50 



M. RUTA 



antenor 

 thecal excavation 



A 



^i 



T/l 



E 

 R 

 I 



O 

 R 



P 

 O 

 S 

 T 

 E 

 R 



i* 



R 



dcnticulations 



Fig. 1 Juliaecarpus milnewrum gen. et sp. nov. Late Ordovician, Upper Ashgill, Rawlheyan, Upper Ktaoua Formation, Morocco. Schematic 

 reconstruction showing orientation, anatomical nomenclature and plate terminology of the theca. A, ventral aspect. B, dorsal aspect. 



posterior part of the thecal frame, because its marginal plates are 

 elongate and rod-like and because its dorsal and ventral integuments 

 consist of a relatively large number of plates. In Parsley's (1997, 

 1998) recent phylogenetic analysis, the above-mentioned taxa to 

 which the new Moroccan form bears close resemblance, several 

 other almost bilaterally symmetrical and mitrate-like comutes and 

 all the mitrates are set apart from the strongly asymmetrical (and 

 often boot-shaped) comutes and are given a rank of their own, for 

 which Parsley (1997, 1998) proposed the name Ankyroida. The new 

 Moroccan taxon would be an ankyroid according to Parsley's (1997, 

 1998) phylogenetic scheme. 



The order Ankyroida comprises stylophorans in which the thecal 

 outline approaches bilateral symmetry in varying degrees, the poste- 

 rior zygal plate is usually a separate somatic element (i.e. this plate 

 is not part of the marginal frame of the theca) and the styloid bears 

 projections in the form of spikes, cusps or blades. However, 

 stylophorans other than the ankyroids form a paraphyletic assem- 

 blage in Parsley's (1997, 1998) phylogeny. Parsley (1997, 1998) 

 retained the former name Comuta as emended by Ubaghs ( 1968) for 

 this assemblage (not characterizable in the light of shared derived 

 features), but suppressed the Mitrata. 



The present study aims to: 1) reconstruct and describe a late 

 Ordovician Moroccan ankyroid; 2) discuss its morphology from a 

 functional viewpoint, with emphasis placed on the stability of the 

 animal in water currents and on the feeding and locomotory roles of 

 its aulacophore; 3) compare it with several other Ordovician 

 ankyroids. 



The discovery of the new Moroccan taxon prompts a reassessment 



of stylophoran interrelationships as well as a critical analysis of the 

 comute-mitrate transition (Cripps, 1989fl, b, 1991; Daley, 1992; 

 Cripps & Daley. 1994) in the light of more traditional views on 

 homologous skeletal characters in these echinoderms. Such argu- 

 ments will be developed in another paper. 



Material and methods 



Most specimens are preserved as incomplete, slightly to heavily 

 disrupted external moulds of the theca and aulacophore. Incomplete 

 internal moulds of isolated thecal elements (especially marginalia) 

 and, more rarely, of almost complete thecae have also been recorded. 

 In very few individuals, the thecae are fully articulated and the 

 degree of disruption affects mainly the alignment of the left and right 

 marginalia. The dorsal integument is usually completely preserved, 

 both externally and, more rarely, internally. The ventral integument, 

 on the other hand, is invariably poorly preserved, heavily disrupted 

 or missing and the boundaries between the infracentralia are difficult 

 to observe. The posterior end of the theca is generally disrupted. 



The zygal bar is often found broken, but in some individuals, both 

 the posterior zygal plate and the posterior process of M' | are uncrushed 

 and their relative size and proportions with respect to the rest of the 

 theca can be accurately reconstructed. The loss of supracentralia in 

 few specimens allows the dorsal surface of the zygal bar (and 

 especially its suture with the thecal frame) to be observed. Some- 

 times, compaction of the fossils resulted in the squashing of the 

 supracentralia against the zygal bar, thus reproducing the boundaries 

 of the latter and the nature of the contact of the distal end of the 



