Gothograptus obtectus Kozłowska-Dawidziuk, 1990

Kozłowska, Anna, Bates, Denis, Zalasiewicz, Jan & Radzevičius, Sigitas, 2019, Evolutionary significance of the retiolitine Gothograptus (Graptolithina) with four new species from the Silurian of the East European Platform (Baltica), Poland and Lithuania, Zootaxa 4568 (3), pp. 435-469 : 457-462

Gothograptus obtectus Kozłowska-Dawidziuk, 1990

Figs 19–21

Emended diagnosis. Tubarium usually with eight pairs of thecae and appendix. Genicular structures forming reticulated veil, covering thecal orifice and theca. Veil ended by a thicker list located above the lip of the previous theca. Mid-ventral lists are well-developed in proximal and medial thecae while becoming thinner or not developed in distal thecae. It differs from other Gothograptus species in the extreme development of veils.

Material. Material described herein comes from Poland and Lithuania. Material from Poland: Bartoszyce IG- 1 core, upper part of lundgreni Biozone, from 1679.3– 1677.5 m, 20 mature and young specimens, Zawada 1 core, depth 1533.3–1540.0 m, upper part of lundgreni Biozone, several specimens. Material from Lithuania comes from the upper part of the lundgreni Biozone, from two cores: Šiupyliai-69, depth 1012.0 m and Likenai-396 core, depth 577.0 m. It consists of ten mature and 20 young specimens and some fragments of tubaria.

Description. The best preserved mature tubarium from Poland, with seven pairs of thecae, is 9.6 mm long. The tubarium is narrower up to the third pair of thecae, about 1 mm wide, and becomes much wider from the level of th4 1 up to th6 2 and reaches 1.3 mm, and then the width decreases close to the appendix. The appendix is about 1.4 mm long. In mature specimens, the first pair of thecae does not have any genicular structures, whereas the next thecae have typical reticulated veils covering the thecal openings. Veils grow proximally from the geniculum; they are connected to pleural lists and lateral apertural rods. A thick, distinctive transverse list ends every veil ( Fig. 19A ). The veils are longer in distal thecae, ending just above the genicular list of the previous theca. Some thicker transverse lists may be developed in the middle part of the veil. The lips are clearly visible among the meshwork of reticulum. The genicular lists become very thin in distal thecae and difficult to identify. Mid-ventral lists are very thin in distal thecae; in the last thecae it is difficult to distinguish them from other lists of the tubarium ( Fig. 20A ).

Specimens of Gothograptus obtectus from Lithuania are well preserved; three-dimensional, representing different stages of astogenetic growth. There is no complete mature tubarium; they are slightly damaged proximally or distally. The longest specimen, with no proximal end, is 6.4 mm long and has nine pairs of thecae and a fragment of appendix. The specimen is slightly damaged laterally which makes it thinner ( Fig. 21A ). The measurements of tubaria from Lithuania are similar to those of the population from Poland. They are about 0.7 mm wide at the level of the first pair of thecae and slightly wider up to the fourth or fifth pairs of thecae and then become about 1.1 mm wide ( Fig. 21D ).

Mature specimens of G. obtectus from Lithuania show some diversity in genicular process development in proximal thecae. In some specimens the second pair of thecae has remnants of the incomplete rim of the genicular process; the next thecae are covered by genicular reticulated veils with typical thicker lists.

Remarks. Previously, Gothograptus obtectus was known only from the Zawada 1 core of Poland (Kozłowska- Dawidziuk 1990), where it occurred in the upper part of the lundgreni Biozone. Our new data indicate that it is present in a similar biostratigraphic interval in Lithuania.

The G. obtectus tubaria are narrower in the proximal part; from the level of the fourth pair of thecae they becomes much wider. This is caused by a layer outside of the distal thecae, formed by the veils ( Fig. 21D ).

The transverse lists of the G. obtectus veil may represent successive edges of the growing veil during astogeny.

Figures

FIGURE 19. Mature tubarium with appendix of Gothograptus obtectus Kozłowska-Dawidziuk, 1990 from the Bartoszyce IG- 1 core, depth 1677.5 m, lundgreni Biozone, Poland, ZPAL G.55/48. A. enlargement of distal part of specimen. B. enlargement of proximal end. C. whole specimen, reverse view. Abbreviations: lat. ap.—lateral apertural.

FIGURE 20. Tubaria of Gothograptus obtectus Kozłowska-Dawidziuk, 1990, Zawada 1 core, depth 1533.3–1540.0 m, Poland, upper part of lundgreni Biozone. A. tubarium, ZPAL G.XIII/23. B. gerontic tubarium, holotype, ZPAL G.XIII/23. Abbreviations: conn.—connecting, v.—ventral.

FIGURE 21. Mature tubaria of Gothograptus obtectus Kozłowska-Dawidziuk, 1990 from Lithuania, Šiupyliai-69 core, depth 1012.0 m, lundgreni Biozone. A, B, E. finite tubarium with no proximal end, ventro-lateral view, VU RET-19; A, whole specimen; B, enlargement of last thecae; E. enlargement of thecal hood. C–D. proximal part of tubarium, obverse view, VU RET-20; C. enlargement of medial part; D. whole specimen; F–H. fragment of tubarium, lateral-ventral view, VU RET-21; F. whole specimen; G. enlargement of proximal part; H. enlargement of distal part. Abbreviations: genic.—geniculum, lat. ap.— lateral apertural, m–v.—mid-ventral.