Silicotintinnabulum, Dumitrică, 2024

Dumitrică, Paulian, 2024, New Middle Triassic Bell-Shaped Nassellarian Radiolaria From Alpine And Carpathian Areas, Acta Palaeontologica Romaniae 20 (1), pp. 51-75 : 54-55

Genus Silicotintinnabulum n. gen.

Type species. Silicotintinnabulum formosum n. sp.

Diagnosis. Test dicyrtid, bell-shaped bearing a tricostate apical horn with hollow cylindrical or conical base, globular cephalis and wide conical thorax. Initial spicule without dorsal ray, and all the other rays of the initial spicule, except the apical one, not extended outside.

Etymology. From Latin silica – silicon dioxide, and tintinnabulum – small bell, neuter gender.

Remarks. Morphologically, this genus resembles, partly, the upper Ladinian to upper Triassic genus Pseudosaturniforma Kozur & Mostler from which it differs by having a well-developed apical horn, thorax with pores of various locations and sizes, instead of only a row of large pores separated by longitudinal rays, and the initial spicule without dorsal ray. The two genera differ also by having different stratigraphical distributions, the genus Silicotintinnabulum being known only in the Middle Triassic (uppermost Anisian to Lower Ladinian), whereas the species of the genus Pseudosaturniforma are known from the upper Ladinian ( Tekin & Mostler, 2005) to Upper Triassic according to Kozur & Mostler (1979), Blome (1984), Tekin (1991), Bragin (2007) and Ozsvárt et al. (2017).

The morphological similarities and differences between the two genera and their different ranges suggest that the genus Silicotintinnabulum was the forerunner of the genus Pseudosaturniforma . The trend toward the transition to the genus Pseudosaturniforma started in the lower Ladinian with the appearance of the transitional species Silicotintinnabulum transitum n. sp., represented until present by a single specimen, and continued during the lower part of upper Ladinian (Longobardian), a period when many evolutionary changes took place in the families Oertlispongidae ( Kozur & Mostler, 1996b) , Muelleritortiidae ( Kozur, 1988; Kozur & Mostler, 1996a) and many other radiolarian groups ( Tekin & Mostler, 2005; Kozur & Mostler, 2006). For Silicotintinnabulum this change signified: a) the reduction and, finally, disappearance of the apical horn, b) the reduction of the number of thoracic pores to a single row of 5-8 and, finally, to only 6 wide circumferential windows between cephalis and the distal ring of the thorax, and c) the appearance of the dorsal ray in the initial spicular system. The apical horn is completely missing in the Upper Triassic (upper Carnian to lower or upper Middle Norian) species Pseudosaturniforma carnica Kozur & Mostler 1979 , P. latimarginata Kozur & Mostler 1979 and P. minuta Blome 1984 , but it is present to a small three-bladed spine at the oldest upper late Ladinian species P. ladinica Tekin & Mostler 2005 from the Dinarides of Bosnia and Herzegovina. The reduction of the thoracic pores to a single row of 5-8 large circumferential windows between cephalis and the distal circle determined the appearance of 5-8 straight thoracic rays that represent intervening bars of these large pores, and are remnants of the thorax of Silicotintinnabulum transitum n. sp. A trend towards the appearance of such windows can be seen at the species S. transitum n. sp. (fig. 1e) of the present study, where the thorax has a distal circumferential row of large pores resembling the pores of Pseudosaturniforma ladinica . Moreover, the continuation of this trend toward reduction of the thoracic elements to six can be remarked if we compare species P. ladinica with the younger species ( P. carnica , P. latimarginata and P. minuta ) that have only six pores or windows and radial thoracic bars, respectively, situated in the space between the base of the cephalis and the distal circle (see Blome, 1984, pl. 13, figs. 11, 18).

Due to its simple shell, the genus Pseudosaturniforma was initially considered a monocyrtid nassellarian by Kozur & Mostler (1979) and Blome (1984), but Tekin & Mostler (2005) emended this assignation and considered it as a dicyrtid genus, an opinion with which I completely agree. Its relationship with Silicotintinnabulum , described in this study, confirms this opinion. It proves that Pseudosaturniforma is a true dicyrtid resulted by a trend toward simplification of the thorax of its forerunner and reduction and finally, complete disappearance of the apical horn.

Stratigraphic range. Upper Anisian to lower Ladinian so far as known.