Plectograptus robustus ( Obut and Zaslavskaya, 1983 )

Kozłowska-Dawidziuk, Anna, 2002, Agastograptus, a synonym of Plectograptus (Retiolitidae, Graptolithina), Acta Palaeontologica Polonica 47 (3), pp. 459-467 : 462-463

Plectograptus robustus ( Obut and Zaslavskaya, 1983)

Fig. 2C, D .

Agastograptus robustus n. sp.; Obut and Zaslavskaya 1983: 108, pl. 24, tab. 24: 1–3.

Agastograptus robustus Obut and Zaslavskaya ; Obut and Zaslavskaya 1986: 210, fig. 1a–c.

non Agastograptus robustus Obut and Zaslavskaya ; Lenz 1994a: fig. 2: 1–7, fig. 3: 1–9.

non Agastograptus robustus Obut and Zaslavskaya ; Lenz 1994b: fig. 2: 1–5, fig. 3: 1–4, 6.

non Agastograptus robustus Obut and Zaslavskaya ; Rickards, Packham, Wright and Williamson 1995: 32, fig. 18I, J.

Emended diagnosis.—Similar to P. macilentus , but differs in possession of paired reticulofusellar apertural processes. Reticular lists of the ancora sleeve and thecal walls are absent.

Description.—The longest (3 mm) specimen contains 3 pairs of thecae, and represents growing rhabdosome ( Fig. 2C ). Width of ancora umbrella is about 1 mm. Some lists of ancora umbrella are present, forming almost complete rim on the ancora. Rhabdosome width increases up to 2 mm on the level of 3rd pair of thecae. Apertural processes absent in two first pairs of thecae. Length of first apertural processes is 0.5 mm. Virgella is not preserved.

Discussion.—The holotype of A. robustus number UIGGM SB RAS 251/42−4/1 (there is also number 251/42−44 for the same specimen, see Obut and Zaslavskaya 1983: 108, 175) illustrated on Fig. 2D represents a mature specimen built of relatively strong lists with the distinctive zigzag pattern of the ancora sleeve wall, and mid−ventral lists on the ventral side of rhabdosome, as well as paired reticulofusellar spines. This specimen, shown in reverse view, possesses a large lateral orifices outlined by the ancora sleeve lists located close to lip of th 11 and in region of the lip of the th 12. The specimen UIGGM SB RAS 251/42−2/2 from type collection ( Obut and Zaslavskaya 1983: fig. 24: 3) represents early stage of growth with first theca developed. It is similar to the specimen representing early growth stage of the species from the author’s collection.

Specimens from Arctic Canada assigned to A. robustus by Lenz (1994a, b) have a unique preservation or different development of the thecal and ancora sleeve membranes. These specimens, although similar in size to P. robustus , are assigned to Spinograptus because of possession of an appendix, proximal lateral orifices of similar position and size (see Kozłowska−Dawidziuk 1997: fig. 4) to Spinograptus , as well as paired reticulofusellar apertural processes. The prime first zigzag list of the sleeve above the ancora umbrella is connected to the ancora umbrella lists ( Lenz 1994a: fig. 2: 1; fig. 3: 3). The lateral proximal orifices in the holotype of P. robustus , as in P. macilentus ( Fig. 2 ), are larger than those in A. robustus from the Arctic, and the prime first zigzag lists are connected to ventral lists on the levels of the first theca apertures. The features described above, as well as occurrence in the Colonograptus ludensis Biozone , show that the Arctic forms belong to Spinograptus , and assigned to a new species, S. praerobustus Lenz and Kozłowska−Dawidziuk, 2002 . Specimens from Australia assigned to A. robustus by Rickards et al. 1995 show similar features as specimens from the Arctic Canada ( Lenz 1994a, b; Lenz and Kozłowska−Dawidziuk 2002), as well as developed reticulum which is absent in P. robustus . Australian specimens do not have the specific membrane present in the Arctic species.

Material.— One immature specimen ( Fig. 2C ) and several fragments from depth 1627.0 m, Bartoszyce borehole, Poland; co−occur with Colonograptus jaegeri , C. praedeubeli , C. gerhardi ?, Pristiograptus dubius , Spinograptus munchi .

Occurrence.— Plectograptus robustus occurs in Neodiversograptus nilssoni Biozone of North Kaliningrad, Russia, and late Homerian of Poland.

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.