72 



M. RUTA 



also believe that most, if not all of the stylophorans used their 

 aulacophores as a locomotory organ. 



I agree with Jefferies & Prokop (1972) that the hollow nature and 

 perforated stereom texture of the ossicles, paired plates and styloid in 

 such stylophorans as Reticulocarpos may have represented further 

 adaptations for reducing body weight. However, as already ex- 

 plained above, I question the validity of their arguments concerning 

 the occurrence of a short aulacophore. As discussed in the anatomi- 

 cal description, none of the observed specimens of Juliaecarpus 

 shows the inside of the styloid and of the ossicular spikes. Therefore, 

 their hollow nature, although plausible, cannot be corroborated by 

 direct observations. In addition, their stereom texture is less porous 

 than that of Reticulocarpos. 



I concur with Jefferies & Prokop (1972), Jefferies (1975, 1981, 

 1984, 1986), Jefferies et al, 1987, Woods & Jefferies (1992) and 

 Cripps & Daley (1994) that several morphological features of the 

 stylophorans indicate that they may have moved in the direction of 

 the appendage (i.e. anteriorly according to the anatomical orienta- 

 tion adopted in the present work). As pointed out by Cripps & Daley 

 (1994). the articulated appendage may also have had an anchoring 

 function. As in the case of Reticulocarpos and Prokopicystis (and 

 most other comute stylophorans), the distal part of the aulacophore 

 of Juliaecarpus must have behaved as a rigid, rod-like structure in 

 life, as suggested by the presence of flat interossicular articular 

 surfaces. 



The aulacophore of Juliaecarpus could probably move horizon- 

 tally and vertically thanks to the high degree of flexibility of its 

 proximal region. As in other stylophorans, the bulk of the muscula- 

 ture was probably concentrated in the lumen of the articulated rings 

 of the proximal part of the aulacophore, which probably represented 

 the main motor during locomotion (Jefferies & Prokop, 1972; 

 Jefferies, 1984, 1986; Jefferies et al., 1987; Woods & Jefferies, 

 1992). 



Contrary to Jefferies & Prokop ( 1 972) and Jefferies ( 1 986), how- 

 ever, I consider vertical thrusts of the styloid and ossicular spikes 

 within the sediment to have been poorly effective for movement (as 

 indicated by the fact that the spikes are mostly flattened laterally and 

 show a slightly pronounced keel posteriorly), but highly effective to 

 cut a way open through the mud. Lateral movements of the partially 

 buried intermediate and distal parts of the aulacophore, on the other 

 hand, may have resulted in a more effective lateral pushing action 

 against the substrate. This is because during the lateral thrusts, the 

 entire lateral surfaces of the styloid and ossicular processes were in 

 contact with the mud. The power strokes were made more effective 

 by the rigid articulations between adjacent ossicles and between the 

 most proximal ossicle and the styloid. This functional interpretation 

 is applied also to Reticulocarpos. 



To sum up, left and right lateral thrusts can be visualised as power 

 strokes exerting an alternating clockwise and anticlockwise 'rowing' 

 action, whereas return strokes consisted mainly of vertical, upward 

 and downward movements enabling the animal to free its aulacophore 

 from mud and to lower it down within sediment. Ruta & Bartels 

 (1998) have recently suggested that a similar alternating series of 

 vertical thrusts (presumably not actively involved in locomotion) 

 and lateral thrusts (exerting a dragging action) occurred in the 

 locomotory cycle of the anomalocystitid mitrate Rhenocystis 

 latipedunculata Dehm. 1932 from the Lower Devonian of Germany. 



1 hypothesize that, at the beginning of each lateral push within the 

 sediment, the theca of Juliaecarpus rotated slightly in a direction 

 opposite to that of the thrust and slightly forward (anatomically 

 anterior). As explained below, this yawing component of the move- 

 ment was probably reduced by the elongation of the theca and by the 

 fact that its centre of mass was likely to be close to the theca/ 



aulacophore insertion. The position of the theca was perhaps read- 

 justed at the beginning of the successive power stroke, when the 

 'rowing' action of the aulacophore exerted a lateral thrust in the 

 opposite direction with respect to that of the preceding power stroke. 



