70 



M. RUTA 



slightly compressed laterally, with blunt anterior and posterior mar- 

 gins, elliptical in cross-section throughout most of its height and 

 roughly circular near its apex. Its lateral surfaces are gently concave 

 in the upper two-thirds and merge smoothly into the posterior part of 

 the styloid. Its posterior margin meets the posterior part of the styloid 

 along a smooth, hyperbolic curve in lateral view. 



Distal part. Isolated ossicles and paired plates are observed in 

 some specimens (Fig. 7D-F; PI. 2, fig. 3; PI. 3, figs 1, 3, 5; PI. 5, figs 

 3-4, 6; PI. 7, figs 1,6; PI. 8, figs 1-2). In a few individuals, the distal 

 part of the aulacophore is preserved articulated and straight and 

 consists of two or three segments. However, it is not absolutely 

 certain that a short distal aulacophore represents a genuine feature of 

 Juliaecarpits. Jefferies & Prokop (1972) argued in favour of the 

 presence of a reduced distal aulacophore (their hind tail) in 

 Reticulocarpos hanusi (see also below). Although it is possible that 

 the aulacophores of both Reticulocarpos and Juliaecarpus consisted 

 of a greatly reduced number of segments in comparison with those of 

 other stylophorans, it is risky to deduce their original length from the 

 available material. 



As noted by Lefebvre er a/. ( 1 998 ), the distal part of the sty lophoran 

 aulacophore is extremely fragile and often incompletely preserved. 

 In addition, because the distal aulacophores of both Reticulocarpos 

 and Juliaecarpus seem to have been rigid, it is likely that they broke 

 up rather easily during diagenesis or burial and that its various 

 segments were dragged away from the rest of the body. It is possible, 

 therefore, that more distal ossicles were present in life. 



As in the case of the styloid, each ossicle consists of an upper, 

 massive part and a lower, recurved spike. The massive part is just 

 wider than long and composed of anterior and posterior articular 

 surfaces, two lateral surfaces and an upper surface. The anterior and 

 posterior articular surfaces are subequal in shape and size and 

 roughly crescentic. The posterior surface is flat except for the 

 presence of a poorly developed ridge occupying the central third of 

 its lower margin. The anterior surface is slightly depressed in its 

 lower half and shows no ridges. The lower margins of both the 

 anterior and the posterior surfaces are accurately semicircular. Their 

 upper margins are complicated by the intersection of both articular 

 surfaces with the system of longitudinal grooves and thickenings 

 present on the upper surface of the massive part. The lateral surfaces 

 of the massive part are very gently convex in transverse section and 

 broadly rectangular in outline, and diverge slightly lateralward in 

 dorsal direction. They merge gradually into the lateral surfaces of the 

 spike from which they are separated by a very low, almost indistinct 

 subhorizontal ridge disappearing rapidly anteriorly. The upper sur- 

 face of the massive part is deeply concave and carries a median 

 longitudinal furrow (Ubaghs, 1968) with a broadly parabolic cross- 

 section. The furrow is flanked by a left and a right thickening, about 

 half as wide as the groove and with a blunt, gently convex top. The 

 thickenings are asymmetrical in cross-section, for they slope slightly 

 upward in a medio-lateral direction. The most lateral parts of the 

 upper surface are generally poorly preserved. However, it is possible 

 to see the presence of left and right bump-like projections about half 

 as long as the ossicles and slightly displaced anteriorly. Immediately 

 posterior to these projections are very shallow, straight transverse 

 channels, slightly diverging posteriorly from the median longitudi- 

 nal furrow and apparently intersecting the lateral thickenings. 



The ossicular spike is almost twice as high as the massive part and 

 laterally compressed for most of its height, so that its cross-section 

 appears to be subelliptical. Near its apex, the spike is more rounded 

 in section and ends in a subconical point. The anterior and posterior 

 margins of the spike are narrowly acute in section, although never 

 acutely sharp, and concave in lateral profile, the posterior margin 



more so than the anterior margin. In lateral view, the uppermost part 

 of the posterior margin forms a small angle with the posterior 

 articular surface of the massive part. As a result, the spike points 

 almost exactly ventralward rather than posteriorly as in 

 Reticulocarpos. In their upper third, the lateral surfaces of the spike 

 are gently concave, whereas at the level of their lower third they are 

 mostly flat and subparallel. The distance between the anterior and 

 the posterior margin of the spike decreases rapidly dorso-ventrally. 

 The paired plates of the distal aulacophore are not preserved in 

 place. However, I interpret as disarticulated plates several small, 

 subcircular to plectrum-shaped elements with a characteristic radial 

 striation pattern. These are visible in proximity to the ossicles in 

 some specimens (e.g. PI. 8, fig. I). The margin of the plates towards 

 which the striae radiate out may correspond to their anterior side, 

 although this is not certain. 



Stereom. The plates of the proximal part of the aulacophore 

 display a retiform stereom similar to that of the centralia and 

 marginalia, although the average size of the perforations is much 

 smaller. The perforations are apparently distributed regularly on the 

 surface of the plates. Those plates which are here interpreted as 

 inferolaterals show a radiating pattern of trabeculae separated by 

 elongate pores in their posterior half. 



The stereom texture of the ossicles (PI. 5, fig. 3; PL 7, fig. 1; PI. 8, 

 fig. 1 ) differs in the massive part and along the height of the spike. As 

 far as the massive part is concerned, its anterior, posterior and upper 

 surfaces, as well as the upper parts of its lateral surfaces, show a 

 compact stereom, with no apparent texture pattern. The stereom 

 texture is reticulate at the level of the upper half of the lateral surfaces 

 of the spike, whereas its anterior and posterior margins and the apex 

 show an irregularly perforated to coarsely granular texture consist- 

 ing of short trabeculae and shallow pits. The stereom of the paired 

 plates is mostly reticulate, but becomes compact or granular along 

 their margins. 



FUNCTIONAL MORPHOLOGY 



General considerations 



Introduction. Despite several claims to the contrary (e.g. 

 Jefferies, 1984, 1986 and references therein), a consensus on the 

 functional morphology of the stylophorans (or on any other aspect of 

 their paleobiology) has not yet been reached (Ruta, 1998, in press). 

 Although some inferences are plausible (e.g. Jefferies & Prokop, 

 1972, Daley, 1992, Woods & Jefferies, 1992, Donovan, in press), 

 they do not necessarily imply the correctness of the life-style inter- 

 pretations proposed for these animals. Ubaghs (1968) provided the 

 most comprehensive summary of the various functional hypotheses 

 put forward so far (see also Ubaghs, 1981, Jefferies, 1986 and Kolata 

 etal, 1991). 



In this section, the functional adaptations of the theca (including 

 its dorsal and ventral integuments) and suranal plate of Juliaecarpus 

 are analyzed and the significance of certain anatomical features 

 (especially the lateral profile of the theca) in relation to the stability 

 of the animal in water currents is examined in detail. 



A brief account of the possible functions of the aulacophore 

 (feeding organ; locomotory device) is dealt with in the next two 

 secfions and a reconstrucfion of the locomotory cycle oi Juliaecarpus 

 is presented. 



Functional adaptations of the theca. Before discussing the 

 external anatomy of Juliaecarpus from a functional viewpoint, it is 

 necessary to consider briefly the ankyroid Reticulocarpos hanusi. 



