A NEW STYLOPHORAN ECHINODERM 



71 



the biomechanics and locomotion of which were analyzed in great 

 detail by Jefferies & Prokop ( 1972). It is reasonable to apply some of 

 Jefferies & Prokop's (1972) arguments also to Juliaecarpus, given 

 the fundamental anatomical resemblance between the latter and 

 Reticulocarpos (see also the section on morphological comparisons 

 below). 



According to Jefferies & Prokop (1972), Reticulocarpos was 

 fundamentally adapted to staying up on a soft sea-bottom, aided in 

 this by the weight-bearing capacity of the muddy substrate rather 

 than by compensation through lateral displacement of the sediment 

 (see also Jefferies, 1975, 1981 for a discussions of the biomechanical 

 constraints imposed by the two methods in different stylophorans). 

 The following morphological features of Reticulocarpos suggest 

 such a life-style: 1) flat ventral surface of the theca; 2) nearly 

 bilaterally symmetrical outline; 3) perforated texture of the stereom: 

 4) presence of a peripheral flange; 5) small size. Jefferies & Prokop 

 (1972) considered these features to be adaptations for reduction of 

 the total skeletal mass of the animal, reduction of its weight per unit 

 area and even distribution of the body weight to the sediment through 

 the flat ventral side of the theca. 



The fact that the thecal outline of Juliaecarpus is more bilaterally 

 symmetrical than that of Reticulocarpos implies that in Juliaecarpus, 

 the thecal weight may have been distributed to the sediment more 

 uniformly than in Reticulocarpos and in approximately equal amounts 

 on the left and on the right sides of the theca with respect to the oro- 

 anal axis. A more bilaterally symmetrical outline may also have 

 involved greater stability in the water currents and a more effective 

 manoeuvrability of the theca during locomotion, when the animal 

 pulled itself along dragged by its aulacophore (see below for a 

 reconstruction of the locomotion in Juliaecarpus). 



The lateral profile of the theca of Juliaecarpus, as reconstructed in 

 this paper, appears to be very low and decreases progressively in 

 height from the aulacophore insertion to the anal opening (see 

 anatomical description above ). In addition, neither the supracentralia 

 northe marginalia show projections or irregularities of any sort. With 

 a certain approximation, such a low, almost smooth lateral profile is 

 similar to the cross-section of a hydrofoil. 



This observation suggests that, perhaps, water turbulence was 

 drastically reduced or even absent all around the theca of Juliaecarpus. 

 A regularly laminar, or almost laminar, water flow may have contrib- 

 uted to stabilize the animal in a regime of currents, perhaps 

 supplementing the anchoring action of its styloid and ossicular 

 spikes (see below). Slowing down of the flow at the level of the theca/ 

 aulacophore junction (where the theca reached maximum thickness) 

 or even production of eddies immediately above this region may 

 have resulted in a slight increase in the fluid pressure (Bernoulli 

 effect), thus preventing forces generated by the water currents from 

 lifting the theca off the sea-floor. A similar hydrodynamic mecha- 

 nism counteracting the action of currents, based on a series of flume 

 tank experiments, was proposed by Daley (1996) for the North 

 American Middle Cambrian solute Coleicarpus sprinklei. 



The smaller size of the lateral denticulations of M', and M, of 

 Juliaecarpus with respect to those of Reticulocarpos and the less 

 developed peripheral flange may have represented further adapta- 

 tions to weight reduction, although this conclusion is not certain. 



Stability in water currents, especially when Juliaecarpus was at 

 rest on the sea bottom, was perhaps achieved in part through the 

 suction forces acting along the flat, ventral surface of the theca. 

 Suction forces, although not hampering the movements of the animal 

 along the sea bottom, probably prevented its theca from tilting (see 

 below). It is possible that, as in the case of other comutes (e.g. see 

 Daley, 1992), the zygal bar of Juliaecarpus may have helped reduce 

 suction forces during the lateral power strokes of the aulacophore 



(see reconstruction of the locomotory cycle below), presumably by 

 interrupting the continuity of the smooth, ventral integument. 



Thus, it is proposed that suction forces may have played an 

 important role in stabilizing the animal in water currents, acting 

 along the flat, ventral projections of the marginalia. At the same time, 

 however, such forces had to be reduced in part when the animal 

 pulled itself along the .sea-floor. Reduction may have occurred 

 essentially along a direction following the course of the zygal bar and 

 was most important at its anterior end, near the theca/aulacophore 

 insertion, where forces exerted by the lateral pushing movements of 

 the aulacophore were transmitted to the theca. 



Integuments. It is not clear to what extent the dorsal and ventral 

 integuments of Juliaecarpus were flexible in life. The occurrence of 

 broad, almost polygonal plates that did not overlap each other 

 (especially on the dorsal integument) certainly accounts for reduced 

 flexibility. The integument is remarkably different from that of 

 certain primitive comutes (e.g. several boot-shaped forms), in some 

 of which it consisted largely or exclusively of small, round or 

 subpolygonal elements, presumably partly embedded in a soft tissue 

 and not abutting against each other. Some degree of flexibility may 

 have developed along the peripheral margins of the dorsal and 

 ventral integuments of Juliaecarpus and in the portion of the dorsal 

 integument lying immediately anterior to the suranal plate. 



However, compression or expansion of the integuments was per- 

 haps limited by the constructional morphology of the supracentralia 

 and infracentralia. In fact, flexibility may have been limited to 

 restricted portions of the integuments (e.g. the periproctal region or 

 the integumental periphery) in most if not all ankyroids. In these, the 

 integument plates usually grew larger than in the asymmetrical 

 comutes and, in some cases (e.g. Cripps, 1989a; Ubaghs, 1991; 

 Cripps & Daley, 1994), the integuments consisted of few, large 

 polygonal elements. 



The suranal plate as a valve. The modalities of articulation of 

 the suranal plate of Juliaecarpus with the posterior part of the thecal 

 frame and with the supracentralia, and the fact that the lateral 

 margins of this plate seem to have been blunt in cross-section, 

 suggest that the suranal plate was probably capable of a certain 

 degree of vertical movement in life, perhaps acting as a flexible lid to 

 seal partially the anal opening and control waste disposal, or as an aid 

 in the regulation of gas exchanges through a pumping action of the 

 gut (Prof. R. L. Parsley, pers. comm.). 



That the suranal plate may have played an important role in gas 

 exchange is plausible, considering the fact that the theca of 

 Juliaecarpus does not show openings other than the posterior anus. 

 Primitive comutes display a diverse array of body openings (e.g. 

 sutural pores, cothumopores, lamellipores. etc.; see Ubaghs. 1968) 

 which are likely to have functioned as respiratory stmctures (see 

 elaboration of this argument in the chordate interpretation of 

 stylophorans provided by Jefferies, 1986). 



Unlike primitive comutes, ankyroids rarely display thecal open- 

 ings (apart from the anus). Thus, the evolutionary history of 

 stylophorans may have witnessed a shift in the gas exchange func- 

 tions from the thecal pores to other parts of the body (e.g. anus, 

 integument, appendage). The perforated stereom of the integuments 

 and, perhaps to a lesser extent, that of the marginalia and of the 

 aulacophore may also have been involved in gas exchange, although 

 this argument is highly speculative. 



The aulacophore as a locomotory device 



As stated in the introduction, I regard the aulacophore as the homo- 

 logue of an echinoderm ambulacrum (Sumrall. 1997). However. I 



