Triactoma tithonianum Rüst, 1885

Dumitrica, Paulian, Dieni, Iginio & Massari, Francesco, 2022, Valanginian Radiolarians Of Ne Sardinia (Italy) In The Frame Of The Weissert Event, Acta Palaeontologica Romaniae 18 (2), pp. 97-159 : 109-110

Triactoma tithonianum Rüst, 1885

Fig. 7s, s 1, t, t1 , t 2

Selective synonymy due to absence of spines:

1885 Triactoma tithonianum Rüst , p. 289, pl. 28, fig. 5.

1973 Triactoma tithonianum Rüst - Foreman, p. 289, pl. 2. fig. 1.

1986 Triactoma sp. C – Aita & Okada, p. 122, pl. 1, fig. 4.

1994 Triactoma tithonianum Rüst – Jud, p. 115, pl. 23, figs. 11, non 10.

1995 Triactoma tithonianum Rüst – Baumgartner et al., p. 592, pl. 3097, figs, 1-3.

Description. Skeleton double shelled, spherical externally to slightly rounded triangular in face view bearing 3 three-bladed spines of unknown length and morphology. Pores circular and polygonally (hexagonally to pentagonally) framed on surface. Medullary shell double consisting of a small spherical microsphere with a few pores and a concentric and smooth outer medullary shell with circular or subcircular pores, both closely interconnected by simple or forked radial bars. Medullary shell interconnected to cortical shell by a few very thin radial bars, no one (it seems) aligned with the external spines.

Studied material. Two partly broken specimens: one in PD120-OZ834-R10-13 (fig. 7s), the other one in PD120- OZ825-R12-09 (7t).

Dimensions. Diameter of cortical shell of both specimens 85 µm, of medullary shell 29 µm.

Remarks. The two illustrated specimens seem to belong indubitably to this species in spite of the absence of the three spines. Interesting is the presence of a double medullary skeleton, although the diagnosis of the genus Triactoma and the family to which it has been assigned (Pessagno et al., 1989, p. 202) mentions that the skeleton consists only of a cortical shell. However, Dumitrică (in De Wever et al., 2001, fig. 63.2) mentioned and illustrat- ed internal centripetally directed radial bars aligned with the spines, which he considered as remains of a medullary shell. This shell, as seen in T. tithonianum would differs from that of the Middle Jurassic species by not being connected with the cortical shell by rays aligned with spines. Outer medullary shell is connected with the cortical one by a very small number (5-6 probably) of thin radial bars, no one extended outside. Cortical shell is spherical or very slightly rounded triangular in face view. Pores are circular or subcircular and polygonally framed, but frames are not of equal thickness in radial direction. Tops of them are irregular, some of them may be higher and slightly protruding. Base of spines is reinforced by an inner triradial support structure rotated 30° around the axis of spines relative to the blade of spines ( Figs. 7 sa, ta). Such a structure is not rare in Spumellaria and Entactinaria radiolarians (Dumitrică, 2013b, p. 85) but its morphology is variable.

Rang e. UAZones 6-22 (mid Bathonian to late Barremian – early Aptian

Figures

Fig. 7 a - Praeconosphaera sphaeroconus (Rüst OZ826. b-d - Praeconosphaera antiqua (Parona): b - OZ834, c - OZ838, d - OZ825. e - Tritrabs ewingi (Pessagno), arm in lateral view, OZ836. f-h - Emiluvia chica Foreman, OZ836; f - face view, g - lateral view from a spine, h - lateral view of a broken specimen showing the medullary test. i - Pantanellium sp. 1, OZ825. j, l-n1 - Pantanellium squinabolli (Tan): j - OZ834; l, m - OZ834, n - OZ836; m1, n1, details of inner wall of pores. k - Pantanellium sp. 2, OZ834. o – Pantanellium cf. squinaboli (Tan), OZ836; p - Praeconocaryomma copiosa Wu, OZ836. r - Praeconocaryomma thurowi Dumitrică n. sp., OZ836. s, t - Triactoma tithonianum Rüst, side views; s - OZ834, broken off specimen showing medullary shell; t - OZ825, equatorial view, t1 – face view; s1, t2 - detail of the base of spines of figs. s and t, respectively, x800. Figs. a-h, l-n x200; i-k, o-r x300; s, t x400, m1, n1 x500.

Fig. 1 Geologic map of the Siniscola area, NE Sardinia (from Dieni & Massari, 1971, updated in 1986). Legend: a - Recent and present alluvial deposits (Holocene). b - Scree deposits (Würm-Holocene). c - Detrital cones mostly composed of gelifraction limestone clasts (“éboulis ordonnés”) (Würm-Holocene). d - Uncemented alluvial cones mostly consisting of limestone clasts (Würm-Holocene). e - Terraced alluvial deposits composed of clasts of basement rocks (Riss?). f - Tightly cemented and dismembered alluvial cones mostly made up of gelifraction limestone clasts (Riss?). g - Cuccuru ’e Flores Conglomerate: polymict, chaotic and thick-bedded breccia and stratified sandstone (middle Eocene). h - Siniscola Marl: grey and grey-yellow marl and, in the upper part, alternating marl/marly limestone with ammonites and very abundant radiolarians and sponge spicules (late Valanginian). i - Schiriddè Limestone: light-brown cherty limestone and limestone with marly interbeds, containing ammonites, radiolarians, sponge spicules and calpionellids (early Valanginian). j - Monte Bardia Limestone: thick-bedded to massive white limestone in places bioconstructed, with corals, nerineids, etc. (early Tithonian - late Berriasian). k - S’Adde Limestone: light-brown to grey fine-grained, well-bedded limestone, condensed with hardgrounds near the base, cherty in the upper part, with ammonites and belemnites in the uppermost part (latest Bathonian - earliest Tithonian). l - Dorgali Formation: well bedded quartz-arenitic and locally ooidal brown dolostone (middle-late Bathonian). m - Crystalline basement (phyllite, micaschist, paragneiss, quartzite, etc.) (Palaeozoic). n - Fault. o - Attitude of beds and schistosity planes. p - Limits of slumped masses of Jurassic formations (Quaternary). q - Concave-upward slip surfaces delimiting rotational slumps (Quaternary). r - Edge of fluvial terrace. s - Quarry. A-A’ Trace of the geological section of Fig. 2. (extracted from Bottini et al., 2018).

Fig. 2 Geological section across the S’Ozzastru area (trace A-A’ in the geological map of Fig. 1). Legend: a - Recent and present alluvial deposits (Holocene). b - Cuccuru ’e Flores Conglomerate: sandstone and breccia (middle Eocene). c - Siniscola Marl: marl (1) and marly limestone (2) (late Valanginian). d - Schiriddè Limestone: cherty limestone (early Valanginian). e - Monte Bardia Limestone: limestone (early Tithonian - late Berriasian). f - S’Adde Limestone: limestone and cherty limestone (latest Bathonian - earliest Tithonian). g - Dorgali Formation: dolostone (middle-late Bathonian). h - Metamorphic basement (Palaeozoic). i - Fault (modified from Bottini et al., 2018).