Pristiograptus auctus RICKARDS , 1965

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

Pristiograptus auctus RICKARDS, 1965

Text-figs 5 i–k, 6a, 7c

1965 Pristiograptus auctus sp. nov.; Rickards, pp. 260–261, text-fig. 2h, pl. 31, figs 2–4.

1970 Pristiograptus auctus RICKARDS ; Palmer, pp. 340–341, pl. 14, figs h–p, pl. 15, figs c–d.

1975 Pristiograptus auctus RICKARDS ; Lenz, pp. 84, 86, text-fig. 3a–d, pl. 1, figs 1–2.

1991 Monograptus auctus ?; Jaeger, fig. 1.

1993 Pristiograptus auctus RICKARDS ; Burns and Rickards, fig. 4b.

1998 Pristiograptus auctus RICKARDS ; Maletz et al., p. 145, textfig. 4/7, pl. 1, fig. 7.

2019 Pristiograptus auctus RICKARDS ; Manda et al., figs 7a, 9d, e.

H o l o t y p e. Specimen no HUR/7W/46 from the lower nilssoni Biozone (lowermost Ludlow) of Wandale Hill, Howgill Fells, England; figured by Rickards (1965: text-fig. 2h, pl. 31, fig. 4).

M a t e r i a l. 8 flattened, mostly complete specimens from the lower – middle ludensis-gerhardi Biozone of the Kosov 1 section (samples K1/31 and K1/32; see Manda et al. 2019: fig. 3).

D i a g n o s i s. After Rickards (1965), emended. Robust pristiograptid rhabdosome with slightly ventrally curved dorsal wall in the proximal part comprising th1– th5. The sicula is gently ventrally curved, in particular near its concave, ventro-proximally facing aperture, and is commonly furnished with a weakly developed rutellum. The sicular aperture is somewhat covered by a short and robust virgella, which is expanded into a prominent globular structure.

D e s c r i p t i o n. The robust, almost 50 mm long, generally straight rhabdosome is weakly ventrally curved in the proximal part comprising th1–th5. The sicula is 1.85– 2.0 mm long, markedly ventrally curved near the aperture, which is slightly concave and faces ventro-proximally, being furnished with a broad, weakly developed rutellum. The sicular aperture is somewhat covered by a short (0.7–0.8 mm) and blunt virgella, which is expanded into a prominent, presumably spatulate or almost globular structure. Ventral wall of th1 is slightly concave, and emerges 0.34–0.42 mm above the sicular apertural margin. Thecae are straight simple tubes with slightly concave apertures, which are either perpendicular to the thecal tube or somewhat everted. Proximal thecae overlap for less than two-thirds of their length, distal thecae overlap for two-thirds or a little more and are inclined at an angle of 34–55° (M = 44°, n = 12) to the rhabdosome axis. The rhabdosome widens from 0.85– 1.02 mm at th1 to 0.94–1.2 mm at th3, 1.28–1.65 mm at th5, 1.65–2.2 mm at th10 and 2.1–2.55 mm at th20 to the distal maximum 2.8 mm. Most proximal thecae are rather densely spaced with 2TRD 1.34–1.6 mm at th2 and 1.53–1.87 mm at th5. Thereafter the 2TRD increased to 2.0– 2.1 mm at th10, and the most distal thecae number 9 in 10 mm with 2TRD 2.2 mm.

R e m a r k s. Specimens from the lower part of the ludensis-gerhardi Biozone of the Kosov 1 section appear like somewhat enlarged versions of the holotype and paratypes of P. auctus from the Neodiversograptus nilssoni Biozone of northern England, which are 0.7–0.75 mm wide at th1 and 2.0 mm wide distally, with thecae numbering 11–18 in 10 mm ( Rickards 1965). Palmer (1970) described “ Monograptus ” auctus from the ludensis Biozone of Ireland with thecal spacing 10–12 in 10 mm proximally and 8–10 in 10 mm distally, which matches our specimens from Kosov 1. Rickards (1965) noted the close similarity of P. auctus to Colonograptus ludensis ( MURCHISON, 1839) and Pristiograptus tumescens ( WOOD, 1900), in having similar thecal spacing and dorso-ventral width, along with a slightly ventrally curved dorsal side of the proximal part of the rhabdosome. The sicula is markedly ventrally curved near the aperture, which is well seen on Palmer’s illustrations (1970: pl. 14), along with the globular virgellar structure which “almost blocks the sicular aperture”. The weak ventral curvature of the dorsal wall of the proximal part of the rhabdosome, ventrally curved sicula, its ventro-proximally facing aperture and bulbous virgella are considered the most diagnostic characters of the species, considering variation in thecal spacing and rhabdosome width that is common in Wenlock and Ludlow pristiograptids and derived colonograptids. The stratigraphic range of P. auctus is restricted to the uppermost Homerian ludensis-gerhardi and lowermost Gorstian nilssoni biozones, according to records from Wales, England, Ireland, Québec ( Canada), Thuringia ( Germany), Albania and the Czech Republic.

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.