Bull. not. Hist. Mus. Land. (Geol.)55(2): 47-79 



Issued 25 November 1999 



A new stylophoran echinoderm, Juliaecarpus 

 milnerorum, from the late Ordovician Upper 

 Ktaoua Formation of Morocco 



\i_ 



'JM 

 MARCELLO RUTA 



Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD . ^ FOV 1999 



CONTENTS i ^^ "^^t tr^'Al^^ 



I GEr i RAL LIBRA RY 



Introduction 47 



The stylophoran controversy 47 



The stylophoran interpretation adopted 48 



Terminology 49 



Aims of the study 49 



Systematic palaeontology 51 



Juliaecarpus gen. nov 51 



/ milnerorum sp. nov 53 



Anatomical description 53 



Theca 53 



Aulacophore 66 



Functional morphology 70 



The aulacophore as a locomotory device 71 



The aulacophore as a feeding organ 73 



Comparisons 73 



Similarities and differences in the thecal plating and aulacophore 73 



Suranal plate and stylophoran orientation 77 



Morphological variation in the zygal bar 77 



Conclusions 78 



Acknowledgements 78 



References 78 



Synopsis. The late Ashgill stylophoran echinoderm Juliaecarpus milnerorum gen. et sp. nov. from the Upper Ktaoua Formation 

 of Morocco is characterized by a high degree of thecal symmetry, a large suranal plate and an almost bilaterally symmetrical 

 arrangement of supracentralia on the anterior half of the theca. Juliaecarpus was adapted to resting on a soft sea bottom by 

 exploiting the weight-bearing capacity of mud. Locomotion probably occurred through alternating left and right lateral thrusts of 

 the styloid and ossicular spikes against sediment, separated by upward and downward return strokes. In most classifications, 

 Juliaecarpus would be placed in the comutes, owing to the presence of a zygal bar extending from M', to the right posterior part 

 of the thecal frame and to the fact that the marginalia are rod-like and surround several supracentralia and infracentralia. Following 

 recent classifications, however, Juliaecarpus is best regarded as an ankyroid, due to its almost bilaterally symmetrical outline, to 

 the possession of styloid processes and to the fact that the posterior zygal plate is a somatic element sutured with the posterior right 

 part of the thecal frame, but not included in the latter. Among ankyroids, Reticulocarpos hanusi, Beryllia miranda and Domfrontia 

 pissotensis most closely resemble Juliaecarpus. 



INTRODUCTION 



The stylophoran controversy 



The debated systematic affinities of the stylophorans, a problematic 

 group of marine metazoans of the Palaeozoic covered with an 

 echinoderm-like skeleton of stereomic calcite (Smith, 1980) and 

 variously allied to the echinoderms (e.g. Ubaghs, 1968) or to the 

 chordates (e.g. Jefferies, 1986), have generated great interest in 

 recent years, resulting in the publication of several papers in which 

 their alpha taxonomy as well as their relationships (both within the 

 group and in the context of deuterostome phytogeny) are discussed. 

 Gee ( 1 996) provided updated bibliographical references and a com- 



prehensive review of the controversy surrounding these animals. The 

 most recent account of the debate is the scientific correspondence 

 between Peterson ( 1 995 ) and Jefferies ( 1 997 ) (but see also Sumrall, 

 1997 for comments). 



Different opinions as to which thecal surfaces and marginal thecal 

 plates are homologous in the two stylophoran groups of the comutes 

 (Jaekel, 1901) and the mitrates (Jaekel, 1918), as well as disagree- 

 ment on their anatomical orientation and on the function of their 

 single and jointed exothecal appendage, have led to a proliferation of 

 different and often contrasting morphological terminologies. An 

 additional difficulty is the lack of a standardized plate nomenclature 

 (Caster, 1952: Ubaghs, 1968. 1969, 1981; Jefferies, 1968, 1973, 

 1981, 1986, 1997; Jefferies & Prokop, 1972; Jefferies & Lewis, 



) The Natural History Museum. 1999 



