Clinoclimacograptus aff. retroversus (Bulman and Rickards, 1968)

Clinoclimacograptus aff. retroversus (Bulman and Rickards, 1968)

Figures 8A View FIGURE 8 ; 9A–B View FIGURE 9 aff. 1968 P. (Clinoclimacograptus) retroversus subgen. et. sp. nov.; Bulman and Rickards 1968, pp. 8–12, text– figs. 3a–c, 4a–c, 5.

aff. 1970 Pseudoclimacograptus (Clinoclimacograptus) retroversus Bulman and Rickards, 1968 ; Rickards, pp. 34–35, text-fig. 14, figs. 1–4.

aff. 1975 Pseudoclimacograptus retroversus (Bulman and Rickards, 1968) ; Bjerreskov, pp. 25–26, figs. 9D–E.

aff. 1991b Clinoclimacograptus retroversus Bulman and Rickards, 1968 ; Loydell, p. 674, pl. 1, figs. 5, 14.

Holotype. Specimen SM A52951 from Tommarp,

Sweden, probably sedgwickii Biozone , figured by

Bulman and Rickards (1968, text-fig. 4b).

Material. 52 flattened specimens, including a number of juvenile tubaria.

Description. The tubarium is 8–10 mm long, exceptionally up to 14.65 mm. The sicula is largely covered, only in one specimen pressed through and showing length of ca. 1.2 mm and apertural width 0.26 mm. Virgella up to 0.44 mm long. Dorso-ventral width varies between 0.6 and 0.74 mm at the first thecal pair, increases to 0.8–0.9 mm at the third thecal pair, and 0.9–1.0 mm at the fifth thecal pair. A maximum width of 1.0– 1.2 mm is attained at the tenth thecal pair. The proximal end is asymmetric but still somewhat rounded, length of th 11 varies within 0.54–0.8 mm. Thecae are strongly geniculated with markedly long (0.65–0.77 mm), parallel-sided supragenicular walls. Deep, broadly slit-like excavations, generally perpendicular to the tubarium axis, occupy approximately one-third of the tubarium width at the level of th5. Strongly everted apertures claimed in pyritized specimens assigned to Clinoclimacograptus are not observed in flattened material. Also, the median septum is not visible in this mode of preservation. The 2TRD increases from 1.48–1.7 mm at th2, through 1.7– 1.85 mm at th5 to 1.8–1.9 mm at th10. The nema is visible, but thread-like.

Remarks. None of the specimens studied can be assigned to C. retroversus with certainty. They differ from the genus Metaclimacograptus by their asymmetric and elongated proximal end and apertural excavations that are partially asymmetric (but not introverted) and slight incisions-like narrow apertural excavations, but deep. In addition, present specimens differ from Metaclimacograptus asejradi by having a significantly narrower flattened tubarium.

Most characters and parameters of the studied specimens correspond to those reported for Clinoclimacograptus retroversus by Bulman and Rickards (1968). However, the tubaria studied here show less densely spaced thecae (higher 2TRD values) than those reported by Bulman and Rickards (1968) and Rickards (1970). Present specimens possess 11–12 thecae in 10 mm, in the original description of C. retroversus this value ranges between 12–13 thecae in the proximal part of the tubarium and decreases to 10 thecae distally. Furthermore, the median septum is not visible in flattened imprints, making it impossible to say with certainty that this is truly a C. retroversus species. On the other hand, the other parameters correspond to the original descriptions of the holotype. Nevertheless, the work of Loydell (1991b), who worked with the extracted 3D tubarium should also be considered. The specimens assigned by them to the species Clinoclimacograptus retroversus show significantly higher dorso-ventral width than the specimens studied in this work. It cannot be overlooked that the width of these specimens preserved in full relief differs from the original descriptions of equally preserved specimens of C. retroversus by Bulman and Rickards (1968). Bjerreskov (1981) described Clinoclimacograptus washingtoni , from North Greenland which can be readily differentiated by its regularly widening tubarium and more concave supragenicular thecal walls. Finally, Bjerreskov (1975) mentions the occurrence of C. retroversus already from the convolutus Biozone, and the holotype of the species C. retroversus is described from the upper sedgwickii Biozone. Its occurrence in the linnaei Biozone in the Prague Synform would therefore continuously extend its stratigraphic range into the younger biozone.