Eospondylus GREGORY 1897

Hotchkiss, Frederick H. C., Prokop, Rudolf J. & Petr, Václav, 2007, Isolated Ossicles Of The Family Eospondylidae Spencer Wright, 1966, In The Lower Devonian Of Bohemia (Czech Republic) And Correction Of The Systematic Position Of Eospondylid Brittlestars (Echinodermata: Ophiuroidea: Oegophiurida), Acta Musei Nationalis Pragae Series B 63 (1), pp. 3-18 : 7-8

Genus Eospondylus GREGORY 1897

The family Eospondylidae contains Eospondylus with five rays and Kentrospondylus with ten rays. Gregory (1897) erected the genus Eospondylus with type species Ophiurella primigenia STÜRTZ, 1886 . Stürtz (1886) described O. primigenia as an ophiuroid of Mesozoic aspect and placed it in the genus Ophiurella AGASSIZ, 1836 . Many excellent photographs of E. primigenius have been published (e.g. Smith 1985, pl. 7.7.2; Bartels, Briggs et Brassel 1998, Text-fig. 194). In a large museum collection of Hunsrück Slate ophiuroids, E. primigenius ranked ninth in number of specimens and comprised 5% of the collection ( Südkamp 1994). Material-based key papers are by Stürtz (1886, 1890, 1893), Spencer (1925), Lehmann (1957) and Boczarowski (2001). The location and registration numbers of the type specimens of E. primigenius , including designating lectotype and paralectotype specimens, is being researched by Alexander Glass (personal communication, May 2005). Starting with its initial description and up to the present, E. primigenius holds an important place in discussions on the origination and ancestry of crown group ophiuroids (especially Stürtz 1893; Spencer et Wright 1966).

Distinguishing morphological characteristics of Eospondylus include: Disk circular, small (incorporates only two arm vertebrae), without oral shields, without radial shields, and without calycinal plates (primary rosette is not evident in post-juvenile stages but is likely in early growth stages). A madreporite is present on the underside of the disk, interradially, near the mouth frame ( Lehmann 1957) (very rarely seen). Arms high, robust, without dorsal or ventral arm plates. Individual segments of the arm long. Ambulacral ossicles completely fused into vertebrae. A median dorsal cleft deeply indents the upper surface of each vertebra. The vertebrae have complex articular processes and facets, including an unpaired ventral median articular process that is perforated by the radial water vessel channel ( Boczarowski 2001). Laterals well separated on the aboral surface and not conspicuous except for the vertical spine ridge. The spine ridges carry long spines, of unequal length, splayed outward from the direction of the arm length, creating a dramatic appearance. Laterals conspicuous on the oral surface of arms, closely approximated, somewhat pearshaped, and bearing spaced minute pointed groove spines.

Eospondylus grew to fairly large size and had fairly large arm vertebrae. Spencer (1925, p. 309) listed “arm high” and “individual segments of the arm long” as generic characters. The arms appear to be about as high as wide. In BMNH E3355 the length of the rays is> 150 mm; the disk diameter (distorted) is 30 mm x 16 mm; the vertebrae are 4 mm to 5 mm wide (width of ray); the arm segment length is 3.1mm (24 arm segments in 75 mm arm length). Spencer (1925, p. 312) reported the length of an arm segment in BMNH E3355 as 4.0mm, and in BMNH E3353 and E3358 as 2.4 mm. In NM S 4767 the disk extends to junction of 2nd and 3rd vertebrae; mouth frame 11 mm x 7 mm (distorted by rock deformation); longest arm is 117 mm and appears not distorted; the arm segment length is 2.5 mm (17 proximal segments in 43 mm arm length); the vertebrae are approximately 3 mm wide (versus distorted arm width of 7 mm). In NM S 4768 the mouth frame is approximately 6mm diameter; the arm segment length is 1.6 mm (proximal 7 arm segments in 11.5 mm arm length); proximal vertebrae approximately 3 mm wide (width of ray at 5th and 6th arm segments).

Eospondylus primigenius and E. primigenius var. compactus occur in the Lower Devonian Hunsrück Slate, Germany ( Lehmann 1957). The age of the Hunsrück Slate is end of Pragian (Siegenian) to middle of the Lower Emsian ( Bartels, Briggs et Brassel 1998, p. 19). Eospondylus tenuis was described by Withers et Keble (1934) from the Silurian Yarravian series, Moonee Ponds, Victoria, Australia ( Clark 1946, p. 165). Gill et Caster (1960, p. 32) listed Eospondylus cf. tenuis from the Lower Devonian, Collins’ Quarry, Kinglake, Victoria, Australia. Australia was included in the distribution records of Eospondylus by Spencer (1950, p. 407) and Ubaghs (1953, p. 821) but not by Spencer et Wright (1966, p. U90), perhaps indicating their opinion that E. tenuis does not belong in this genus. Haude (1998, see also 2004) listed Eospondylus ? n. sp., from the Lower Devonian Talacasto Formation (Lochkovian to Emsian) of the Argentine Precordillera. Boczarowski (2001) described Eospondylus ingens from vertebrae and lateral plates that were chemically extracted from the Middle Devonian, Early Givetian, Skały Beds, Holy Cross Mountains, Poland. Eospondylus sp. was reported from the upper part of the Selscheid Beds (Middle Devonian, Upper Eifelian) of the Attendorn-Olpe area (western Sauerland) by May (2003). We report Eospondylus sp. isolated vertebrae and a lateral plate from the Dvorce-Prokop Limestone (Pragian age), the Loděnice Limestone (Pragian age) and the Třebotov Limestone (Dalejan age, Late Emsian).

