Monograptus ambiguus JAEGER , 1991

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 : 51-53

Monograptus ambiguus JAEGER, 1991

Text-figs 5b, e , g–h, l–m, 6c, e, 7b, d–e

1965 Monograptus subflexilis PŘIBYL ; Jaworowski, text-fig. 3, fig. 6, pl. 1, fig. 6.

1991 Monograptus ambiguus sp. n.; Jaeger, pp. 343, 345, 347, figs 1, 29/8, 10.

2019 Monograptus ambiguus JAEGER ; Manda et al., figs 7a, 9d, e.

H o l o t y p e. Specimen no MBg705.1 from the uppermost lundgreni Biozone of the Gräfenwarth section, Thuringia, Germany; figured by Jaeger (1991: fig. 29/8, 10).

M a t e r i a l. 59 mostly complete flattened rhabdosomes from the uppermost lundgreni Biozone of the Kosov 2 section (samples K2/8–K2/5; see Manda et al. 2019: fig. 2)

D i a g n o s i s. After Jaeger (1991), emended. Rhabdosome ventrally curved with slight dorsal curvature at the proximal end comprising th1–th2. Sicula straight or slightly dorsally curved. Thecae hooked, overlapping for about one-quarter their length, with steeply inclined interthecal septa, curved prothecae and ventral prothecal walls parallel to the rhabdosome axis. Dorso-ventral width increases gradually from about 0.7 mm at th1 to the maximum 1.3 mm most distally. Metathecal hooks comprise half the rhabdosome’s width in proximal thecae and onethird the width in distal thecae.

D e s c r i p t i o n. Rhabdosome is very gently dorsally curved over th1–2. Thereafter, it is moderately ventrally curved, until at least th14–33. Ventral curvature varies among specimens, but becomes almost negligible distally, which is well seen in more than 30 mm long, mature rhabdosomes. The sicula is 1.5–1.85 mm long and 0.22–0.33 mm wide across its concave aperture, which is furnished with a tiny virgella. Sicular apex attains the middle or the dorsal wall of th2. Protheca of th1 begins with a prominent bulge 0.17–0.2 mm from the sicular aperture (e.g. see Text-fig. 5b, e, l ). It is strongly curved in a similar manner to all other prothecae, which have ventral walls either parallel to the rhabdosome axis or very gently (≤2°) inclined. Thecae overlap for about one-quarter their length; interthecal septa are steeply inclined at an angle of 50–85°. Metathecae are hooked, with proximally facing apertures, in proximal thecae probably furnished by short, paired lateral spines. Metathecal hook comprises from one-half the rhabdosome’s dorso-ventral width in proximal thecae to one-third in distal thecae. Dorso-ventral width of the rhabdosome increases gradually from 0.64–0.84 mm at th1 (M = 0.72, n = 10) to 0.72–0.84 at th3 (M = 0.76 mm, n = 10), 0.77–0.93 mm at th5 (M = 0.85, n = 10), and 0.92–1.2 mm at th10 (M = 0.99, n = 9) to the maximum 1.27 mm attained by th40. 2TRD increases from 1.28–1.73 mm at th2 (M = 1.6 mm, n = 10) to 1.45–1.95 mm at th10 (M = 1.71, n = 10). Distal thecae number 10 in 10 mm, with 2TRD ranging from 1.8 to 2.1 mm.

R e m a r k s. Rich material of Monograptus ambiguus found in the uppermost lundgreni Biozone of the Kosov 2 section ( Manda et al. 2019) matches the limited type material of the species described by Jaeger (1991) from the uppermost Testograptus testis Subzone (uppermost lundgreni Biozone) of the Gräfenwarth section, Thuringia in rhabdosome form, thecal form and measured parameters. The present material from the Kosov quarry, however, revealed remarkable variability in the ventral curvature of the rhabdosome, and has shown that specimens from the uppermost lundgreni Biozone of Bartoszyce and Gołdap boreholes in NE Poland, refered to Monograptus subflexilis PŘIBYL, 1941 by Jaworowski (1965), also belong to M. ambiguus . Thus, the species, so far recorded in Germany, the Czech Republic and Poland, qualifies as a useful biostratigraphic marker of the uppermost part of the lundgreni Biozone in Europe. Monograptus ambiguus can be readily distinguished from M. subflexilis by its very short, gently dorsally curved proximal portion of the rhabdosome, more prominent subsequent ventral curvature, much narrower and more slowly increasing dorso-ventral width, relatively longer ventral prothecal walls, almost parallel to the stipe, and much smaller apertural hooks. Maximum width of the rhabdosome does not exceed 1.3 mm in M. ambiguus , in contrast to about 3 mm in M. subflexilis . The simply hooked thecal form of M. ambiguus is closely similar to that of M. subflexilis , which indicates that, despite superficial similarity to Homerian species with S-shaped or ventrally curved rhabdosomes and coiled, streptograptid-like thecae ( Monograptus retroflexus TULLBERG, 1883 and Monograptus serexiguus JAEGER, 1991 ), M. ambiguus is a true, although more derived offshoot from the priodon-flemingii monograptid stem line.

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.