Cyrtograptus hemmanni BOUČEK , 1934

Štorch, Petr & Manda, Štěpán, 2019, Little Known Homerian (Lower Silurian) Graptolites From Kosov Quarry Near Beroun, The Czech Republic, Fossil Imprint 75 (1), pp. 44-58 : 55

Cyrtograptus hemmanni BOUČEK, 1934

Text-figs 7a , 8 a–b

1934 Cyrtograptus hemmanni n. sp.; Bouček, pp. 91–92, pl. 1, figs 3–4.

1952 Cyrtograptus hemmanni BOUČEK ; Münch, p. 138, pl. 52, fig. 3a–b.

2019 Cyrtograptus hemmanni BOUČEK ; Manda et al., fig. 11.

H o l o t y p e. Specimen from the testis (upper lundgreni) Biozone of Lerchenhügel near Ronneburg in Thuringia, designated and figured by Bouček (1934: pl. 1, fig. 4), reportedly housed in the Naturkundemuseum Altenburg .

M a t e r i a l. Two flattened specimens from the upper lundgreni Biozone of Kosov 2 section (samples K2/9 and K2/10; see Manda et al. 2019: fig. 2).

D i a g n o s i s. Loosely coiled main stipe and several strongly dorsally curved cladia of the first and second order, commonly separated by 2–4 thecae. Sicula 0.7 mm long, apex attains dorsal wall of th1. Proximal thecae hooked, high-triangular with small apertural spines, not overlapping. Apertural hooks retained in proximal thecae of primary cladia. Distal thecae are simple, slightly overlapping tubes with dorsally facing apertures. Maximum dorso-ventral width 0.75 mm, attained in pre-cladial part; distal width of main stipe and cladia 0.5–0.6 mm.

D e s c r i p t i o n. A mature, relatively complex rhabdosome from the Kosov quarry possesses a main stipe, which forms a loose helical spiral, with little more than one volution preserved. Diameter of the first whorl is 14 mm, measured between th1 and half of the first whorl. Main stipe bears several (7 in the present specimen) dorsally curved primary cladia, each possessing 1–3 second-order cladia. First primary cladium arises from th10, subsequent cladia of the first order are separated by 1–3 (usually 2) thecae. First cladia of the second order arise from the fourth (occasionally third) theca of the primary cladia. Subsequent second-order cladia are separated by further 3–4 thecae. Sicula is small, 0.7 mm long and 0.28 mm wide across the aperture. Sicula apex reaches a level immediately below the dorsal wall of th1. Theca 1 and th2 are relatively short and high-triangular, with a hooked apertural part; subsequent proximal thecae are somewhat axially elongated, with a rather narrow prothecal part and triangular metatheca, terminated by a small apertural hook with short, presumably paired lateral spines. Dorso-ventral width increases from 0.62 mm at thl to the maximum 0.75 mm attained by th6–7. Proximal thecae are on the convex side of the stipe, until about th16, which gave origin to the third cladium of the first order. Thereafter the main stipe exhibits axial torsion through 180 degrees within subsequent 8–9 thecae, so that the more distal thecae occur on the concave, “inner-facing” side of the stipe. Thecal apertural hook apparently retreats beginning at about th13– 15. Distal thecae, growing on convex part of the procladium are low and triangular, with rather distally facing apertures, and DVW ca. 0.55 mm. Primary cladia commence with triangular, partly isolated thecae having slightly curved, somewhat hooked apertural parts, and apertures still furnished with tiny spines. Subsequent thecae become lowtriangular in outline, with dorsally facing apertures similar to those of the distal part of the main stipe. No overlap has been observed between the thecae. Dorso-ventral width of the cladia is 0.5–0.55 mm. 2TRD is 1.7 mm at th2, 1.92 mm at th5 and 1.87 mm at th10. Distal thecae of both the main stipe and cladia number 8.5 in 10 mm (2TRD dist. = 2.34– 2.38 mm). A second, incomplete and somewhat deformed rhabdosome matches well the specimen described above in all principal features and measured parameters.

R e m a r k s. Bouček (1934) described C. hemmanni based on two specimens from the Thuringian Testograptus testis Biozone, which probably corresponds with the upper lundgreni Biozone of the Kosov 2 section, considering the known stratigraphic range of Testograptus testis ( BARRANDE, 1850) in central Europe. The primary stipe of the type specimen is loosely coiled for about 360 degrees. Cladia of the first and second order are separated by 3–5 thecae, as opposed to 1–4 thecae in the present specimens from Kosov 2. However, distinct variation in the spacing of cladia can be observed even within each of the two specimens from Kosov. Dorso-ventral width is 0.5–0.6 mm in the Thuringian holotype, which is consistent with the distal width of 0.55 mm recorded in the Kosov material. The latter, however, attains a maximum DVW of 0.75 mm in its proximal, hooked triangular thecae. Both Bohemian and Thuringian specimens retain hooked thecae until the level of axial torsion of the main stipe, which begins after the third or fourth cladium. Hooked thecae with minute spines are also developed in the proximal parts of primary cladia. More detailed comparison is barely possible, due to the poor preservation and missing proximal end in the Thuringian specimens.

Cyrtograptus hemmanni has been recorded to date only from the upper lundgreni Biozone of Thuringia, Germany ( Bouček 1934, Münch 1952, Jaeger 1991), and central Bohemia, the Czech Republic ( Manda et al. 2019). The species, which exhibits hooked, relatively high triangular most proximal thecae and slender stipes, resembling those of Cyrtograptus hamatus ( BAILY, 1861) can be readily differentiated from the latter by its loosely coiled rhabdosome, much less abrupt axial twisting of the main stipe, and numerous cladia of first and second order. Cyrtograptus lundgreni TULLBERG, 1883 has a markedly larger and more robust rhabdosome, widely spaced primary cladia, and strongly axially elongated, low triangular metathecae, which become high triangular and considerably isolated before the start of axial twisting of the stipe. Cyrtograptus ramosus BOUČEK, 1931a possesses axially elongated proximal thecae, along with straight cladia of the first order. Secondary cladia have not been recorded. Cyrtograptus mancki BOUČEK, 1931b has a tightly coiled proximal part and densely spaced, generally straight cladia separated by a single theca. Cyrtograptus multirami s differs in having a markedly robust rhabdosome, and somewhat straightened proximal part of the main stipe with low-triangular, axially elongated thecae.

Figures

Text-fig. 7. Monograptid graptolites from the upper lundgreni (a–b, d–e) and lower ludensis-gerhardi (c) biozones of the Kosov quarry. a – Cyrtograptus hemmanni BOUČEK, 1934: PŠ 3902a, sample K2/10. b, d–e – Monograptus ambiguus JAEGER, 1991: b – PŠ 3883, sample K2/6; d – PŠ 3899, sample K2/7; e – PŠ 3851, ventrally curved proximal part of the specimen figured on Text-fig. 5h, sample K2/6. c – Pristiograptus auctus RICKARDS, 1965: PŠ 3839, sample K1/32. All specimens × 10, except for (a) × 2. Black scale bar 1 mm, white scale bar 10 mm.

Text-fig. 8. Cyrtograptus hemmanni BOUČEK, 1934 from the upper lundgreni Biozone of the Kosov 2 section. a – PŠ 3902b, central part of large rhabdosome, for complete counterpart see Text-fig. 7a, sample K2/10; b – PŠ 3886/2, deformed rhabdosome from sample K2/9. Both specimens × 5, scale bar 2 mm.