76 



M. RUTA 



M'4 



B 



Fig. 1 1 Domfmnria pissoteiisis Cripps & Daley. Middle Ordovician, 

 Llandeilo, Pissot Formation, France. Schematic reconstruction of 

 external aspect. A, dorsal view. B, ventral view (redrawn and modified 

 after Cripps & Daley, 1994). 



1. According to the reconstruction provided by Cripps & Daley 

 (1994), Domfrontia possesses almost straight lateral thecal mar- 

 gins. As a result, the anal opening is approximately as wide as the 

 theca. Proportions and relative lengths of the marginalia of 

 Domfrontia are similar to those of Beryllia, but differ remarkably 

 from those of Jidiaecarpus. The convexity of the lateral thecal 

 margins of Jidiaecarpus is somewhat intermediate between that 

 of Beryllia and that of Domfrontia. 



2. The zygal process of M'^ in Domfrontia is comparatively wider 

 and shorter than that of both Beryllia and Juliaecarpus. Likewise, 

 the preserved part of the posterior zygal plate of Domfrontia 

 suggests that this plate is stouter in this ankyroid than in the other 

 two genera. 



3. The styloid and ossicles of Domfrontia are shaped like bulges 

 with a subelliptical to subcircular outline and carry a poorly 

 defined median longitudinal keel with a semicircular to parabolic 

 profile. 



4. The ventral projection of M^ is expanded posteriorly in 

 Domfrontia, but not in Beryllia and only to a very limited extent 

 in Juliaecarpus. 



The number of marginal plates in such genera as Juliaecarpus. 

 Reticulocarpos, Beryllia and Domfrontia, as well as their relative 

 position and shape, gives support to Parsley's (1991, 1997, 1998) 

 suggestion that several lineages within the ankyroids underwent 

 reduction in the number of marginalia through decrease in size and 

 subsequent loss of the rearmost plates. A brief comparison with the 

 ankyroid Nanocarpus dolambii Ubaghs, 1991, from the lower 

 Ordovician of southern France is interesting in this respect (Fig. 12). 



Except for asymmetries in the thecal frame due to the relatively 

 small size of M, and elongation of M' the theca of Nanocarpus is 



M'3 



B 



Fig. 12 Nanocarpus dolambii Ubaghs. Lower Ordovician. Lower Arenig, 

 Schistes de Saint-Chinian, France. Schematic reconstruction of external 

 aspect. A, dorsal view. B, ventral view (redrawn and modified after 

 Ubaghs, 1991). 



similar to that of Juliaecarpus. From a purely morphological point of 

 view, Nanocarpus can be considered as a fairly generalized cornute- 

 like ankyroid in which the theca retained asymmetries as in more 

 primitive comutes. In Nanocarpus. the rearmost marginalia, espe- 

 cially Mj., are still relatively large with respect to more anterior 

 marginalia, as opposed to the situation observed in Reticulocarpos 

 and Juliaecarpus. On the other hand, M' ^ and M_, are shorter than M', 

 and M,, and M\ and M, differ greatly in size. 



An interesting feature observed in some of the specimens of 

 Nanocarpus figured by Ubaghs ( 1 99 1 ) is the presence of a posterior 

 plate visible between M'^ and M^. Although poorly preserved, this 

 element seems to be homologous with the suranal plate of 

 Juliaecarpus. However, it is difficult to say whether it was flexibly 

 articulated with the rest of the thecal frame in life. 



It is possible to hypothesize that, in the evolutionary history of the 

 ankyroid stylophorans, thecal asymmetries were drastically reduced 

 through the loss of the posterior marginalia, subsequent elongation 

 of the marginalia occupying an intermediate position along the 

 lateral margins of the thecal frame and, finally, acquisition of a more 

 bilaterally symmetrical outline. A suranal plate, as explained below, 

 may have been acquired in a number of ankyroids through enlarge- 

 ment and rearward displacement of a posterior supracentral plate 

 which became flexibly articulated both with the dorsal integument 

 and with the medial margins of the dorso-lateral projections of the 

 most posterior marginalia. 



As far as the ventral surface of the theca is concerned, the posterior 

 zygal plate and posterior zygal process of Nanocarpus are similar in 

 shape and general proportions to those of Juliaecarpus, except that 

 in Nanocarpus. the posterior zygal process of M' is slightly curved 



