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Introduction
Calcareous worm tubes are found either loose (free-lying) in sedimentary rocks or fixed to a solid substrate. The latter includes both biotic (e.g., echinoid tests, belemnite guards and oyster shells) and abiotic substrates, in particular, hardgrounds and walls of burrows or borings therein. Calcareous tubes found loose either belong to a minority of species that did not attach permanently to a substrate (obligate encrusters) or represent specimens that were originally fixed to substrates such as algae, sea grass or aragonitic molluscan shells, that have now dissolved, or constitute individuals that became separated from their substrates by mechanical means. The operculum is occasionally found of some species.
In the type Maastrichtian, many sabellids and serpulids possess a sturdy and well-preserved calcitic tube. However, a few taxa had aragonitic tubes which are preserved either as a mould in the host rock or as a negative imprint on the attachment area of epibionts (e.g., oysters) which settled over the serpulids and bioimmured them in this way. In other, rare cases, tubes are silicified. Some species, such as those of the subgenus Pyrgopolon (Pyrgopolon), have both calcitic (inner and outer tube layers) and aragonitic parts (‘middle‘ tube layer and operculum).
Serpulid tubes occur in nearly rock-building frequencies at some intervals within the Nekum and Meerssen members, with one species, Pyrgopolon mosae, predominating. Stratigraphic units in which serpulids are both common and diverse include the lower portion of interval 6 of the Vijlen Member, the Lanaye Member, and, best of all, the lower portion of the Meerssen Member with burrows into hardgrounds and their infills. In these natural shelters, even the most fragile serpulid tubes were protected from abrasion.
Pyrgopolon mosae was the first serpulid to be described and illustrated in 1808 by de Montfort, from levels which subsequently formed part of the definition of the type Maastrichtian, while Goldfuss (1831, 1841) erected numerous new species of serpulids, including a few from that area. Around the same time, Defrance (1827), J. de C. Sowerby (1829), Dujardin (1837), von Hagenow (1840) and Roemer (1841) described species from France, England and Germany, some of which were later recognised by Müller (1847, 1851) and subsequent authors to occur in the type Maastrichtian as well. Some serpulids recorded in the first half of the nineteenth century, especially those which currently are contained in the subgenus Pyrgopolon (Pyrgopolon), were considered to be scaphopods or other invertebrates (see Donovan & Jagt, 2012). De Ryckholt (1852) was among the first to interpret them correctly as serpulids; he also erected a few additional species.
The second half of the nineteenth century, and the first half of the twentieth, brought few new data on serpulids from the type Maastrichtian. Species appeared mostly in extensive faunal lists in works which focused on regional stratigraphy. There are two notable exceptions to this. First, there are the small illustrations in Voigt’s seminal paper of 1929, and secondly, Umbgrove (1925, 1956) provided descriptions and illustrations of a curious taxon which is now known as Dorsoserpula (Pegmaticula) turpificata. This ‘gap’ of one hundred years is all the more remarkable, because in other regions research on fossil serpulids continued unabated.
The modern phase of scientific work on serpulids began in the 1950s. The opercula of Pyrgopolon were described by Wrigley (1952) and, later and in more detail, by Cupedo (1980a, b). Regenhardt’s (1961) monograph was a milestone in the taxonomic assessment of Cretaceous serpulids from central Europe, although it was incomplete and the systematics proposed subsequently raised criticism. Regenhardt introduced several new species from the type Maastrichtian, on the basis of a unique collection of well-preserved mesofauna collected by the late Ehrhard Voigt.
A critical revision of Regenhardt’s systematics was initiated by Lommerzheim (1979) and continued by Jäger (1983). However, both these papers focused primarily on serpulids from Germany; material from the type Maastrichtian was considered only in passing. Later, Pillai (1993) described the internal tube structures of Spiraserpula, while Keutgen (1996) presented detailed stratigraphic data of serpulids of early Maastrichtian age in the Aachen area and direct environs. Numerous specimens from the borehole Thermae 2000 at Valkenburg aan de Geul (the Netherlands) were illustrated by Jäger (1987), who also (1988) presented a detailed overview of serpulid biostratigraphy around the boundary between the Gulpen and Maastricht formations.
A second phase of revisions began with Jäger (1993), who considered only species from the early-middle Danian Houthem Formation in southern Limburg and the Belgian province of Limburg. Subsequently, Jäger (1998) illustrated material from the ENCI-Heidelberg Cement Group quarry (Maastricht) and a list of species occurring at several intervals of the Vijlen Member at Altembroeck (Voer, northeast Belgium) was provided by Jäger in Jagt et al. (1995), who also discussed the stratigraphic value of Nogrobs (Tetraditrupa) canteriata and Conorca trochiformis. Lastly, Jäger (2005) published a monograph in which several new species from the type Maastrichtian were described and illustrated. Since then, a few name changes have proved necessary.