A NEW STYLOPHORAN ECHINODERM 



73 



Fig. 8 Juliaecarpus milnerorum gen. et sp. nov. Late Ordovician. Upper 

 Ashgill, Rawtheyan, Upper Ktaoua Formation. Morocco. Reconstruction 

 of a hypothetical locomotory cycle. The horizontal black line is an 

 imaginary reference direction. A, locomotion begins: the aulacophore is 

 lowered down within mud and represented by a blackened rod. B, first 

 power stroke: the aulacophore is pushed rightward against mud while the 

 theca yaws slightly anti-clockwise and moves forward while pivoting on 

 its left antero-lateral angle. C, first return stroke: the aulacophore 

 (represented by a white rod) is freed from mud and rotated leftward. D, 

 the aulacophore is lowered down within mud. E, second power stroke: 

 the aulacophore is pushed leftward against mud while the theca yaws 

 slightly clockwise and moves forward while pivoting on its right antero- 

 lateral angle. F, second return stroke: the aulacophore is freed from mud 

 and rotated rightward. 



anti-clockwise direction (perhaps bringing the aulacophore parallel 

 to the oro-anal axis), by a downward thrust within the sediment 

 (facilitated by the shape of the styloid and ossicles), and finally by a 

 lateral pushing action within the mud in an anti-clockwise direction. 

 During alternating clockwise and anti-clockwise lateral thrusts, the 

 theca of Juliaecarpus probably yawed very slightly about a vertical 

 axis passing through or close to the theca/aulacophore insertion, so 

 that its antero-lateral angles moved slightly anterior. 



However, yawing, rolling and pitching components of the move- 

 ments were perhaps negligible, so that the theca was relatively stable 

 during locomotion. The lateral flanges of Juliaecarpus may have 

 partially prevented sliding of the theca in a posterior direction, 

 although their poor development in comparison with those of 

 Reticulocarpos indicates that they were probably scarcely effective 

 as friction-generating devices. 



I point out that, as in previous reconstructions of the locomotory 

 cycle of stylophorans (see especially Jefferies, 1984 and Woods & 

 Jefferies, 1992), no accurate estimate of the speed and of the distance 

 covered by the animal at the end of each power stroke can be 

 provided. Therefore, the position occupied by Juliaecarpus at the 

 end of a cycle is only speculative. 



Based on the relative proportions of the theca and of the preserved 

 part of the aulacophore 1 conclude that, as in the case of other 

 cornutes (and perhaps of all stylophorans), locomotion in 

 Juliaecarpus was probably not very advantageous from an energy 

 point of view, although it may have allowed the animal to explore 

 adjacent, food-rich patches of the sea-floor (see the next section) and 

 to have changed its position with respect to the water currents. In 

 summary, the life-style of Juliaecarpus was perhaps that of a rela- 

 tively sedentary, suspension and/or detritus feeding, bottom dwelling 

 organism (see also Parsley, 1988, 1991, 1994, 1997, 1998). 



The aulacophore as a feeding organ 



With regards to the feeding function of the aulacophore, I accept 

 Parsley's (1988, 1991 ) view that this structure was probably oriented 

 upstream in the main direction of the water current when the animal 

 fed. Juliaecarpus may have exploited food sources in the proximity 

 of its body by moving the rigid, distal part of the appendage laterally, 

 a few microns off the substrate. 



With its aulacophore held straight in the water current, y(//(rtfra/77M.v 

 perhaps fed by allowing food particles to enter through the slit-like 

 spaces present between adjacent pairs of distal aulacophore plates. 

 Mitrates, on the other hand, seem to have been able to exploit food 

 particles in suspension more actively than cornutes. 



The aulacophore of initrates (including its distal part) was charac- 

 terized by a higher flexibility than that of cornutes. According to 

 Parsley (1988, 1991; but see comments in Kolata et at.. 1991 and 

 Ruta, 1 998, in press), mitrates could arch the distal aulacophore so as 

 to increase the spaces between consecutive segments (the paired 

 cover plates of their distal aulacophores were apparently fused along 

 their medial margins, thus strengthening the whole appendage and 

 making it more effective as a locomotory organ when it pushed 

 against mud). 



As explained in the anatomical section, whether the cover plates 

 of the distal aulacophore of Juliaecarpus could open in life (as in the 

 case of the comute genus Phyllocystis Thoral, 1935 and few other 

 stylophorans) is difficult to ascertain (Ubaghs, 1968, 1969, 1981; 

 Jefferies, 1968, 1986). It is also proposed that the stirring action 

 exerted by the styloid and ossicular spikes may have brought 

 particulate food in suspension. 



The recurved styloid and ossicular spikes of Juliaecarpus prob- 

 ably enhanced the gripping action of the aulacophore by pointing in 

 the direction opposite to that of the water current flow (i.e. the spikes 

 would point in a downstream direction) when the animal fed by 

 holding its appendage in the currents. 



Waste disposal was perhaps facilitated by slight vertical move- 

 ments of the suranal plate (see above) and by the fact that, given the 

 position of the animal in the water currents, its faeces were dragged 

 away from the body at the rear end of the theca. 



COMPARISONS 



Similarities and differences in the thecal plating 

 and aulacophore 



In this section. Juliaecarpus is compared in detail with a number of 

 other ankyroid stylophorans. Such comparisons do not aim to iden- 

 tify shared derived features, although they provide a taxonomic 

 framework for a revised phylogeny of these organisms. As specified 

 in the introduction, a formal numerical character analysis of the 

 stylophorans is not presented in this work. 



Among the comute-like stylophorans placed by Parsley (1997, 

 1998) in his order Ankyroida, the genera Reticulocarpos. Beiyllia 

 and Domfrontia are discussed at length here, for they show closer 

 resemblance to Juliaecarpus than other ankyroids do. I also briefly 

 compare Juliaecarpus with Nanocarpus, which represents a gener- 

 alized ankyroid with several skeletal features foreshadowing the 

 morphological condition found in later, presumably more derived 

 stylophorans. 



Juliaecarpus is similar to Reticulocarpos hanusi from the Llanvim 

 of Bohemia (Fig. 9) in the general aspect and proportions of the 

 theca. in the shape and relative lengths of the marginalia and in the 

 morphology of the styloid and ossicles. However, external anatomi- 

 cal differences between these two taxa are numerous and can be 

 summarized as follows: 



1 . The thecal outline of Juliaecarpus is more bilaterally symmetri- 

 cal than that of Reticulocarpos. Corresponding pairs of marginalia, 

 especially the more posterior ones, are almost equal in shape and 

 length and the oro-anal axis is almost aligned with the longitudi- 

 nal axis of the aulacophore. In addition, the lateral thecal margins 



