Rhagasostoma minuens Brydone, 1936

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 : 16-20

Rhagasostoma minuens Brydone, 1936

Figs 6–7 , Table 3

Rhagasostoma inelegans var. minuens Brydone, 1936: 74 , pl. 35, figs 9–10.

Onychocella cf. inelegans ( Lonsdale, 1850) – Voigt 1949: 26, pl. 8, figs 1–2.

Material studied

Lectotype (here designated)

UNITED KINGDOM • England, East Sussex, between Brighton and Roedean; late Santonian ( Uintacrinus socialis Zone / lower Marsupites testudinarius Zone ); SM B 36883 ( Fig. 6 A–D; figured by Brydone 1936: pl. 35, fig. 10).

Paralectotype (here designated)

UNITED KINGDOM • England, Norfolk, Trimingham; early Maastrichtian ( Ostrea lunata Zone , Porosphaera Beds ); SM B 36882 ( Fig. 6 E–F; figured by Brydone 1936: pl. 35, fig. 9).

Additional figured material

UNITED KINGDOM • 2 specs; England, Kent; Coniacian of Chatham; SMF 29921 ( Fig. 7 A–B), 29922 ( Fig. 7 C ) • England, Wiltshire, SW of Salisbury, Harnham; early Campanian; SMF 29920 ( Fig. 7D ) • England, Norfolk, Trimingham; early Maastrichtian ( Ostrea lunata Zone ) [middle Campanian ( Belemnitella mucronata Zone ), Weybourne, Norfolk, in SM database]; SM B 36678, specimen (a) ( Fig. 7 H ) • 2 specs; England, East Sussex, Newhaven, Meeching Quarry; early Campanian; SMF 29943 ( Fig. 7 E ), 29944 ( Fig. 7 F ).

FRANCE • Hanches ; Campanian; SMF 29918 ( Fig. 7 G ) .

Other material

UNITED KINGDOM • 3 specs; England, East Sussex, Newhaven, Meeching Quarry ; early Campanian ; SMF 29919, 29942 , 29945 .

GERMANY • Schleswig-Holstein, Lägerdorf; early Campanian ; SMF 29917 .

Description

Colony encrusting sheet-like, fragments about 10 mm in diameter, or erect with flattened bifoliate branches, fragments up to 2.0–10.0 mm long by 2.0–5.0 mm wide. Ancestrula ( Fig. 6C ) about 0.27 mm in diameter, rounded rhombic. Autozooids variable in shape, often broad and subrectangular with rounded distal ends; zooidal boundaries raised. Gymnocyst lacking. Cryptocyst extensive, finely pustulose,

depressed or slightly convex centrally, sometimes with proximolateral peripheral caverns, up to 0.22 mm long ( Fig. 7 B–F), rarely peripheral caverns surrounding the cryptocyst ( Fig. 7H ). Opesia terminal, rarely subterminal, semielliptical with slight shelf in the distal part ( Fig. 6D, F ), formed by two walls, a thin inner wall delimiting the distolateral part and a salient, thickened outer wall delimiting the proximolateral part. Proximal edge of opesia straight, smooth, thickened, with outgrowths near the two proximolateral corners delimiting, small roundish opesiules (outgrowths are often broken thus giving the opesiules the appearance of opesiular indentations, or they may be obscured by sediment infills of the opesia). Septula not observed. Ovicells endozooidal, ooecium is formed by the distal zooid, recognizable as a small swelling of its proximal cryptocyst ( Figs 6D , 7C , E–F). Avicularia interzooidal, elongate, longer than autozooids. Rostrum channeled with elevated wing-like walls and pointed tip, falciform (almost the same width along the whole length of the rostrum or slightly expanding near apex, and pointed tip curved, sickleshaped), asymmetrical, dextral or sinistral. Proximal part rounded, shorter and wider than the rostrum. Cryptocyst pustulose, concave, without peripheral caverns. Opesia large, roundish or oval, with thickened articular ridges bearing two short teeth proximally and a long, slit-like opesiular indentation between the teeth; opesiules lacking. Kenozooids rounded, rare, located at the margins of a colony. Cryptocyst finely pustulose. Opesia round. Intramural reparative budding of avicularia within host avicularia sometimes observed ( Fig. 7B ), all with the same polarity as the host zooid. Intramural reparative kenozooidal buds sometimes observed within host autozooids and avicularia ( Fig. 7 D–E). Closure plates not observed.

Remarks

Brydone (1936) introduced the subspecies Rhagasostoma inelegans minuens for unilaminar colonies lacking ovicells. We have restudied Brydone’s syntypes in the SM collection, choosing a lectotype (the specimen figured by Brydone 1936: pl. 35, fig. 10); endozooidal ovicells are present in this colony. Voigt (1949) mentioned one specimen of “ Onychocella cf. inelegans ” from the early Campanian of Lägerdorf, Schleswig-Holstein of Germany but this belongs to Rhagasostoma minuens .

Rhagasostoma minuens can easily be distinguished from R. inelegans , R. brydonei sp. nov., R. aralense sp. nov. and R. operculatum sp. nov. in having a falciform avicularian rostrum. The species differs from R. angliae , specimens of which may have similar falciform avicularian rostra, in the avicularian cryptocyst having large roundish or oval opesia without opesiules instead of small subcircular opesia and two opesiules.

Distribution

Coniacian United Kingdom: Chatham, Kent, England. Late Santonian United Kingdom: between Brighton and Roedean, East Sussex, England. Early Campanian United Kingdom, England: Harnham, SW of Salisbury, Wiltshire; Meeching Quarry, Newhaven, East Sussex. Germany: Lägerdorf, Schleswig-Holstein. Campanian France: Hanches. Early Maastrichtian United Kingdom: Trimingham, Norfolk, England.

