Monograptus subflexilis PŘIBYL , 1941

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 : 53-55

Monograptus subflexilis PŘIBYL, 1941

Text-figs 5 c–d, f,?a; 6b, d

1941 Monograptus subflexilis n. sp.; Přibyl, pp. 3–4, pl. 1, figs 2–4,?1.

1943 Monograptus subflexilis PŘIBYL ; Přibyl, p. 10, pl. 1, figs 4–5.

? 1990 Monograptus sp. aff. M. flexilis ELLES; Lenz and Melchin, fig. 3J–L.

? 1991 Monograptus aff. flexilis ELLES; Lenz and Melchin, pp. 229–230, fig. 17f–h.

2019 Monograptus subflexilis PŘIBYL ; Manda et al., fig. 8h, i.

H o l o t y p e. Specimen no NM-L 30948 from the testis (upper lundgreni) Biozone exposed in a small quarry south of Bykoš, figured by Přibyl (1941: pl. 1, fig. 3).

M a t e r i a l. 15 complete flattened rhabdosomes and 14 incomplete rhabdosomes and fragments from the upper lundgreni Biozone of the Kosov 2 section (samples K2/15– K2/11; see Manda et al. 2019: fig. 2) and material figured by Přibyl (1941, 1943).

D i a g n o s i s. Rhabdosome doubly curved, with proximal part comprising 4–6 thecae dorsally curved, thereafter ventrally curved with curvature decreasing distally. Sicula very slightly dorsally curved, with tiny virgella. Hooked thecae of priodon - type, with relatively broad ventroproximally facing aperture and only slightly inclined ventral prothecal wall. Dorso-ventral width increases steadily from 0.7 mm at th1 to 1.6– 1.95 mm at th20, thereafter slowly increases to 2.9–3.0 mm at th50–60. Distal thecae number 10 in 10 mm, proximal thecae more tightly spaced.

D e s c r i p t i o n. The rhabdosome is characteristically doubly curved, in excess of 100 mm long. The proximal part, comprising the first 4–6 thecae is slightly but distinctly dorsally curved. Thereafter, curvature gradually switches to ventral, which decreases distally, so that the distal part becomes straight from th20–30. The sicula is very gently dorsally curved, 1.6–1.7 mm long and 0.34–0.45 mm wide across the aperture. The apex attains the level of the dorsal wall of th2. First theca originated 0.2–0.25 mm from the sicular aperture. The thecae are hooked, with metathecal hook comprising about two-fifths the rhabdosome’s dorso-ventral width. Thecal apertures are directed ventroproximally; thecal overlap is one-quarter to one-third in proximal thecae and slightly exceeds one-half in distal thecae. Ventral prothecal wall is only slightly inclined to the rhabdosome axis, as opposed to the steeply inclined interthecal septum. Dorso-ventral width of the rhabdosome increases steadily from 0.7–0.8 mm at th1, to 0.8–0.85 mm at th3, 0.9–1.1 mm at th5, 1.3–1.4 mm at th10 and 1.6–1.95 mm at th20. Thereafter the rate of increase in dorso-ventral width slows down, until a width of 2.9 mm is attained at about th50. Distal fragments questionably assigned to M. subflexilis are ca. 3.0 mm wide. 2TRD is 1.53–1.7 mm at th2 and 1.7– 0.8 mm at th10. Distal thecae number 10 in 10 mm, with 2TRD ranging from 1.85 to 2.1 mm.

R e m a r k s. Monograptus subflexilis is known solely from the lundgreni Biozone of the Prague Synform, the Czech Republic. The species was most likely derived from the priodon-flemingii stem line, from which it is primarily distinguished by having a more or less moderately sigmoidal rhabdosome. Teller (1986: 55) lumped M. subflexilis in Monograptus flemingii ( SALTER, 1852). The present material of M. subflexilis suggests that this assessment was not correct.

Monograptus subflexilis can be consistently distinguished from the Sheinwoodian and Homerian M. flemingii in that the latter has a markedly higher rate of rhabdosome widening in the proximal and mesial parts of the rhabdosome, a longer sicula, and proximal dorsal curvature of the generally straight rhabdosome commonly limited to the 2–4 most proximal thecae. Distal fragments of M. flemingii are still wider than those of M. subflexilis , and apertural hooks are more curved so that thecal apertures face proximally rather than proximo-ventrally.

The middle-upper Sheinwoodian Monograptus belophorus ( MENEGHINI, 1857), although rather variable in general shape of the rhabdosome, is markedly less robust and generally more dorsally curved in the proximal part. It has a longer sicula, a characteristically long and robust virgella, and less developed apertural hooks. Monograptus belophorus itself is a senior synonym of M. flexilis. Lenz and Melchin (1990, 1991) illustrated and briefly described the smoothly sigmoidal Monograptus aff. flexilis ELLES, 1900 from the middle – upper Sheinwoodian of the Canadian Arctic islands, and Obut and Sennikov (1977) reported similar specimens assigned to M. flexilis flexilis in association with Cyrtograptus tchukotkaensis OBUT et SENNIKOV, 1977 (= Cyrtograptus multirami s TÖRNQUIST, 1910) from the Chukotka Peninsula, Russia. Despite their stratigraphically earlier occurrence, the Canadian specimens match M. subflexilis in all parameters, including thecal spacing, used for their differentiation by Lenz and Melchin (1991). The only difference is that the dorsally curved proximal part of M. aff. flexilis, sensu Lenz and Melchin (1991) encompasses a greater number of proximal thecae (12–15 compared to 4–6 in M. subflexilis ). The Canadian form is tentatively assigned to M. subflexilis in this study.

The early Telychian Stimulograptus utilis LOYDELL, 1991 exhibits rhabdosome curvature closely similar to that of M. subflexili s, but can be readily distinguished by having considerably lesser DVW, negligible thecal overlap and prominent apertural spines.

Figures

Text-fig. 5. Monograptid graptolites from the upper lundgreni (a–h, l–m) and lower ludensis-gerhardi (i–k) biozones of the Kosov quarry.?a, c–d, f – Monograptus subflexilis PŘIBYL, 1941: a – PŠ 3874, slender juvenile specimen from sample K2/11; c – PŠ 3871/1, sample K2/13; d – PŠ 3871/2, sample K2/13; f – PŠ 3864, sample K2/12. b, e, g–h, l–m – Monograptus ambiguus JAEGER, 1991: b – PŠ 3899, sample K2/7; e – PŠ 3896, least ventrally curved specimen, sample K2/7; g – PŠ 3873, specimen showing small apertural spines in proximal thecae, sample K2/6; h – PŠ 3851, most ventrally curved specimen from sample K2/6; l – PŠ 3883, sample K2/6; m – PŠ 3866, sample K2/7. i–k – Pristiograptus auctus RICKARDS, 1965: i – PŠ 3848/2, sample K1/32; j – PŠ 3848/1, sample K1/32; k – PŠ 3839, sample K1/32. All specimens × 6, scale bar 1 mm.