It is possible that the comparatively more elongate theca of 

 Juliaecarpus and its higher degree of bilateral symmetry with re- 

 spect to Reticulocarpos prevented excessive yaw while the animal 

 pulled itself along on the surface of the sea floor (for a comparison 

 with the functional adaptations in mitrates see the comprehensive 

 discussion of the biomechanics of these animals in Jefferies, 1984). 

 That yaw was reduced during locomotion is also suggested by the 

 fact that the centre of mass of the theca oi Juliaecarpus probably lay 

 somewhere at the level of its anterior half, where the thecal frame 

 reaches its maximum height and width and the where the marginalia 

 are comparatively thicker than in more posterior parts of the theca. 

 This region, being close to the theca/aulacophore insertion, was 

 perhaps less likely to swing laterally about a hypothetical vertical 

 axis passing through the anterior thecal excavation. 



Rolling and pitching components of the movement were presum- 

 ably greatly reduced or even absent. This is mainly due to the fact that 

 if, as I believe, locomotion was achieved essentially through lateral 

 thrusting actions of the intermediate and distal parts of the 

 aulacophore {contra Jefferies & Prokop, 1972 and Jefferies, 1986 

 but in partial agreement with the conclusions presented by Jefferies 

 et al., 1987, Daley, 1992 and Woods & Jefferies, 1992), then no 

 vertical components of the reaction forces of the sediment to the 

 movements of the aulacophore could be transmitted to the theca. 



It is possible that such vertical components were slightly more 

 effective when the aulacophore was lowered down within the mud or 

 extracted from it. However, in the downward thrust, the pointed 

 processes of the styloid and ossicles presumably penetrated through 

 the superficial layers of mud with minimum effort. Furthermore, the 

 aulacophore moved essentially through the uppermost layers of the 

 sediment in the upward thrust, where cohesion forces acting between 

 particles of mud were strongly reduced by the high water content. 



Thus, friction was negligible during the vertical movements of the 

 aulacophore, because of the slashing action of the spikes and because 

 of the relatively high fluidity of the sediment. The reaction forces to 

 the vertical thrusts exerted by the latter, therefore, were presumably 

 weak. In addition, the flat ventral side of the theca and the occurrence 

 of lateral flanges in Juliaecarpus probably prevented or greatly 

 reduced the tilting of the theca (see also above), thus reducing the 

 risk of it sinking into the substrate (for analogous adaptations in 

 other cornutes see Jefferies & Prokop, 1972, Jefferies et al, 1987, 

 Daley, 1992, Woods & Jefferies, 1992 and Cripps & Daley, 1994) 



In conditions of maximum ventral flexion of the proximal part of 

 the aulacophore, the spike-like processes pointed backward, or 

 backward and slightly downward. Thus, Juliaecarpus may have 

 anchored itself more firmly, for the whole appendage was inserted 

 within relatively dense layers of sediment well below the level of the 

 sea bottom. Such a posture may have been effective in a regime of 

 occasional and particularly strong water currents. Friedrich (1993) 

 proposed a similar anchoring function for the appendage of cinctan 

 echinoderms. 



In conclusion, the locomotory cycle of Juliaecarpus can be visu- 

 alized as a series of clockwise and anti-clockwise lateral thrusts of 

 the stiff, intermediate and distal parts of the aulacophore within mud, 

 alternating with vertical movements in the water column, perhaps a 

 few microns off the surface of the sea-floor (Fig. 8). A wide variety 

 of movements could be performed through combination of horizon- 

 tal and vertical flexions of the tetramerous rings. A clockwise lateral 

 thrust was probably followed by an upward lift of the appendage 

 (which was thus released from the mud), by its partial rotation in an 



