134
Journal of Paleontology 91(1):116–145
Remarks.—This is the biostratigraphically lowest retiolitine species in Arctic Canada (as well as globally), and is considered the most primitive (see Phylogenetic analysis herein). Owing to the poor preservation, resulting in lack of detail on thecal form, internal structure, evidence of stomata, it is not possible to assign this species confidently to Pseudoretiolites, although the zigzag distal thecal floors and proximal structure suggest affinities with those species. This appears to be a newspecies, butmaterial is not complete enough for full description of a new taxon. An unusual attribute of these specimens is that the prosicula
and metasicula are both completely preserved. Some specimens exhibit fusellar remnants of the base of theca 11 (Fig. 11.3, 11.4) as well. In this respect, this taxon appears to be primitive in relation to the condition in all other known species of Pseudoretiolites, in which the prosicula is commonly preserved but the metasicula, if represented at all, is normally incomplete. Later retiolitines normally preserve only the prosicula or just the virga (e.g., Bates et al., 2005).
Genus Pseudoplegmatograptus Příbyl, 1948
Type species.—Retiolites perlatus obesus Lapworth 1877, p. 137, by original designation.
Diagnosis (emended from Příbyl, 1948).—Ancora umbrella shallow, saucer-shaped, with coarse, hexagonal-pentagonal meshes. Prosicula present. Nema attached to connecting rods throughout. Thecal mid-ventral lists present only in more distal thecae, forming ventral wall of thecae; transverse rods throughout. Thecal framework also includes pleural lists and zigzag thecal framework lists. Thecae with straight, outward- inclined ventral walls. Ancora sleeve may form double layer in mature specimens (see Bates and Kirk, 1992). Thecal lips are horizontal lists to which are connected long, paired spines that may be distally extended by fine, or lace-like bifurcations or multifurcations (see Lenz and Kozłowska, 2007). Stomata present.
Pseudoplegmatograptus obesus (Lapworth, 1877) Figure 12.3, 12.4
1877
Retiolites perlatus obesus Lapworth, p. 137, pl. 6, fig. 29.
1944 Plegmatograptus obesus obesus; Bouček and Münch, p. 6, text-figs. 1a–g, 2a–b, pl. 1, figs. 1, 2.
1952 1982 1984
Pseudoretiolites (Plegmatograptus) obesus; Münch, p. 75, pl. 12, figs.1a–c.
Pseudoplegmatograptus obesus obesus; Lenz, p. 41, figs. 16E, 17A, B.
Pseudoplegmatograptus obesus; Chen, p. 50, pl. 7, figs. 5, 8, pl. 8, figs. 1, 2.
1987a Pseudoplegmatograptus obesus obesus; Lenz and Melchin, pl. 1, fig. 1.
1990 1991 1992
Pseudoplegmatograptus obesus; Ge, p. 80, pl. 9, fig. 3, pl. 10, fig. 6.
Pseudoplegmatograptus obesus; Tomczyk, Urbanek, and Teller, p. 300, pl. 185, fig. 1.
Pseudoplegmatograptus obesus; Bates and Kirk, p. 176, figs. 171–183, 193–250 (non figs. 184–192).
1993
Pseudoplegmatograptus obesus; Loydell, p. 59, text: fig. 13, fig. 24 (see for additional synonymy).
Materials.—One mature and well-preserved specimen without an ancora umbrella, one immature specimen with ancora umbrella and two thecal pairs, and four other fragmentary specimens.
Occurrence.—Snowblind Creek, Cornwallis Island, from S. guerichi Zone. Found in the S. sedgwickii, S. halli, S. guerichi, S. turriculatus and S. crispus zones (upper Aeronian to mid Telychian) in Arctic Canada and Yukon, Canada, Britain, Czech Republic, Siberia, Germany, Poland, Denmark (Bornholm), and south China.
Remarks.—The single mature specimen lacking the ancora umbrella was previously illustrated by Lenz and Melchin (1987a, pl. 1, fig. 1), but the image is included here for the purpose of demonstrating the complete retiolitine fauna occurring in the Aeronian and lower Telychian of the Canadian Arctic. Unlike the occurrences of Pseudoretiolites, Pseudoplegmatograptus is rare in the Canadian Arctic. Species of Pseudoplegmatograptus are distinguished from those of Pseudoretiolites by a simple, shallow ancora umbrella with hexagonal/pentagonal meshes, paired thecal apertural spines, lack of zigzag thecal ventral floors, and development of mid-ventral lists only in the distal part of the tubarium. P. obesus is characterized by a uniform and moderately dense reticulum as compared with the much greater reticulum density of P. reticulatus Bouček and Münch, 1944 (e.g., Hutt, 1974; Lenz et al., 2003). The morphological details of this species were described
by Bates and Kirk (1992). Some of the specimens illustrated as P. obesus by Bates and Kirk (1992, figs. 184–192), however, clearly show zigzag ventral thecal floors and no thecal apertural spines, suggesting that these specimens represent a species of Pseudoretiolites rather than Pseudoplegmatograptus. The morphometrics our specimens of P. obesus are as
follows: ancora umbrella shallow (see Fig. 12.4), of hexagonal/ pentagonal meshes without spiral lists; maximum tubarium width ~4.0mm, exclusive of spines; eight pairs of thecae spaced the rate of ~11 in 10mm (=2TRD 1.82 mm); paired thecal aperture spines ~1mm long; at least two large, slightly ovate stomata are present; and, ancora sleeve a coarse meshwork. Prosicula and virgella ~1mm long. Pleural lists are approximately vertical, and lateral apertural rods almost horizontal. As far as we are aware, no specimens of this species have been reported that show continuous membranes of preserved tissue on the thecal walls.
Pseudoplegmatograptus? sp. Figure 12.8, 12.9
Materials.—Two well-preserved distal fragments.
Occurrence.—L. convolutus Zone, Cape Manning and unknown locality, Cornwallis Island.
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