48 



M. RUTA 



1978; Philip, 1979; Kolata & Jollie, 1982; Kolata et ai, 1991; 

 Parsley, 1988, 1991, 1994, 1997, 1998; Ruta, 1998, in press). 



Parsley (1997, 1998) and Lefebvre et al. (1998) addressed the 

 problematic nature of the stylophorans in a number of papers 

 focussing on their skeletal homologies and interrelationships. In 

 particular, the elegant synthesis by Lefebvre ei al. ( 1 998) adds to the 

 arguments put forward by earlier authors (e.g. Ubaghs, 1968, 1969, 

 1981; Parsley, 1988, 1991; Kolata et al. 1991) in support of the 

 homology between the plano-concave thecal surface of the mitrates 

 and the flat thecal surface of the comutes. Such arguments are based 

 on a detailed comparative morphological study of the following 

 skeletal structures in representatives of both stylophoran groups: 1 ) 

 the tripartite, jointed exothecal appendage; 2) the apophyses, which 

 project medially from the most anterior pair of marginal thecal 

 plates, M'l and M,; 3) the left and right adoral plates, LA and RA, 

 which contribute to the excavation for the insertion of the articulated 

 appendage together with the apophyses; and 4) the zygal/septum 

 complex, which runs obliquely on the flat side of the theca and, at 

 least in its anterior portion, is associated with the same marginal 

 plate (M'|) in all known stylophorans (see also section on the 

 terminology below). 



The constructional and topological similarity of the above-men- 

 tioned structures in comutes and mitrates leads to the unequivocal 

 conclusion that all stylophorans show the same basic anatomical 

 organization (Ubaghs, 1981; Kolata et al, 1991; Peterson, 1995; 

 Parsley, 1997, 1998; Lefebvre et al, 1998), contrary to the opinion 

 expressed by such workers as Jefferies ( 1 986 and references therein), 

 Cripps (1989a) and Cripps & Daley (1994). 



The recent discovery of such primitive mitrates as Vizcainocarpus 

 dentiger Ruta, 1997 and a morphological comparison between this 

 species and the cornute Nanocarpus dolambii Ubaghs, 1991 

 strengthen the commonly held view that the flat side of the cornute 

 theca corresponds to the plano-concave side of the mitrate theca. 

 However, this view was formerly rejected by me (Ruta, 1997; see 

 also discussion in Ruta, 1998, in press) and I did not compare 

 Vizcainocarpus with Nanocarpus. In addition, I presented a charac- 

 ter-based hypothesis of interrelationships of the mitrocystitid 

 mitrates using several cornute taxa as outgroups, but I neglected 

 most skeletal similarities between cladistically derived comutes 

 and basal mitrates. 



Additional arguments favouring the homology of the flat thecal 

 surfaces in all stylophorans were presented by Ubaghs (1994) in his 

 comparison between the comute Lyricocarpus courtessolei and the 

 mitrate Chinianocarpos thorali Ubaghs, 1961. 



After the publication of my 1997 paper on Vizcainocarpus, Dr B. 

 Lefebvre (pers. comm.) and Prof R. L. Parsley (pers. comm.) drew 

 my attention to the fundamental resemblance (especially evident in 

 the zygal/septum complex) between the flat thecal surface of 

 Vizcainocarpus and that oi Nanocarpus as well as to the fact that the 

 distinction between comutes and mitrates, as proposed by those 

 workers who interpret stylophorans as chordates, relies on ad hoc 

 hypotheses of character transformation between these two groups 

 (see also below), as previous authors have already pointed out 

 (notably Ubaghs, 1981, Kolata et al, 1991 and Parsley, 1991). 



In fact, as noted by Parsley (1997, 1998) and especially Lefebvre 

 et al. (1998), the distinction between comutes and mitrates is no 

 longer valid and ought to be abandoned in favour of the resolution of 

 phylogenetic relationships of all stylophorans as a group. Revision- 

 ary work in this field has just begun (Dr. B. Lefebvre, pers. comm.; 

 Ms M. Marti-Mus, pers. comm.). In particular. Parsley ( 1997, 1998) 

 showed that the mitrates, as originally conceived, may not have a 

 common origin after all (i.e. mitrates would be polyphyletic). These 

 results prompt a re-evaluation of current ideas on the evolution and 



character distribution of these fossils (however, see below for com- 

 ments on the results of Parsley's (1997, 1998) analysis). 



