ray radial, in turn supports the anal series 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; basals 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. Lower order internodals may be course, would have been permanently coiled and made more pentastellate, giving the column a ladder-like appearance inflexible by having wedge-shaped columnals (e.g. Dono- (Pl. 1, figs 3, 5). Column heteromorphic, N434243414342434. van 1986, text-fig. 12K). Nodals highest with nodose angles and convex latera. Interno- This leads to the question of where are the long and dals progressively lower, all with convex latera except quartin- complex anal sacs that are so typical of Iocrinus? None is ternodals. Articular facet of columnals with a central, rounded apparent on any of the latex casts, even as fragments. It is pentagonal lumen (Text-figs 2D, 3A,B). Five areola petals devel- conservative to assume that both well preserved crowns oped in angles of columnal, each adjacent pair separated by (Pl. 1, figs 2, 4) are in the same orientation with the more one or three large crenulae. Articulation symplectial, best devel- anterior part of the crown preserved. So, the posterior oped as a perilumen and between columnal angles. Raised part of the crown, including the anal sac, would be on circumference of columnal may have lacked a crenularium. Latera unsculptured. the counterpart of the parent sandstone slab, but this is Dorsal cup poorly seen. Monocyclic, basals deeply infolded as not preserved. Alternatively, it may be that the anal tubes a continuation of the structure of the proxistele (Pl. 1, fig. 4). were autotomised as a reaction to disturbance (compare Basals about half height of cup, wide. Radials more convex, with Lane 1984) and their hydrodynamic characteristics about as high as basals, but wider than high, with broad radial led to transport to another site. These are the oldest Iocri- facets about three quarters of plate width. Plates of crown un- nus specimens known, but that is no reason to suppose sculptured. that the anal sac was any less well developed than in the Tegmen and anal series not preserved. North American and Welsh species. Arms long, uniserial, apinnulate, branching isotomously prox- Ausich (in review comments, forwarded to S. K. D. on imally, both isotomously and heterotomously more distally. 18 February 2010) has brought to our attention the simi- Arms branching at least seven times, branches becoming more larity of the Oman material to Llanvirn Heviacrinus me- gracile after each bifurcation. Primibrachials robust, slightly lendezi Gil Cid, Domıńguez Alonso and Silvań Pobes, higher than wide, branching at IBr3. Secundibrachials more slen- 1996 (see also Ausich et al. 2002, 2007) from Spain, a der, branching about IIBr7. 528 PALAEONTOLOGY, VOLUME 54

A BC

TEXT-FIG. 2. Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865), latex casts of specimens in BMNH collection. A, BMNH EE 13822, details of more distal parts of arms, Scale bar represents 10 mm. B, BMNH EE 13823, curved pluricolumnal (compare with Pl. 1, fig. 1) Scale bar represents 10 mm. C, D, BMNH EE 13824, lateral and facetal views, respectively, of a short pluricolumnal, Scale bar represents 5 mm. All specimens coated with ammonium chloride. disparid genus lacking an anal sac. We have assumed Heviacrinus, then Heviacrinus is an iocrinid. We speculate above that the anal sac of the Oman specimen was not that Iocrinus may have evolved from a Heviacrinus-like preserved (=Iocrinus), whereas perhaps it was not present ancestor by a paedomorphic retention of a larval ⁄ juvenile (=Heviacrinus). This poses an obvious quandary and the anal sac or, vice versa, Heviacrinus is an Iocrinus that has Gondwanan occurrence of H. melendezi strengthens its lost these features (McNamara 1986). We prefer to call case. Although we are sympathetic to this suggestion, we our material Iocrinus (but certainly an iocrinid) and to consider the systematic position of H. melendezi equivo- suggest that Heviacrinus is a primitive member of the Ioc- cal. Gil Cid et al. (1996, p. 20) originally assigned the rinidae. Furthermore, the Oman specimens differ from genus Heviacrinus to the family Iocrinidae, whereas Aus- H. melendezi in having arms that branch at least seven ich et al. (2002, p. 989) moved it to the family Maennilic- times (see above) rather than four (Ausich et al. 2002, p. rinidae Ausich of the order Maennilicrinida Ausich in 990) and also have some heterotomous branches, unlike strict accordance with Ausich’s (1998) system of classifica- the Spanish taxon. Less significantly, but in the same tion. Yet, the morphological similarities of Heviacrinus description, Ausich et al. (2002, p. 990) speculated that and Iocrinus are surely too many to dismiss as conver- the ‘lumen was probably pentalobate,’ although the artic- gence; Iocrinus is a Heviacrinus with an anibrachial and ular facet geometry remains unknown; the Oman speci- an anal sac. To put it more plainly, if these specimens are mens have a rounded pentagonal lumen.

EXPLANATION OF PLATE 1 Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865). Latex casts of specimens that could not be collected and were left in the field. All specimens coated with ammonium chloride. The scale bar 10 mm applies to all figures. Fig. 1. BMNH EE 13820, open coiled pluricolumnal, probably coiled post-mortem rather than part of a distal spiral attachment structure (compare with Kelly 1978, pl. 1, figs 7, 8). Fig. 2. BMNH EE 13821(1), well preserved crown and proxistele. Column tapering just beneath cup, cup poorly seen, arms uniserial and branching isotomously at least seven times. Fig. 3. BMNH EE 13821(2), straight length of mesistele (probably distal to that seen in Pl. 1, fig. 2). Fig. 4. BMNH EE 13821(3), crown and proxistele, cup particularly well seen. Fig. 5. BMNH EE 13821(4), long, straight length of mesistele (from same region as specimen in Pl. 1, fig. 3). PLATE 1 3