Figures

Fig. 6. Rhagasostoma minuens Brydone, 1936. A–D. Lectotype, SM B36883, late Santonian (Uintacrinus socialis Zone/lower Marsupites testudinarius Zone), between Brighton and Roedean, East Sussex, England, UK. A. Overview of encrusting colony. B. Growing edge. C. Early astogeny, ancestrula (arrowed), autozooids and avicularia. D. Autozoods with endozooidal ovicells (arrows) and avicularia. E–F. Paralectotype, SM B36882, early Maastrichtian (Ostrea lunata Zone, Porosphaera Beds), Trimingham, Norfolk, England, UK. E. Overview of encrusting colony. F. Autozooids and avicularia. Scale bars: A, E = 1 mm; B = 200 µm; C–D, F = 100 µm.

Fig. 7. Rhagasostoma minuens Brydone, 1936. A–C. Coniacian, Chatham, Kent, England, UK. A–B. SMF 29921. A. Overview of erect bifoliate colony. B. Autozooids with proximolateral peripheral caverns (Cv) and broken ooecia, avicularia, and intramural avicularia within host avicularia (arrowed). C. SMF 29922, autozooids with proximolateral peripheral caverns (arrowed) and broken ooecia (some with reparative sutures). D. SMF 29920, early Campanian, Harnham, SW of Salisbury, Wiltshire, England, UK; non-ovicellate autozooids, some with proximal peripheral caverns, and avicularia; intramural kenozooids observed within host autozooid and avicularia (arrowed). E–F. Early Campanian, Meeching Quarry, Newhaven, East Sussex, England, UK. E. SMF 29943, ovicellate (with broken ooecia) and non-ovicellate autozooids, some with proximal peripheral caverns, avicularia, and intramural kenozooid within a host avicularium (arrowed). F. SMF 29944, ovicellate (with broken ooecia) and non-ovicellate autozooids with proximal peripheral caverns and avicularia. G. SMF 29918, Campanian, Hanches, France, overview of bifoliate colony showing autozooids and avicularia. H. SM B36678, specimen (a), late Campanian to early Maastrichtian (Ostrea lunata Zone) [middle Campanian (Belemnitella mucronata Zone), Weybourne, Norfolk, in SM database], Trimingham, Norfolk, England, UK; autozooids with peripheral caverns surrounding the cryptocyst and avicularia. Scale bars: A = 2 mm; B, E–G = 1 mm; C = 500 µm; D = 300 µm; H = 200 µm.

Tables

Table 3. Summary of measurements of Rhagasostoma minuens Brydone, 1936. 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		Southern North Sea Basin Coniacian – Early Maastrichtian Sussex, Trimingham, Kent, Wiltshire, Hanches	North German Basin Early Campanian Lägerdorf	Total
	AzL	360–830 (53) 575.09 ± 115.42	550–780 (6) 705.00 ± 83.61	360–830 (59) 588.31 ± 118.80
	AzW	260–560 (53) 393.40 ± 71.12	480–670 (6) 521.67 ± 74.14	260–670 (59) 406.44 ± 80.85
Autozooids	CvL OpL OpW	30–220 (29) 122.41 ± 48.82 120–300 (53) 191.51 ± 48.77 140–280 (53) 201.13 ± 32.62	60–180 (3) 133.33 ± 64.29 200–260 (5) 232.00 ± 22.80 230–260 (5) 242.00 ± 13.04	30–220 (32) 123.44 ± 49.29 120–300 (58) 195.00 ± 48.35 140–280 (58) 204.66 ± 33.41
	OoL	350–360 (2) 355.00 ± 7.07	170–380 (4) 277.50 ± 91.79	170–380 (6) 303.33 ± 81.65
	OoW	250–260 (2) 255.00 ± 7.07	250–320 (4) 285.00 ± 28.87	250–320 (6) 275.00 ± 27.39
	AvL	520–1080 (46) 760.87 ± 172.51	840–1030 (5) 916.00 ± 76.35	520–1080 (51) 776.08 ± 171.52
	RL	270–740 (46) 481.96 ± 112.81	500–590 (5) 546.00 ± 32.09	270–740 (51) 488.24 ± 109.12
	RCL	0–330 (47) 152.77 ± 73.74	200–270 (5) 236.00 ± 25.10	0–330 (52) 160.77 ± 74.62
Avicularia	RW PrL	100–330 (47) 180.21 ± 48.12 150–470 (45) 278.44 ± 74.16	160–210 (5) 190.00 ± 20.00 300–440 (5) 370.00 ± 52.92	100–330 (52) 181.15 ± 46.13 150–470 (50) 287.60 ± 77.05
	PrW	160–400 (46) 260.00 ± 59.10	260–340 (5) 300.00 ± 30.82	160–400 (51) 263.92 ± 58.00
	OpL	80–220 (42) 141.90 ± 37.24	160–230 (5) 200.00 ± 27.39	80–230 (47) 148.09 ± 40.36
	OpW	60–200 (42) 104.29 ± 30.21	120–170 (5) 138.00 ± 20.49	60–200 (47) 107.87 ± 30.99
Kenozooids	KzL KzW OpD	340–430 (4) 372.50 ± 40.31 190–260 (4) 245.00 ± 51.96 60–100 (4) 80.00 ± 16.33	550 (1) 350 (1) 150 (1)	340–550 (5) 408.00 ± 86.72 190–350 (5) 266.00 ± 65.04 60–150 (5) 94.00 ± 34.35