The stylophoran interpretation adopted 



In the present paper, the stylophorans are interpreted as echinoderms, 

 according to the traditional view and contrary to the opinions ex- 

 pressed in some of my earlier works, in which a chordate interpretation 

 was followed and in which the jointed appendage of these animals 

 was regarded as a posterior, locomotory organ, homologous with a 

 chordate tail (references in Ruta, 1998, in press). I now concur with 

 several other researchers that the segmented appendage of comutes 

 and mitrates, henceforth referred to as the aulacophore (Ubaghs, 

 1968), is primarily an anterior feeding organ of ambulacral origin, 

 although, as Sumrall (1997: 269) pointed out, the resemblance 

 between the aulacophore and the ambulacmm "... does not allow for 

 the falsification of the hypothesis that [these two stmctures] are 

 homologous'. 



The interpretation of the aulacophore as an echinoderm arm fully 

 satisfies criteria of morphological similarity. Thus, for Lefebvre et 

 al. (1998: 104) the aulacophore '. . . may be homologous to a single 

 crinoid arm'. However, this view has been emphatically rejected by 

 some authors, notably Jefferies (1986) and Gee (1996). The argu- 

 ments in support of the echinoderm affinities of the stylophorans are 

 thoroughly discussed by Ubaghs ( 1 968, 1 98 1 ), Parsley ( 1 988, 1 99 1 , 

 1994, 1997, 1998) and Lefebvre f fa/. (1998) and will not be repeated 

 in this paper. 



Unlike Ubaghs (1981) and in partial agreement with other au- 

 thors, I also regard the aulacophore as a locomotory device which 

 may have functioned in different ways in different stylophorans (e.g. 

 Jefferies & Prokop, 1972; Kolata & Jollie, 1982; Jefferies, 1984, 

 1986;Parsley, 1988, 1991;Cripps, 1989^; Kolata era/.. 1991;Daley, 

 1992; Woods & Jefferies, 1992; Ruta & Bartels, 1998). 



A third inteipretation of the aulacophore, considered only as a 

 locomotory device but not homologous with a chordate tail, was 

 discussed by Philip (1979), Kolata & Jollie ( 1982) and Kolata etal. 

 (1991) (synthesis in Jefferies, 1986 and Ruta, 1998, in press). 



The calcichordate theory of Jefferies and co-workers (see espe- 

 cially Jefferies, 1986) deserves a final, brief comment. It represents 

 a thorough elaboration of Gislen's (1930) hypothesis that the ances- 

 try of the vertebrates is rooted into the extinct assemblage of the 

 'carpoid echinoderms', encompassing today the groups solutes, 

 comutes, mitrates, cinctans and ctenocystoids, and was dealt with at 

 length by Gee (1996). According to the calcichordate theory, most 

 solutes (see Daley, 1996) and all comutes are seen as stem-group 

 chordates. all mitrates as stem-group members of the three modem 

 chordate subphyla, Tunicata, Acraniata and Craniata, and cinctans 

 and ctenocystoids as stem-group echinoderms. 



As far as the stylophorans (comutes and mitrates) are concerned, 

 the calcichordate theory requires radical internal and external ana- 

 tomical changes in passing from the comutes to the mitrates, notably 

 the loss of the mid and distal portion of the comute aulacophore and 

 a re-organization of the remaining, proximal portion into the very 

 similar tripartite structure observed in the mitrates. Ubaghs (1981), 

 Parsley ( 1 99 1 ) and Lefebvre et al. ( 1 998) emphasized the difficulties 

 associated with the anatomical modifications demanded by Jefferies' 

 theory. Such modifications are supported by dubious or, more often, 

 no fossil evidence at all. Thus, as Lefebvre et al. ( 1998; 104) put it, 

 acceptance of Jefferies' (1986) arguments implies that '. . . the 

 median and distal parts of the aulacophore suddenly disappeared in 

 the comutes to reappear exactly identical but upside-down in 

 mitrates'. 



Despite my own initial convictions (references in Ruta, 1998, in 



