78 



M. RUTA 



sions are correct, the character distribution in above-mentioned 

 ankyroids will have to be reconsidered, especially when the origin of 

 mitrates from their cornute ancestors is taken into account. 



CONCLUSIONS 



Together with Beryllia, Domfronria and Reticulocarpos, the new 

 stylophoran Juliaecarpus milnerorwn gen. et sp. nov. from the late 

 Ashgill (late Rawtheyan) Upper Ktaoua Formation of Morocco 

 ranks among the most bilaterally symmetrical stylophoran 

 echinoderms. Following recent phylogenetic work by Parsley ( 1 997, 

 1998), Juliaecarpus is an ankyroid, that is, a stylophoran with a 

 separate zygal plate not belonging to the thecal frame and with well- 

 developed, spike-shaped styloid and ossicular processes. 



The most outstanding skeletal feature of Juliaecarpus is the 

 presence of a large suranal plate flexibly articulated with the rear- 

 most part of the thecal frame and dorsal integument. Such a plate 

 may have enabled Juliaecarpus to regulate the pumping action of the 

 gut by acting as a vertically movable, lid-like structure and to control 

 waste disposal. A suranal plate was probably present also in the 

 ankyroid genera Beryllia and Domfrontia, as suggested by the occur- 

 rence of a large, apparently continuous dorsal area revealing stereom 

 structure at the posterior end of the theca. 



A comparison with several ankyroids strengthens the view that, 

 during the evolutionary history of the stylophorans, asymmetries in 

 the thecal outline and in the marginalia, inherited from primitive 

 (and often boot-shape) comutes, became progressively less accentu- 

 ated and were eventually lost in many forms (Ubaghs, 1 968; Jefferies, 

 1986;Cripps, 1989/?; Parsley, 1991, 1994, 1997, 1998;Ruta, 1998, in 

 press). The acquisition of a progressively more bilaterally symmetri- 

 cal thecal outline presumably preceded a complete symmetrization 

 of the paired marginalia. Several stages of this process are exempli- 

 fied by a number of comute-like ankyroids. For instance, Juliaecarpus 

 displays a high degree of bilateral symmetry in the theca, but 

 asymmetries can still be observed at the level of some of the 

 marginalia, especially M^, M'^, M, and M',. 



Acquisition of bilateral symmetry probably occurred several times 

 independently in the stylophorans, as recent phylogenetic analyses 

 and re-assessment of stylophoran thecal plate homologies suggest 

 (Parsley, 1997, 1998; Lefebvre et al., 1998; Dr B. Lefebvre, pers. 

 comm.; Ms M. Marti-Mus, pers. comm.). 



As in the case of Reticulocarpos, Juliaecarpus was probably 

 capable of resting on the surface of the muddy sea-floor aided in this 

 by the weight-bearing capacity of the sediment (Jefferies & Prokop, 

 1972). Nearly bilaterally symmetrical shape, small size, flat ventral 

 surface of the theca, peripheral flange (albeit reduced in size in 

 comparison with that of Reticulocarpos) and highly perforated 

 skeletal plates all seem to account for this life-style. 



Movement perhaps involved a downward thrusting and a lateral 

 pushing action of the aulacophore within the sediment followed by 

 an upward return stroke enabling the animal to free its aulacophore 

 from the mud. Alternating clockwise and anti-clockwise lateral 

 thrusts represented the active component of the locomotory cycle, 

 whereas upward and downward movements, as well as lateral move- 

 ment in the water column just above the sea-floor, allowed 

 Juliaecarpus to readjust the position of its aulacophore between two 

 successive power strokes. Several skeletal features of the theca and 

 aulacophore suggest that yawing, rolling and pitching components 

 of the movement were absent or much reduced. 



Stability in the water currents was probably achieved through a 

 combination of factors: 1 ) suction forces acting along the flat ventral 



side of the theca; 2) reduction of water turbulence due to the low, 

 gently convex lateral profile of the theca and possible production of 

 eddies immediately above the theca/aulacophore junction, both ef- 

 fects resulting in water pressure increase (Bernoulli effect); 3) 

 anchoring function of the recurved styloid and ossicular spikes, 

 perhaps enhanced by the ability of the animal to bend the proximal 

 part of its aulacophore ventralward and rearward to a considerable 

 degree. 



Juliaecarpus probably fed by holding its aulacophore upstream in 

 the water current (see also Parsley, 1988, 1991 ). Food particles were 

 brought in suspension by the stirring action of the aulacophore and 

 forced to pass through the narrow spaces between adjacent pairs of 

 cover plates. 



Juliaecarpus adds to the taxonomic similarities between middle 

 to late Ordovician faunas from Morocco and coeval faunas from 

 several European regions. It also adds to the diversity of early 

 comute-like ankyroids, combines features of several previously 

 described taxa and prompts a reassessment of the polarity of many 

 characters within the stylophoran echinoderms. It is hoped that a 

 revised character analysis of the whole group can yield information 

 on the phylogenetic significance of this ankyroid. 



Acknowledgements. Dr A. R. Milner (Birkbeck College, University of 

 London) and an anonymous referee reviewed the manuscript and made 

 helpful suggestions. Dr B. Lefebvre (Universite Claude Bernard, Lyon, 

 France), Ms M. Marti'-Mus (Uppsala University. Uppsala, Sweden) and Prof. 

 R. L. Parsley (Tulane University, New Orleans, USA) exchanged useful 

 information on several aspects of stylophoran anatomy and interrelationships. 

 Mr D. N. Lewis (Department of Palaeontology. Natural History Museum, 

 London) provided constant encouragement and curatorial expertise. Dr A. B. 

 Smith (Department of Palaeontology, Natural History Museum, London) 

 supplied bibliographical information. Drs A. C. Milner and P. D. Taylor and 

 Profs S. J. Culver and S. K. Donovan (Department of Palaeontology, Natural 

 History Museum, London) read the text. Mr P. Crabb (Photographic Unit, 

 Natural History Museum, London) photographed the specimens. 



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