1 4

DONOVAN et al., Iocrinus sp. cf. I. subcrassus 530 PALAEONTOLOGY, VOLUME 54

TEXT-FIG. 3. A, B, Iocrinus sp. cf. A C I. subcrassus (Meek and Worthen, 1865), articular facets. A, BMNH EE 13825. B, BMNH EE 13826. C, D, circular columnals from same deposit. C, BMNH EE 13827, pentagonocyclic columnal with planar articular facet (synostosis). D, BMNH EE 13828, columnal with marginal symplexy of short crenulae, circular areola and indistinct lumen. Scanning electron micrographs of specimens coated with gold-palladium. Scale bar represents 0.5 mm. B D

A finer grained and more calcareous sedimentary rock an attachment structure, Iocrinus did not have a radicu- was treated in 10 per cent acetic acid for conodont ana- lar holdfast for attachment as discussed above. Illustrated lysis. The heavy fraction (>2.80 specific gravity) of the examples include one columnal that is pentagonocyclic sediment, following sodium polytungstate separation, has with a synostosial articulation and may well represent yielded columnals of Iocrinus and a second taxon. The part of an immovable, radicular attachment structure morphology of the columnals of Iocrinus is well known (Text-fig. 3C). A second is more likely to represent a and the specimens figured herein (Text-fig. 3A,B) are columnal from a gracile disparid or other small crinoid, comparable to those figured from other species (Kelly with a marginal symplectial articulation, circular areola 1978, pl. 2, fig. 7; Donovan and Gale 1989, text-fig. and central lumen of indeterminate outline (Text- 4A,B,D). Other, cyclic columnals probably represent one fig. 3D). or two other crinoid taxa not known from more complete The Oman material is important for the following rea- material (BMNH EE 13827, EE 13828, EE 13831, EE sons. These are the oldest Iocrinus specimens to have been 13832). Although small enough to be radice ossicles from documented and Dapingian ⁄ Darriwilian crinoids continue

TEXT-FIG. 4. Simplified palaeogeography of part of the Southern Hemisphere during the Dapingian ⁄ Darriwilian (redrawn after Torsvik 2009). Iocrinus localities are: 1, Oman; 2, Wales; 3, New York; 4, Ohio and Illinois; and 5, Ontario. Key to continental blocks: Av, Avalonia; Ba, Baltica; La, Laurentia; NC, North China; SC, South China; Si, Siberia; and U, Urals. Coastlines stippled. DONOVAN ET AL.: IOCRINUS HALL FROM OMAN 531 to be poorly known. It is also the first Dapingian ⁄ Darriw- throughout, becoming circular in section distally. Perma- ilian crinoid to be described from Arabia; others are nent attachment by distal, non-planar, spiral coil. mainly known from North America and northern Europe. These specimens also add an unexpected extension of pal- Remarks. The smooth, conical cup and pentagonal trans- aeogeographical range to that of Iocrinus (Text-fig. 4). verse section of the proxistele of these three species unites Iocrinus has hitherto been limited to either side of the them as a single clade (Donovan and Gale 1989, text-fig. Iapetus Ocean between Avalonia (Wales) and, particu- 6) and differentiates them from Iocrinus sensu stricto. larly, Laurentia (North America); Iocrinus-like columnals Iocrinus has a broad geographical range – North America, are also recorded from the Upper Ordovician of Baltica Wales, Baltica(?) and now Oman – whereas Margoiocrinus (Briskeby 1981, pp. 115–116, figs 17, 39, pl. 6, figs 7–9). is limited spatially to Wales (I. brithdirensis) and the Hitherto, it was considered more probable that Iocrinus Welsh Borders, on the Avalonian plate (Text-fig. 4). was a Laurentian genus that migrated to Avalonia in the Although with a similar geometry of cup plating to Iocri- Middle Ordovician (Donovan 1989). But Oman (Gond- nus sensu stricto, the morphology of the cup and proxis- wana) was at a similar palaeolatitude to Avalonia during tele of Margiocrinus is distinct, and there is no possibility the Arenig. Based on what is now known and recognizing of confusing the genera on the basis of these diagnostic that further, well-dated specimens are needed from inter- features. mediate land areas, it is tempting to speculate that migra- tion from Gondwana to Laurentia would have been via Range. Ordovician (Llanvirn to Caradoc) of Wales and the Avalonia. (For a recent and thorough review of peri- Welsh Borders of England. Gondwanan palaeogeography in the Ordovician, see papers in Bassett 2009.) Acknowledgements. This work was supported in part by NERC It is also relevant to briefly note the longevity of Iocri- grant NE ⁄ B503576 ⁄ 1. We thank the Photographic Unit nus Hall sensu stricto, ranging as it does through most of (BMNH) for providing the images in Plate 1 and Text-figure 2. The considered inputs of Martha Richter (BMNH) and Felicity the Middle and Upper Ordovician. A Dapingian ⁄ Darriw- Heward (Oman) are gratefully acknowledged. S. K. D. thanks ilian (=lower Middle Ordovician) Iocrinus is not surpris- Professor David Harper (University of Copenhagen) for discus- ing stratigraphically, although better preserved material is sions of Ordovician palaeogeography. We gratefully acknowledge still desirable to confirm its existence biogeographically the thought-provoking reviews of Professors William I. Ausich and to answer the Iocrinus ⁄ Heviacrinus question. (The Ohio State University, Columbus) and Thomas E. Guens- burg (Rock Valley College, Illinois).

Genus MARGOIOCRINUS Donovan gen. nov. Editor. George Sevastopulo

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