Plectograptus wimani ( Eisenack, 1951 )

Bates, Denis E. B., Kozłowska, Anna, Maletz, Jörg, Kirk, Nancy H. & Lenz, Alfred, 2006, The Silurian retiolitid graptolite Plectograptus: New observations and new species, Acta Palaeontologica Polonica 51 (3), pp. 525-540 : 538-540

Plectograptus wimani ( Eisenack, 1951)

Fig. 10.

1951 Retiolites wimani sp. nov.; Eisenack 1951: 145, pl. 25: 8.

1951 Retiolites sp. indet. Eisenack 1951: 146, figs. 6, 7.

1995 Plectograptus wimani ( Eisenack, 1951) ; Kozłowska−Dawidziuk 1995: 318, fig. 27E.

Emended diagnosis.—Ancora sleeve with secondary lists, quadruple list junctions at ends of apertural lists. Thecal walls inclined inwards from genicular to apertural lists. Singular reticulated genicular hood as wide as a length of genicular list.

Material.—Three mature specimens, partly damaged, and one at ancora stage of growth ( Kozłowska−Dawidziuk 1995), Mielnik Borehole, Poland, depth 1044.9 m, Gorstian, Neolobograptus nilssoni Biozone.

Description.—Largest specimen with 12 pairs of thecae, compressed almost flat, resulting in outward bulging of the thecal lips and genicular lists ( Fig. 10). Ancora umbrella and distal end broken off ( Fig. 10A). Width of ancora sleeve gradually increasing from 2.05 mm at the level of the first geniculum to 2.62 mm at the 7 th thecal pair, gradually decreasing to 2.05 mm at the level of the 10 th thecal pair. Total length estimated at 11.7 mm. Thecal orifices calculated as 1.00 mm wide by 0.36 mm from lip to geniculum, and cross−sectional obverse to reverse width of rhabdosome 2.08 mm. Primary ancora sleeve lists joining the lateral apertural rod/ thecal lip/pleural list junction. Secondary (reticular) lists of the sleeve form a stout meshwork between the primary lists. Genicular processes complex and loop−shaped ( Fig. 10F), inclined proximally at about 30 ° from the horizontal ( Fig. 10A).

The genicular processes are formed of primary curving lists, each bearing an insertion seam on its inner face, and secondary lists which are oblique to the primary lists, having seams on their inner or outer sides.

Discussion.—The illustrated specimen ( Fig. 10) represents a more mature rhabdosome than the holotype ( Eisenack 1951: pl. 25: 8), which has thinner lists and only incipient genicular processes. Two fragmentary rhabdosomes illustrated by Eisenack (1951: figs. 6, 7) represent more mature stages of growth, with better developed genicular processes, similar to the Polish material.

The insertion seams on the inner face of the primary lists making the genicular processes, show that the processes were formed initially with a (fusellar) membrane. The secondary lists with their inner or outer seams therefore were formed on the outer and inner faces respectively of the process.

Geographic and stratigraphic range.—EEP, lower Gorstian, Neolobograptus nilssoni Biozone.