Suttonium Schaaf, 1976

Dumitrica, Paulian, 2019, Cenozoic Spumellarian Radiolaria With Eccentric Microsphere, Acta Palaeontologica Romaniae 15 (1), pp. 39-60 : 46

Genus Suttonium Schaaf, 1976 , emend. Dumitrica, 1983a

Type species. Suttonium praedicator Schaaf, 1976

Remarks. In my article on the genus Suttonium ( Dumitrica, 1983a) I had mentioned that the structure of the arms of this genus resembles perfectly the arm structure of some Cretaceous and Jurassic Hagiastridae , as for example Higumastra Baumgartner . On the basis of this similitude I had supposed that the origin of this genus should be in one of the late Cretaceous higumastrins. At present, when we know that the hagiastrid have a special type of microsphere, which is never spherical, and a pyloniacean mode of growth in their early ontogenetical stages (Dumitrica in De Wever et al., 2001), whereas the microsphere of Suttonium and of all Suttoniidae is a simple sphere, and shell growth is simple, it is difficult to sustain such a filiation. Consequently, despite the structural similarity of arms any relationship between Suttonium and the higumastrins is excluded. Until present its origin remains somehow questionable. The spherical microsphere with dense circular pores of similar size and rather regular disposition, and the crescent shape deuteroconcha attached to it on the antapical side suggest a close relationship with the genus Homunculodiscus n. gen., and especially with Homunculodiscus tainemplecta (Caulet) . The presence of the three primary rays and of the arms around them seem to make this idea questionable although quite possible. If so, the three spines of this genus that constitute the centre around which the arms are built, plus the apical spine present usually on the microsphere of Suttonium and especially on the older species could have derived from 4 of the many rays lying in the equatorial plane and forming the radial structure of skeleton of Homunculodiscus . In this situation, the short bars connecting the central rays of the arms of Suttonium with the cortical shell in planes perpendicular to the disc, and that form a structure resembling that of the arm of some hagiastrids, are similar to the bars perpendicular to the disc connecting the multiple rays with the two cortical plates. A good candidate for this origination is until present the specimen illustrated by Nishimura (2001, pl. 2, fig. 17) as Suttonium riedeli Dumitrică from the late Paleocene Bekoma bidartensis Zone . Certainly, this specimen does not belong to S. riedeli , which is a species with slender arms. Nishimura’s specimen has short and very broad arms and belongs to a new, undescribed species. With this morphology Nishimura’s specimen suggests that the arms of Suttonium appeared by the reduction of the two cortical plates of the disc of the genus Homunculodiscus in the interradial portions after the appearance of the three rays. This change seems to have been rather fast at geological scale, somewhere in the middle Paleocene, since the specimen illustrated by Nishimura comes from the late Paleocene and the three primitive species of Homunculodiscus herein described are early Paleocene in age.

Once appeared, the evolution of the genus Suttonium followed a migration trend of the microsphere from the deuteroconcha and of the latter from the three-rayed cortical shell ( Dumitrică, 1983a, p. 39-40, fig. 2). The consequence of this trend is the appearance of a nassellarian morphology having the microsphere as a cephalis, deuteroconcha as thorax and cortical shell as abdomen, and the magnification of the angle between the lateral arms from around 120° in Suttonium riedeli Dumitrică to 120° or greater in S. anomalum (Sutton) and finally to 180° in S. praedicator Schaaf.

Range. Late or middle Paleocene to Holocene.