Glomerula sp.

Richard Pokorný, Jaroslav Kaše, Jiří Kvaček, Kamil Zágoršek, Kočí, Tomáš & Žítt, Jiří, 2012, Fossils In Late Cretaceous To Early Palaeocene Flint Nodules Embedded In Pleistocene Glaciofluvial Sediments Near Fukov (Děčín District, Northern Bohemia), Acta Musei Nationalis Pragae Series B 68 (3 - 4), pp. 119-131 : 122

Glomerula sp. ex gr. lombrica ( DEFRANCE, 1827) / serpentina (GOLDFUSS, 1831)

Pl. 2, figs 1 – 4

M a t e r i a l: Five specimens preserved either as longitudinal sections of the tubes or as sections through the “glomerulate knot”, collection numbers FZP/P/12/2-3/004, FZP/P/12/2-3/005, FZP/P/12/2-3/006, FZP/P/12/2-3/007, FZP/P/12/2-3/008.

D e s c r i p t i o n: Tube and lumen circular in cross-section, strongly and irregularly curved to form a knot. Outer surface of the tube bears no ornamentation. The tube diameter is 0.5 to 1.2 mm.

R e m a r k s a n d r e l a t i o n s h i p s: Gába and Pek (1999) mentioned the occurrence of sporadic remains of sabellid worm tubes belonging to Glomerula gordialis (VON SCHLOTHEIM, 1820) in facetted flint pebbles of Late Cretaceous age in the Moravian-Silesian area.

Systematics of species within the genus Glomerula is extremely problematic. Jäger (personal communication to Tomáš Kočí) believes that the genus Glomerula consists of a large number of species, but states that it is nearly impossible to distinguish between them, because the construction of the tube is so simple and the morphological variation between specimens, even within one sample, is exceptionally wide.

According to the partly artificial scheme published by Jäger (2005) and later somewhat modified in Jäger (2012), due to their relatively young age, the either latest Cretaceous or early Palaeocene tubes, must belong to the phylogenetically advanced set of species including G. lombrica ( DEFRANCE, 1827) and G. serpentina (GOLDFUSS, 1831) . These are characterized by the occasional occurrence, present in only a small percentage of specimens, of trilobate narrowing of the tube lumen, a construction presumably enabling the animal to fix itself inside its tube as a protection against being drawn out of it by a predator. Unfortunately, the cross-section visible in the specimens examined in the present study do not show any distinct trilobate narrowing. A third species of this phylogenetically advanced set, G. plexa (J. C. SOWERBY, 1829) , can be ignored here because it represented by clusters formed by various numbers of tubes, whereas the specimens dealt with in the present paper each consist of a single tube (or, perhaps, a limited number of tubes).

The most important difference between G. lombrica and G. serpentina is simply that the former is smaller and the latter is larger in tube diameter, with 0.7 to 1.0 mm being the approximate boundary. Due to this criterion, some specimens found in the flint nodules could belong to G. lombrica , some are intermediate between the two species, and only the largest specimen could belong to G. serpentina .

Moreover, practical experience has shown that only in fine-grained offshore sediments and only in the Cretaceous period, it is logical to differentiate between the small G. lombrica and the larger G. serpentina . In contrast, in all coarse-grained nearshore sediments, from the Cretaceous as well as from the Palaeocene and Eocene, and also in Palaeocene and Eocene fine-grained offshore sediments, all Glomerula tubes, independent of their size, should belong to a single species, G. serpentina . While there is no doubt that the flints dealt with in the present study are derived from fine-grained offshore sediments, without closer investigation of the cooccurring fauna it can not be stated with certainty if the flint is derived from Late Cretaceous or Palaeogene sediments as the genus Glomerula occurs in both. It is this unanswered question regarding the definite age of the flint nodules which prevents taxonomic determination of the Glomerula tubes down to species level.

Echinodermata KLEIN, 1734