54 



M. RUTA 



width ratio of about 1 .72. The maximum thecal width is at about two- 

 fifths of the way posteriorly along the oro-anal axis. 



A comparison between the maximum thecal sizes reported by 

 Cripps & Daley (1994) in several solutes, cornutes and mitrates and 

 the thecal sizes of some of the most complete and least disrupted 

 specimens of Juliaecarpus shows that the latter ranks among the 

 smallest stylophorans recorded to date. Estimated thecal length 

 values in the best preserved specimens range between 7 mm and 10 

 mm, whereas thecal width values in the same specimens are com- 

 prised between 4 mm and 6 mm (see also Table 1). As discussed 

 below, morphological variation, both in the general thecal propor- 

 tions and in the shape and size of the individual plates, is small. 



The lateral thecal margins are only slightly convex for most of 

 their length, subparallel or gently diverging in their anterior half, 

 slightly converging posteriorly and merging smoothly into the antero- 

 lateral, lobe-like angles of the theca. These are blunt and shaped 

 approximately like an arc of about 90° in plan view. The left margin 

 of the theca is slightly longer than the right margin, which suggests 

 that the anal opening may have faced slightly rightward in life (see 

 also Cripps & Daley, 1994). As a result of this difference in length, 

 the oro-anal axis is not aligned with the longitudinal axis of the 

 aulacophore, but is displaced slightly to the right of the latter. 



A dorsal and a ventral integument, each composed of several 

 polygonal plates of different shapes and sizes, are attached to the 

 medial margins of the dorso-lateral and ventral projections of the 

 marginalia. A large, irregularly polygonal supracentral element, the 

 suranal plate, roofs over the anal opening and is interposed between 

 the rearmost parts of the lateral thecal margins. 



The boundaries of the anal opening could not be observed directly 

 in any of the specimens examined, due to the extreme dorso- ventral 

 flattening of the theca in its posterior third, to the generally poor 

 preservation of the rearmost infracentralia and to the fact that the 

 suranal plate is often found collapsed onto the internal side of the 

 ventral thecal surface. 



In some individuals, however, both the dorsal and the ventral 

 thecal surfaces are preserved more or less fully articulated and a 

 morphological comparison between part and counterpart suggests 

 that the suranal plate either extended slightly beyond the posterior 

 margin of the ventral integument or corresponded in position with 

 the latter (see also description below). Thus, the anal opening faced 

 either slightly ventralward or posteriorly in life and, as in most other 

 ankyroids, was almost certainly slit-like, although the reconstruction 

 of its outline is only tentative. 



In longitudinal section, the theca of Juliaecarpus is shaped ap- 

 proximately like a hydrofoil (see also discussion on the 

 morpho-functional adaptations of the theca in relation to the stability 

 of the animal in water currents). The dorsal integument is likely to 

 have been smoothly convex in lateral view and to have reached its 

 maximum curvature at a point situated presumably slightly anterior 

 to its central part. It probably sloped gently rearward and ventralward 

 in life, as suggested by the fact that the height of the marginalia 

 decreases uniformly antero-posteriorly, being about twice greater at 

 the level of the anterior thecal excavation than at the level of the anus. 



Transverse sections of the theca at various levels along the longi- 

 tudinal axis of the aulacophore are probably not very dissimilar in 



Fig. 3 Juliaecarpus milnerorum gen. et sp. nov. Late Ordovician, Upper 

 Ashgill, Rawtheyan, Upper Ktaoua Formation, Morocco. Cross-section 

 of the marginalia at different levels along the thecal frame. 



outline. Major morphological differences are due to the different 

 curvature and width of the dorso-lateral projections of the marginalia, 

 both of which decrease antero-posteriorly (Fig. 3). 



Variation. Morphological differences between the most com- 

 plete specimens of Juliaecarpus (see Table 1) as well as between 

 isolated, homologous skeletal elements of its theca (especially 

 marginalia, notably M',) are negligible. Ontogenetic variation (if 

 any) could not be detected, since the most complete individuals 

 differ little in size and, therefore, their ontogenetic ages are assumed 

 to be approximately the same. 



Table 1 Estimated maximum thecal widths and lengths in some 



individuals oi Juliaecarpus milnerorum gen. et sp. nov. Late Ordovician, 

 Upper Ashgill. Rawtheyan. Upper Ktaoua Formation, Morocco. 



Specimen 



Thecal length 



Thecal width 



BMNH EE 3069 



10 



6 



BMNH EE 3070 



7 



4 



BMNH EE 3072 



9.5 



5.5 



BMNH EE 3083 



9 



5 



BMNH EE 3101 



10 



5.5 



BMNH EE 3119 



7.5 



4.5 



BMNH EE 3121 



10 



6 



Differences can be observed in the general outline of the theca (i.e. 

 in its length/width ratio and in the degree of divergence of the lateral 

 margins from the oro-anal axis in a posterior direction), in the 

 curvature of M', and M, in plan view, in the shape and curvature of 

 the posterior zygal process of M' ^ and of the posterior zygal plate, in 

 the shape and arrangement of the supracentralia, in the more or less 



PLATE 1 



Juliaecarpus milnerorum gen. et sp. nov. Late Ordovician, Upper Ashgill, Rawtheyan, Upper Ktaoua Formation, Morocco. All latex casts coated with 

 ammonium chloride. 1, EE 3101, x 9; poorly preserved dorsal aspect of theca, aulacophore and obliquely oriented zygal bar. 2, EE 3072. x 20; 

 arrangement of supracentralia on anterior half of theca and slightly disrupted aulacophore. 3, EE 3 144, x 10; almost complete dorsal aspect of theca, 

 arrangement of supracentralia and obliquely oriented zygal bar. 4, EE 3077, x 8; poorly preserved theca in dorsal aspect with well preserved aulacophore 

 insertion. 5, EE 3 1 19, x 10; well preserved theca in dorsal aspect showing almost undisrupted arrangement of supracentralia and suranal plate. 



