Plectograptus trijunctus, Bates & Kozłowska & Maletz & Kirk & Lenz, 2006

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

Plectograptus trijunctus sp. nov.

Fig. 9A .

2002 Plectograptus robustus ( Obut and Zaslavskaya, 1983) ; Kozłowska−Dawidziuk 2002: 462 , fig. 2c.

Derivation of the name: From the Latin tri and junctus, for a three junction.

Holotype: Holotype ZPAL G.27/2 ( Fig. 9A ) .

Type locality: Bartoszyce borehole, 1627.0 m, Poland.

Type horizon: Gorstian, Neolobograptus nilssoni Biozone.

Material.— One immature specimen and several fragments from Bartoszyce borehole, 1627.0 m, Poland .

Diagnosis.—Ancora sleeve only primary lists, mid−ventral lists vertical. The ancora sleeve list junctions with pleural lists triple. Thecal walls inclined inwards from genicular to apertural lists, similar to Plectograptus mobergi and P. wimani . Paired reticulofusellar genicular processes similar to those of P. robustus .

Description.—Proximal obverse and reverse orifices as well as thecal profiles similar to Plectograptus robustus . Lateral profile with thecal walls from genicular list to lips slightly undulating. Pleural lists in two sections, similar to P. mobergi . Genicular processes on immature specimen with three thecal pairs, developed only on third pair of thecae. See also Kozłowska−Dawidziuk (2002) for additional description.

Discussion.—The new species is similar to Plectograptus robustus in having similar apertural processes and thecal profiles, but in contrast to Plectograptus robustus , the ancora sleeve list junctions with the pleural lists are triple rather than quadruple. Geographic and stratigraphic range.—EEP, lower Gorstian,

Neolobograptus nilssoni Biozone.

Figures

Fig. 9. A. SEM micrographs of Plectograptus trijunctus sp. nov., holotype ZPAL G.27/2, Bartoszyce borehole, depth 1627 m, EEP, Poland, late Homerian; A1, whole rhabdosome, reverse view; A2, enlargement of apertural process. B, C. Plectograptus robustus (Obut and Zaslavskaya, 1983). B. Light photograph of reverse of mature rhabdosome with nematularium, SMD−SAS 217, Grube “Frisch Glueck” near Zwickau, south of Stenn, Germany, Ludlow; not to scale. C. SEM micrographs of a mature rhabdosome, MB.G 1081, Spandau bei Berlin, Germany, Ludlow: C1, obverse view; C2, paired apertural process; C3, ancora hub; C4, enlargement of ancora umbrella fragment.