Graptoloidea Lapworth, 1873

Lenz And Anna Kozłowska, Alfred C., 2007, New and unusual upper Llandovery graptolites from Arctic Canada, Acta Palaeontologica Polonica 52 (3), pp. 489-502 : 492-494

Order Graptoloidea Lapworth, 1873 Family Retiolitidae Lapworth, 1873 Subfamily Retiolitinae Lapworth, 1873 Genus Pseudoplegmatograptus Příbyl, 1948b

Type species: Retiolites perlatus var. obesus Lapworth, 1877 , Gala beds, Elwand Water , Melrose, Scotland; Telychian .

Biostratigraphic range: Middle Llandovery Demirastrites pectinatus – Dm. triangulatus to mid Wenlock Monograptus bellerophorus – M. riccartonensis biozones.

Species included: Pseudoplegmatograptus obesus ( Lapworth, 1877) ; P. reticulatus ( Bouček and Münch, 1944) ; P. hexagonalis ( Bouček and Münch, 1944) ; P. wenlockianus Štorch, 1992 ; P. relictus ( Bouček and Münch, 1944) ?; P. longispinus ( Bouček and Münch, 1944) ?; P. flaccidus Wang, 1978 ?; Sinostomatograptus mui Huo, 1957 ?.

Pseudoplegmatograptus cf. obesus ( Lapworth, 1877) Figs. 1, 2A, C, E.

1877 cf. Retiolites perlatus obesus sp.nov.; Lapworth 1877: 137, pl.6: 29.

1944 cf. Plegmatograptus obesus obesus ( Lapworth, 1877) ; Bouček and Münch 1944: 6, figs. 1a–g, 2a, b; pl. 1: 1, 2.

1982 Pseudoplegmatograptus obesus ( Lapworth, 1877) ; Lenz 1982: 41, figs. 16e; 17a, b.

1987 cf. Pseudoplegmatograptus obesus obesus ( Lapworth, 1877) ; Lenz and Melchin 1987: 163, pl. 1: 1 (see Fig. 2C herein).

1992 cf. Pseudoplegmatograptus obesus ( Lapworth, 1877) ; Bates and Kirk 1992: 176, pls. 15–20: 171–182, 212–217, 239–249. Non figs. 183–191, 235–238 (= Pseudoretiolites sp. )

Material.—Two mature, isolated specimens, lacking the proximal−most regions ( Fig. 1).

In addition, the holotype specimen of Lapworth (1877) and one isolated specimen of P. obesus from the Canadian Arctic ( Fig. 2A and C, respectively), and one flattened specimen of P. cf. obesus from Yukon Territory ( Fig. 2E), are illustrated for comparison of the delicate structures.

Description.—Longest specimen 10 mm, proximal end missing; maximum width about 2.6 mm, exclusive of spines. Thecal framework medial region with distinct zigzag lists ( Fig. 1A 1). Transverse rods close to nema ( Fig. 1A 2). Ancora sleeve well developed, well separated from thecal framework, composed of moderate−size polygonal meshes, and some larger, somewhat ill−defined, medially positioned polygonal meshes, probably stomata. Apertural lips thickened ( Fig. 1A 3), with paired, robust and proximolaterally−positioned spines; occasional spine bifurcating or trifurcating distally ( Fig. 1B 3). Arising primarily near the junction of the thecal framework and ancora sleeve are numerous, very fine, frilly, cobweb−like or lacey reticular lists, mostly disorderly; some forming secondary fringelike meshes ( Fig. 1B); these structures are clearly similar to those seen in flattened specimen shown in Fig. 2E. List micro−ornament smooth ( Fig. 1B 7, B 5) or with weak, parallel striae ( Fig. 1B 2).

Discussion.—The genus Pseudoplegmatograptus obesus Příbyl, 1948b appears to be one of the few retiolitids that occasionally generated delicate structures such as multi−furcated thecal spines that may partially fuse distally, cobweb−like or lacey reticular networks and, still more rarely, apparent membraneous structures (see Elles and Wood 1908: figs. 223a, b; Lenz 1982: fig. 17a–c; Bates and Kirk 1992: figs. 241–243; and holotype specimen of P. obesus illustrated in Fig. 2A).

The only other non−retiolitid group featuring such structures relatively commonly are some species of Petalolithus Suess 1851 (see some examples in Koren’ and Rickards 1996). This parallelism is considered very significant, since the petalolithids and the retiolitids constitute the ancorate graptolites, which are considered to share a common ancestry ( Melchin 1998). It is also considered that the retiolitids were derived from some petalolithid ( Lenz 1994; Melchin 1999; Kozłowska−Dawidziuk 2004; Bates et al. 2005).

The delicate, multi−furcating thecal spines are only intermittently seen on the two illustrated specimens. However, their relative density where present (preserved) suggests that they were probably originally present on all, or most, of the spines, and that they were broken off during the acid extraction. The presence of thecal spines and rim structures on the species type specimen, Pseudoplegmatograptus obesus from Spain ( Štorch 1998), P. reticulatus from Argentina ( Fig. 2A, B, respectively), P. wenlockianus described from the Prague region ( Štorch 1992), and on some specimens of Pseudoplegmatograptus obesus illustrated in Bates and Kirk (1992) supports this conclusion. Whether, however, similar multifurcations were commonly present in all species of the genus is unknown. As a further observation, it appears that Pseudoplegmatograptus is one of the few genera to more or less consistently feature border fringes and thecal spine complications.

The study specimens, like the specimens described as Pseudoplegmatograptus obesus in Lenz (1982) and illustrated in Fig. 2E, differ from the type species in being somewhat narrower and widening more gradually proximally. In other respects, they are very similar and, accordingly, are identified as Pseudoplegmatograptus cf. obesus .