Palaeogeographic implications of a new iocrinid crinoid (Disparida) from the Ordovician (Darriwillian) of Morocco

Geological setting and stratigraphy

Ordovician outcrops are very well developed and exposed in the Anti-Atlas Mountains of Morocco (Destombes, Hollard & Willefert, 1985). Many units yield well-preserved specimens of echinoderms, a number of which are currently under study (e.g., Hunter et al., 2010; Van Roy et al., 2010; Van Roy, Briggs & Gaines, 2015; Sumrall & Zamora, 2011; Martin et al., in press), and these faunas occur throughout sections from the Lower to Upper Ordovician. Numerous clades of echinoderms have been documented, including stylophorans, solutes, blastozoans, crinoids, asteroids, edrioasteroids, and cyclocystoids.

The Ordovician succession in the Anti-Atlas region is divided into the following lithostratigraphic units: the Outer Feijas Shale Group, the First Bani Sandstone Group, the Ktaoua Clay and Sandstone Group, and the Second Bani Sandstone Group (Choubert, 1943; Choubert & Termier, 1947; Destombes, Hollard & Willefert, 1985). The Outer Feijas Shale Group includes the Lower and Upper Fezouta formations (Tremadocian–Floian) and the Tachilla Formation (Darriwilian) (Fig. 1). These units are characterized by siltstones that are rich in graptolites, with some thin sandstone interbeds, and contain exceptionally preserved Burgess Shale-type faunas in places (Van Roy et al., 2010; Van Roy, Briggs & Gaines, 2015; Martin et al., in press). The overlying First Bani Group spans the Darriwilian to Sandbian and is subdivided into five formations (Taddrist, Bou-Zeroual, Guezzart, Ouine-Inirne, and Izegguirene formations) that are chiefly comprised of sandstones with interbedded shales. This group is the thickest, most constant, and most extensive sandstone group in the Anti-Atlas Mountains (Destombes, Hollard & Willefert, 1985). The fossil taxa recovered from the First Bani Group were reviewed by Gutiérrez-Marco et al. (2003), and there are no reports of crinoids from this time interval.

The First Bani Group is overlain by the Ktaoua Clay and Sandstone Group (Sandbian–Katian), which is comprised of siltstones interbedded with two or three sandstone units, depending on the exact position within the Anti-Atlas Mountains. It is divided into three units: the Sandbian to Katian Lower Ktaoua Formation, the Katian Upper Tiouririne Formation, and the Katian Upper Ktaoua Formation. The Ordovician ends with the Second Bani group, which is Hirnantian in age.

The new locality yielding Iocrinus africanus sp. nov. lies in the Taddrist Formation, low in the First Bani Group, close to the village of Battou (south Alnif, eastern Anti-Atlas) (Figs. 1 and 2). This locality was recently described by Rábano, Gutiérrez-Marco & García-Bellido (2014), who provided detailed information about the faunal content and age based on the presence of key graptolites and trilobites. In this area, the Taddrist Formation has been excavated predominantly by local collectors and has yielded a rich faunal assemblage preserved in carbonate concretions (Fig. 3). Rábano, Gutiérrez-Marco & García-Bellido (2014) suggested that the levels containing fossiliferous concretions belong to the Didymograptus murchisoni Biozone (Gutiérrez-Marco et al., 2003), which is assigned to the upper Oretanian, a regional stage roughly equivalent to the upper Darriwilian 2/basal Darriwilian 3 stage slices of the global chronostratigraphic scale (Gutiérrez-Marco, Sá & Rábano, 2008; Bergström et al., 2009). According to Rábano, Gutiérrez-Marco & García-Bellido (2014), the fossiliferous concretions have yielded the trilobites Caudillaenus nicolasi (Rábano, Gutiérrez-Marco & García-Bellido, 2014), Morgatia? rochi (Destombes, 1972), Placoparia (Coplacoparia) sp. nov., Colpocoryphe sp., Parabarrandia aff. crassa (Barrande, 1872), and an undetermined cheirurid (Eccoptochile? sp.). Other non-trilobite fossils include molluscs (e.g., a cyrtonellid tergomyan, bivalves such as Praenucula sp., and orthoconicnautiloids), hyoliths (Elegantilites sp.), echinoderms (Diploporita and Asterozoa indet.), conularids (Exoconularia sp.), and rare graptolites (Didymograptus sp.). In addition to the crinoid described herein, new cyclocystoids, the first ever reported from Africa, were recently presented from this locality and await formal description (Sprinkle, Reich & Lefebvre, 2015).

Terminology

The terminology used below follows Moore (1962), Webster (1974), Ubaghs (1978), and Ausich et al. (1999); the classification follows Ausich (1998). Note, the terminology used for the aboral plates differs from that of Ausich, Gil Cid & Domínguez Alonso (2002). In addition, superradial and inferradial are used to designate radially positioned plates where two plates are in the C ray portion of the radial circlet. This usage recognizes the homologies of these plates (essential for phylogenetic analysis) rather than using unique names that obscure homology, such as anibrachial and brachianal as outlined in Ubaghs (1978). The present usage is consistent with many recent studies (e.g., Ausich, 1998; Ausich & Copper, 2010; Ausich et al., 2015; Ausich, Rozhnov & Kammer, 2015), although the Ubaghs (1978) terminology for these plate is also used (e.g., Guensburg, 2010).

Nomenclatural acts

The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is: http://zoobank.org/References/3F137EAC-ECB5-4FF8-9BA5-8B1A0E1BC986. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

Description

Crown small in size. Aboral cup medium bowl-shaped; smooth plate surfaces; radial and basal plates sharply convex.

