Rhagasostoma tchvanovi ( Favorskaya, 1992 )

Koromyslova, Anna V., Taylor, Paul D., Martha, Silviu O. & Riley, Matthew, 2018, Rhagasostoma (Bryozoa) from the Late Cretaceous of Eurasia: taxonomic revision, stratigraphy and palaeobiogeography, European Journal of Taxonomy 490, pp. 1-66 : 38-41

Rhagasostoma tchvanovi ( Favorskaya, 1992)

Fig. 15 , Table 9

Onychocella tchvanovi Favorskaja, 1992: 126 , pl. 68, figs 6–8.

? Rhagasostoma gibbosum var. weybournensis Brydone, 1930: 48 , pl. 26, fig. 12.

non Rhagasostoma gibbosum var. weybournensis – Brydone 1930: 48, pl. 26, figs 13–14.

Material studied

Holotype

UZBEKISTAN • Republic of Karakalpakstan ; late Campanian ( Cibicidoides voltzianus Zone ; LS 15) of the southern Aral Sea Region; TsNIGR Museum 33/12582 ( Fig. 15 A–C; figured by Favorskaya 1992: pl. 68, fig. 6).

Additional figured material

UZBEKISTAN • Same data as for holotype; TsNIGR Museum 34/12582 ( Fig. 15 D–E; figured by Favorskaya 1992: pl. 68, fig. 7) • Republic of Karakalpakstan ; middle Campanian ( Brotzenella monterelensis Zone ; LS14) of the southern Aral Sea Region; TsNIGR Museum 32/12582 ( Fig. 15F ) .

Description

Colony erect with flattened bifoliate branches (3–4 mm wide). Ancestrula and early astogeny not observed. Autozooids variable in shape, often broad and subrectangular with rounded distal ends; zooidal boundaries raised. Gymnocyst lacking. Cryptocyst extensive, finely pustulose, slightly depressed centrally, sometimes with proximolateral peripheral caverns ( Fig. 15F ). Opesia terminal, semielliptical, formed by two walls, a thin inner wall forms the distolateral part of the opesia, and a projecting thickened outer wall forms the proximolateral part ( Fig. 15C, E ). Proximal edge of opesia straight, smooth, with outgrowths at the two proximolateral corners which delineate small opesiules; outgrowths often destroyed giving the opesiules the appearance of opesiular indentations. Septula not observed. Ovicells endozooidal, ooecium is formed by the distal zooid, well-recognizable, with cryptocyst-like surface and arch-like proximal edge with elongated proximolateral processes extending along the cryptocyst of the maternal zooid ( Fig. 15 A–B). Avicularia interzooidal, smaller than autozooids, elongate. Rostrum channeled with elevated wing-like walls and pointed tip, narrow, almost straight, asymmetrical, dextral or sinistral. Top of rostrum reaches opesia of distal autozooid. Wedge-like structures not observed. Proximal part rounded, shorter and wider than rostrum, cryptocyst pustulose, concave. Opesia round with short slit proximally, opesiules lacking ( Fig. 15C ). Closure plates, kenozooids, intramural reparative budding of autozooids and avicularia not observed.

Remarks

Rhagasostoma tchvanovi differs from the closely related species R. gibbosum in the avicularian rostrum being long and almost straight instead of short and tilted to the right or left. The specimen described by Brydone (1930: pl. 26, fig. 12) as Onychocella gibbosum weybournensis differs from R. gibbosum ( Marsson, 1887) in having a long and straight avicularian rostrum and is conspecific with R. tchvanovi . Specimen 32/ 12582 in the collections of the TsNIGR Museum and labelled as “ O. subgibbosum Brydone, 1930 ” is not the one that was depicted by Favorskaya (1992: pl. 67, fig. 3) and belongs to R. tchvanovi .

Distribution

Middle to late Campanian Republic of Karakalpakstan, Uzbekistan: six localities between Chimboy/ Shımbay (Чимбой/Шымбай) and the Aral Sea.

Figures

Fig. 15. Rhagasostoma tchvanovi (Favorskaya, 1992). A–E. Late Campanian (Cibicidoides voltzianus Zone; LS15), southern Aral Sea Region, Republic of Karakalpakstan, Uzbekistan. A–C. Holotype, TsNIGR Museum 33/12582. A. Overview of erect bifoliate colony. B. Ovicellate autozooids and avicularia. C. Non-ovicellate autozooids and avicularia. D–E. TsNIGR Museum 34/12582. D. Overview erect bifoliate of colony. E. Non-ovicellate autozooids and avicularia. F. TsNIGR Museum 32/12582, middle Campanian (Brotzenella monterelensis Zone; LS14), southern Aral Sea Region, Republic of Karakalpakstan, Uzbekistan; overview of erect bifoliate colony, cryptocyst of autozooids with proximolateral caverns. Scale bars: A = 500 µm; B, E = 200 µm; С = 250 µm; D, F = 1 mm.

Tables

Table 9. Summary of measurements of Rhagasostoma tchvanovi (Favorskaya, 1992). For each parameter the range is given with the number of measurements in brackets. The arithmetic mean is given ± standard deviation. All measurements in µm.

Basin Stratigraphy Locality		North Ust-Yurt–Aral Basin Early to middle late Campanian Southern Aral Sea Region
	AzL	340–550 (18) 418.89 ± 50.98
	AzW	230–360 (18) 264.44 ± 22.29
Autozooids	CvL OpL OpW	70–100 (4) 80 ± 14.14 110–140 (14) 127.86 ± 9.75 110–140 (14) 120.00 ± 2.96
	OoL	140–230 (5) 198.00 ± 35.64
	OoW	180–230 (5) 210.00 ± 18.71
	AvL	290–530 (11) 381.82 ± 71.25
	RL	160–350 (11) 254.55 ± 58.20
Avicularia	RW PrL PrW	80–120 (11) 93.64 ± 13.62 110–190 (10) 137.00 ± 25.84 70–120 (11) 94.55 ± 17.53
	OpL	70–100 (8) 77.50 ± 10.35
	OpW	30–70 (8) 53.75 ± 11.88