A feature of the Hunsrück Slate ophiuroid Eospondylus primigenius ( Text-fig. 1 ) is that the rays often are bent into strong curves ( Spencer 1925, Pl. 21, Text-fig. 9 ; Lehmann 1957, Pl. 15). Usually the bending of the rays is horizontal but sometimes is vertical ( Spencer 1925, p. 310, Bonn specimen, Lehmann 1957, Pl. 16). A clump of Eospondylus with tangled arms was reported by Lehmann (1957). During this study we obtained a specimen with arms curved ventrally under the disk in euryalid fashion (NM S 4764; Text-fig. 2 ) and one with aboral arm flexure (NM S 4765). Such flexibility and attitudes of the rays is not seen in other Hunsrück ophiuroids ( Lehmann 1957) and differentiates the behavioral ecology of Eospondylus from other ophiuroids. It also has been noticed that Eospondylus and crinoids sometimes are preserved in intimate association and may have lived together ( Stürmer 1968, figs 2, 3; Bartels, Briggs et Brassel 1998, figs 151, 155; Bartels et al. 1997, figs 48, 56). Haude (2004, p. 418) reported from the Lower Devonian of Argentina “an eospondylid that lived as a commensalist on large rheophilic crinoids.”

The above observations on the behavioural ecology of Eospondylus , especially the vertical bending and flexibility of the rays and the intimate association with crinoids, were used by Spencer et Wright (1966) to classify Eospondylus in the Order Euryalida , which contains the epizoic gorgonocephalid and euryalid basket-stars of modern oceans. Thus Spencer et Wright (1966, p. U27-28) wrote [we paraphrase] that by Devonian time two ophiuroid groups had developed, one with arms that allowed for active movement on the sea floor, the other with arms that allowed for a commensal life attached to crinoids and other hosts. The second of these groups had arms that could climb up and grip onto hosts such as crinoids by means of the vertical rolling of the arms and small hooked spines. This group consists of suspension-feeders collecting organic particles by their own ciliary action, aided by that of their hosts. With this prologue, Spencer et Wright (1966, p. U90) reclassified Eospondylus into its own Family Eospondylidae , and placed that family in the Suborder Euryalina . They listed the content of the Euryalina as the Palaeozoic families Eospondylidae and Onychasteridae , and the Recent families Asteronychidae , Asteroschematidae , Gorgonocephalidae and Euryalidae ( Spencer et Wright 1966, p. U38). Fell et Pawson (1966, p. 36) suggested from retained metapinnular structure in Asteronyx that the Euryalida “derived from some stock nearer to the somasteroids than the Oegophiurida ” and thus are of great antiquity. Latest classifications treat the Euryalina as the Order Euryalida ( Smith, Paterson et Lafay 1995) (see also Clark et Courtman-Stock 1976, p. 127 [footnote]). Spencer et Wright (1966) clearly recognized the appearance of Eospondylus in the Lower Devonian as the stratigraphic first occurrence of the Euryalida . This status is an important reason to review the classification of Eospondylus using the new evidence from isolated vertebrae.

Figures

Text-fig. 1. Eospondylus primigenius (STÜRTZ) Bundenbach, Eschenbach-Bocksberg quarry, Lower Devonian, Lower Emsian (Zlichovian), Hunsrück Slate, specimens PWL 1996 57LS (NMM) top image (A) x 1.5, and PWL 1996 66LS (NMM) bottom image (B) x 1.7. Specimen with rays bent into strong curves. Such flexibility and attitudes of rays differentiate the behavioral ecology of Eospondylus from all other Hunsrück Slate ophiuroids. Delicateness of the top specimen and robustness of the bottom specimen is perhaps the combined result of real phenotypic differences, differences of preserved detail, and differences of distortion of the rock. [courtesy of Herbert Lutz and Thomas Engel; photos by Alexander Glass].

Text-fig. 9A. Eospondylus cf. primigenius (STÜRTZ) "Červený lom" quarry near Praha-Klukovice, Loděnice Limestone, Lower Devonian, Pragian, NM L 36910, x 40. Left lateral plate. Inner surface view from below. The spine ridge faces distally (posteriorly) and has blunt pointed denticles. Anterior and parallel to denticled part of spine ridge is internal rounded ridge. Ventrolaterally ossicle has slit-like sockets for groove spines.

Text-fig. 2. Eospondylus primigenius (STÜRTZ) Bundenbach, Eschenbach-Bocksberg quarry, Lower Devonian, Lower Emsian (Zlichovian), Hunsrück Slate,, NM S 4764, x 3. Specimen with ventral arm coiling. The specimen is on its dorsum in slate with all five rays curled ventrally inward toward mouth area on underside of disk. Barely visible are tips of two jaws; slightly exposed are proximal parts of rays in oral view extending outward from disk. The location of abrupt ventral bending of rays is indicated by emergence from slate of five rays in aboral view that point inward toward buried disk. Based on ventral bending of rays and intimate association with crinoids Eospondylus has been interpreted as stratigraphic first occurrence of Order Euryalida, which contains epizoic gorgonocephalid and euryalid basket-stars of modern oceans. This status is rejected using new evidence from isolated vertebrae. [Photo by Alexander Glass].