Basal circlet 27 percent of aboral cup height; five basal plates, approximately two times wider than high, much smaller than radial plates. Radial circlet 73 percent of aboral cup height; radial plates five, maximum height approximately 1.25 times higher than maximum width; maximum width of radial plate at mid-height, radials narrow sharply proximally, maximum width more than 10 times proximal width; maximum width 1.6 times distal width. Radial facets peneplenary, approximately as deep as wide. A, B, D, E radial plates simple, C radial compound; C inferradial approximately same size as simple radials; C superradial much smaller than C inferradial, wider than high, distal heterotomous division with anal plates to left and C-ray arm to right.

All anal plates above aboral cup; column of 16 stout anal sac plates preserved from the left facet on the C-ray superradial, plates very convex, successive plates with bend yielding a sinuous appearance for this column of plates; each plate higher than wide, otherwise very similar to shape of brachials. Other anal sac plates disarticulated and collapsed within the crown, presumably sac plates were lightly calcified or uncalcified, except for the column of plates from the C superradial.

Arms robust, primaxil varies from third to fifth primibrachial (45553; ABCDE), secundaxil fourth or fifth secundibrachial; where known, tertaxil positioned on the seventh or thirteenth tertibrachial; as many as 16 unbranched quartibrachials on an incomplete branch of the A-ray arm. Brachials strongly convex aborally with flattened lateral, abambulacral extensions, rectangular uniserial, deep ambulcacral groove, more proximal brachials approximately 1.7 times wider than high. Brachial facet with two, merging aboral ligament fossae. Primaxial approximately the same size as non-axillary primibrachials; remaining brachials diminish in size distally.

Column strongly pentastellate, holomeric, heteromorphic, proximal column N3231323; nodals higher than priminternodals, obvious heteromorphic pattern lacking in mesistele, large portion of columnal facets presumably a petaloid articulation (but details not preserved). Preserved column higher than crown height and preserved in an open coil.

Remarks

Characters differentiating genera within the Iocrinidae are listed in Ausich, Rozhnov & Kammer (2015). The combination of visible basal plates, three to five primibrachials, no fixed interradial plates, pentalobate/pentastellate columnal shape, holomeric column construction, and a petaloid facet clearly align the new crinoid described herein with the genus Iocrinus. Another feature that identifies the specimen as belonging to Iocrinus is the preservation of the column in an open coil. This is similar to Iocrinus subcrassus, which is thought to have had a holdfast that could coil around erect objects (Kelly, 1978; Brett, Deline & McLaughlin, 2008; Meyer & Davis, 2009).

Species-level characters within Iocrinus include: the height of the basal plates, the height of the radial plates, radial plate height versus width, presence and character of the transverse ridge between adjacent radial plates, primibrachial shape, number of primibrachials, number of secundibrachials, number of tertibrachials, maximum number of in-line bifurcations in a ray, anal sac plating, and the shape of the proximal columnal (Table 1). Iocrinus africanus sp. nov. is distinguished from other Iocrinus species based on the shape of the radial plates, the number of tertibrachials, the number of bifurcations in-line per ray, and the lack of or very light plating of most of the anal sac.

Donovan et al. (2011) reported the only other putative Iocrinus known from Gondwana, I. sp. cf. I. subcrassus from the Middle Ordovician of Oman. Assuming that this taxon does belong to Iocrinus, which cannot be confirmed without further information about the CD-interray and C-ray morphologies, the new Moroccan species differs from the Donovan et al. (2011) specimen as follows. Iocrinus africanus sp. nov. has a basal plate height approximately 37 percent of radial plate height; a broad transverse ridge; primibrachials 1.5 times wider than high; four to five secundibrachials; and three in-line bifurcations per ray. In contrast, I. sp. cf. I. subcrassus has a basal plate height approximately 50 percent of radial plate height; a narrow transverse ridge; primibrachials slightly higher than wide; seven secundibrachials; and as many as seven in-line bifurcations per ray.

Taxonomic assignments within the Iocrinidae have received some attention in the last three decades (Warn, 1982; Guensburg, 1984; Donovan, 1985; Donovan, 1989; Ausich, Rozhnov & Kammer, 2015); with the new species described herein, a total of eight species and one subspecies are currently recognized for Iocrinus (Webster & Webster, 2014). These include the Laurentian species: I. crassus (Meek & Worthen, 1865); I. similis (Billings, 1865); I. subcrassus (Meek & Worthen, 1865); I. subcrassus torontoensis (Fritz, 1925); and I. trentonensis (Walcott, 1884); and the Avalonian species: I. llandegleyi (Botting, 2003); I. pauli (Donovan & Gale, 1989); and I. whitteryi (Ramsbottom, 1961) (Table 2). Additional Iocrinus identifications left in open nomenclature are known from Avalonia, Laurentia, and Gondwana (for the previous potential Gondwanan occurrence, see Donovan et al., 2011). Iocrinus africanus sp. nov. is Darriwilian in age, and thus it is among the oldest members of the genus (Table 2). In terms of morphology, it is equally dissimilar to species from both Laurentia and Avalonia. The occurrence of I. africanus sp. nov. in Morocco confirms the presence of Iocrinus in Gondwana and demonstrates that Iocrinus, together with Merocrinus, is the most geographically widespread Ordovician crinoid genus.

The use of micro-CT was essential for describing the morphology of Iocrinus africanus sp. nov. in full. The posterior interray is buried below the surface of the concretion and is hence not visible in the latex casts (Fig. 4); however, the posterior interray and the C-ray can be clearly seen in the micro-CT scans (Fig. 5; Data S1, S2, http://dx.doi.org/10.5523/bris.uv7qt4c6kpat1befj0937ooml). Without an understanding of these characters, it would not have been possible to confidently assign the specimen to the genus Iocrinus.