A NEW STYLOPHORAN ECHINODERM 



49 



press ), I now remain very sceptical when it comes to a consideration 

 of the fossil evidence favouring the calcichordate interpretation of 

 the stylophorans. Such an interpretation discounts obvious morpho- 

 logical and topological similarities between the anatomical 

 organization of comutes and that of mitrates and makes poorly 

 supported or unsubstantiated claims as to the morphological changes 

 which occurred at the cornute-mitrate transition (summary in 

 Jefferies. 1986 and Cripps, 1989a). 



The wealth of morphological data produced by Jefferies and co- 

 workers is impressive. However, the morphological comparative 

 analysis of the relevant fossils poses certain problems, especially as 

 far as the anatomical evidence in support of the transition from 

 comutes to mitrates is concerned. The theory is, therefore, rejected in 

 favour of widely accepted and well-corroborated views on skeletal 

 homologies in the stylophorans. 



Terminology 



Morphological nomenclature, plate notation and anatomical orienta- 

 tion of the stylophorans are those proposed by Ubaghs (1968) in the 

 Treatise on Invertebrate Paleontology (see also Ubaghs, 1969) and 

 adopted by subsequent workers (e.g. Parsley, 1988, 1991, 1994, 

 1997, 1998, Sumrall et ai, 1997, Lefebvre et al., 1998, etc.). The 

 identification of the marginal plates in the new stylophoran de- 

 scribed here is based on a recent character analysis by Parsley ( 1 997, 

 1998) and on a comparative study of morphological data gleaned 

 from various sources, especially Ubaghs (1968, 1969), Jefferies & 

 Prokop ( 1 972), Cripps ( 1 989fl), Cripps & Daley ( 1 994) and Lefebvre 

 effl/. (1998). 



A list of the morphological terms applied to the stylophorans and 

 a detailed description of their external and internal anatomical 

 organization were provided by Ubaghs (1968). I emphasize the fact 

 that, in most cases, such a terminology does not imply anatomical or 

 functional interpretations of the observed structures. As stated by 

 Ubaghs (1968), the interpretations are often entirely conjectural 

 (see, however, Jefferies, 1986 and discussion in Ruta, 1998, in press). 



The most frequently cited morphological terms used in the ana- 

 tomical description (Fig. 1 ) are briefly explained below and appear 

 in bold in this section of the paper only. Most of them apply to all 

 stylophorans, a few are restricted to the vast majority of these fossils 

 and some are introduced here in conjunction with the description of 

 the new taxon. 



A stylophoran is divided morphologically into a posterior theca 

 and an anterior aulacophore, the latter representing an arm (i.e. a 

 feeding organ of ambulacral origin extending beyond the oral sur- 

 face; see Sumrall, 1997). The convex and flat surfaces of the theca 

 consist of a dorsal integument and a ventral integument respec- 

 tively. The main thecal opening, at the opposite end of the theca with 

 respect to the aulacophore insertion, is the anus. The main axis, or 

 longitudinal axis of the body, is the intersection between the plane 

 of bilateral symmetry of the aulacophore and the plane of flattening 

 of the theca. The oro-anal axis runs from the centre of the aulacophore 

 insertion to the anus. Proximal and distal indicate the position of a 

 structure close to or away from the junction between the theca and 

 the aulacophore. Median, admedian and lateral (or marginal) 

 indicate the position of thecal structures (e.g. integument plates) 

 with respect to the longitudinal axis. 



The theca is framed by marginal plates, or marginalia, sur- 

 rounding several central plates, orcentralia, termed supracentralia 

 and infracentralia on the dorsal and ventral surfaces respectively. 

 The marginalia are labelled antero-posteriorly (i.e. from the thecal 

 excavation for the aulacophore insertion to the anus) as M', , M',, M',, 

 . . ., etc. on the left side, and as M , M„ M,, . . ., etc. on the right side 



of the theca. Each marginal plate consists of a dorso-lateral projec- 

 tion, sloping laterally and ventrally, and a ventral projection. The 



two projections meet at the lateral margins of the theca. The two 

 anterior supracentral plates articulated with M'^ and M, and contrib- 

 uting to the excavation for the aulacophore insertion are the left and 

 right adoral plates, or adoralia (LA and RA respectively). 



A large, flexibly articulated plate (perhaps an enlarged supracentral 

 element) present between the rearmost ends of the thecal margins 

 and roofing over the anal opening in the new stylophoran described 

 here is termed the suranal plate, or suranal. 



The zygal bar, orzygal (apparently absent in such cornute genera 

 as Ceratocystis Jaekel, 1901 and, perhaps, also Nevadaecystis 

 Ubaghs, 1968 and Protoc-v.s//rey Hicks, 1872; see also Ubaghs, 1963, 

 1967, 1987, Jefferies, 1969, 1986 and Jefferies t-r a/., 1987). is a strut- 

 like bar connecting M'| with the posterior part of the right margin of 

 the theca on the ventral surface and is homologous with the septum 

 of mitrates (Ubaghs, 1968, 1969;Kolata& Jollie, 1982; Kolataeffl/., 

 1991; Parsley, 1991, 1997, 1998; Lefebvre et ai, 1998). In several 

 stylophorans, such as the new form described in this work, the zygal 

 consists of a posterior zygal plate (a somatic element sutured with 

 the right posterior part of the thecal frame but not included in the 

 latter) and a posterior process of M'^. 



The proximal part of the aulacophore usually consists of 

 tetramerous rings, each composed of a pair of dorsal plates, termed 

 the tectals. and a pair of ventral plates, or inferolaterals. The robust 

 ventral element present in the intermediate part of the aulacophore is 

 the styloid or stylocone. The term styloid is preferred over stylocone 

 when the structure in question bears well-developed projections 

 (spikes, cusps, blades, etc.), as in mitrates and in several mitrate-like 

 comutes (Jaekel, 1918; Parsley, 1991, 1997, 1998; Lefebvre et al, 

 1998). However, the distinction between stylocone and styloid has 

 no morphological basis, as correctly pointed out by Ubaghs ( 1968, 

 1981), for these terms refer to the same (homologous) skeletal 

 element in all stylophorans. Such a distinction was adopted by 

 Jefferies (1986) who denied the homology between the comute 

 stylocone and the mitrate styloid (but see rebuttal by Lefebvre et al., 

 1998), as explained above. 



Each segment of the distal part of the aulacophore consists of a 

 ventral ossicle articulated with a dorsal pair of cover plates. The 

 stmctures visible on the internal surface of the aulacophore ossicles 

 and those present on the dorsal (internal with respect to the inside of 

 the theca) surface of the most anterior portions of M'^ and M| will be 

 discussed separately in the descriptive anatomical section of this 

 paper. 



AIMS OF THE STUDY 



Background 



In the collections of the Department of Palaeontology, The Natural 

 History Museum, London, there is abundant material of a new 

 stylophoran echinoderm from upper Ashgill mudstones in Morocco, 

 collected in the early 1990s. Several features of its dorsal and ventral 

 integuments, thecal frame and aulacophore justify its attribution to a 

 new genus and species, although it is similar in several respects to 

 such mitrate-like comutes as Reticulocarpos luinusi Jefferies & 

 Prokop, 1972, Domfrontia pissotensis (Chauvel & Nion. 1977) and 

 Beryl I ia miranda Cripps & Daley, 1994 (see section on morphologi- 

 cal comparisons below). 



In traditional classifications of the stylophorans (e.g. Ubaghs, 

 1 968), the fossils described herein would be assigned to the comutes, 

 because it possesses a zygal bar extending from plate M' , to the right 



