Plectograptus macilentus ( Törnquist, 1887 )

Kozłowska-Dawidziuk, Anna, 2002, Agastograptus, a synonym of Plectograptus (Retiolitidae, Graptolithina), Acta Palaeontologica Polonica 47 (3), pp. 459-467 : 460-461

Plectograptus macilentus ( Törnquist, 1887)

Fig. 2A, B .

Retiolites macilentus n. sp.; Törnquist 1887: 491, fig. 3.

Plectograptus macilentus (Törnquist) ; Moberg and Törnquist 1909: 13, pl. 1: 1–12.

Plectograptus tetracantus n. sp.; Eisenack 1951: 140, pl. 23: 6–8, pl. 25: 9, text−figs. 4, 5.

Plectograptus macilentus (Törnquist) ; Bouček and Münch 1952: 120, fig. 7a–f, pl. 1: 1–4.

Plectograptus (Plectograptus) macilentus (Törnquist) ; Lenz 1993 a: 13–14, pl. 1: 6–8.

Plectograptus macilentus (Törnquist) ; Kozłowska−Dawidziuk 1995: 317, fig. 33.

non Retiolites (Plegmatograptus) obesus var. cf. macilentus ; Elles and Wood 1908: 343, pl. 34; 13a, b, fig. 224.

Emended diagnosis.—Ancora sleeve and thecal walls without reticulum, as in P. robustus . Differs from P. robustus in absence of apertural processes.

Description.—The new material represents the same features as the material described before

Discussion.—The first detailed description and discussion of the species was by Bouček and Münch (1952). The authors carefully studied the previously described material of Plectograptus macilentus , and other species incorrectly included in this species. They noticed the presence of large openings above the ancora, central position of nema, which in mature specimens projects distally as a thickened “ramification”, which was regarded by them as a “teratological phenomenon”. This feature is regarded as a three−vaned nematularium ( Kozłowska−Dawidziuk 1995).

Bouček and Münch (1952) also mentioned the variation in the development of the proximal part of rhabdosome. These differences are also observed by the author, and are expressed in the location of the first lists of the ancora sleeve walls, which may appear at the level of the either the first or second theca, accounting for differences in the size of the ancora sleeve proximal orifices. More material is needed to study the significance of this feature.

The ancora umbrella with incomplete lists is similarly developed in P. macilentus from a Baltic erratic boulder ( Fig. 2A, B ) (see also Kozłowska−Dawidziuk 1995: fig. 33B) and in P. robustus from the Kaliningrad area ( Fig. 2D ), whereas the specimen of P. robustus from the EEP possesses betterdeveloped ancora umbrella lists ( Fig. 2C ), forming an almost complete rim on the ancora. The primary difference between P. macilentus and P. robustus lies in the presence of paired apertural processes in the latter.

Material.— Five specimens representing fragments of rhabdosomes come from Baltic erratic boulder 46 from Jarosławiec, Poland. It co−occurs with N. balticus , Saetograptus chimaera , Bohemograptus bohemicus , Neodiversograptus beklemishevi , Pristiograptus dubius , Monoclimacis sp. , and Crinitograptus crinitus ?.

Occurrence.— P. macilentus is restricted to the Lower Ludlow, Neodiversograptus nilssoni Biozone in Sweden ( Törnquist 1887), Czech Republic ( Bouček and Münch 1952; Kozłowska−Dawidziuk et al. 2001), Kyrgyzstan (Koren' 1991), Arctic Canada ( Lenz, 1993), Poland ( Kozłowska−Dawidziuk 1995), China ( Lenz et al. 1996) and Iberia ( Gutiérrez−Marco et al. 1996).

Figures

Fig. 2. A, B. Plectograptus macilentus (Törnquist, 1887), ZPAL G. 27/1; Baltic erratic boulder 46 Jarosławiec (photo D. Bates). A. Reverse view of proximal fragment of immature rhabdosome. B. Inside view showing shape of ancora umbrella and shape of rhabdosome in cross section. C, D. Plectograptus robustus (Obut and Zaslavskaya, 1983). C. ZPAL G.27/2, EEP, Bartoszyce borehole, depth 1627.0 m, obverse view of immature rhabdosome. D. UIGGM SB RAS 251/42−4/1, holotype, Kaliningrad District, North Kaliningrad borehole, depth 2094.0–2105.5 m (courtesy of N. Sennikov). Scale bars 0.5 mm. Abbreviations: a, aperture; ap, apertural process; au, ancora umbrella; l, lip; mv, mid−ventral list; po, proximal orifice; v, virgella.