An unusual conchorhynch from the upper Maastrichtian of the southeast Netherlands and the distinction between nautiloid and ammonoid conchorhynchs (Mollusca, Cephalopoda)☆
Introduction
Conchorhynchs and rhyncholites are the pointed calcareous elements of the jaw tips in several cephalopod groups, such as nautilids (Nautilida de Blainville, 1825) and two ammonoid suborders: Phylloceratina Arkell, 1950 and Lytoceratina Hyatt, 1889. In view of the fact that in extant cephalopods, such elements occur exclusively in the jaws of representatives of the order Nautilida (see Saunders et al., 1978), the long-held assumption was that calcareous tips in the fossil record could also be interpreted as parts of nautiloid jaws (Saunders et al., 1978). However, pointed calcareous elements were subsequently recognised in the jaw apparatuses of Cretaceous ammonoids, including in situ findings, preserved in the body chambers of ammonoids (Kanie et al., 1978; Tanabe et al., 1980; Kanie, 1982). Following these discoveries, a new type of ammonoid jaw was identified, the rhynchaptychus type (see Tanabe et al., 2015).
Although there are a number of records of Palaeozoic rhyncholite-like structures, these are no longer considered to constitute cephalopod jaw elements (see Yochelson, 1971). The earliest undoubted rhyncholites and conchorhynchs are of Middle Triassic age (Riegraf and Schmitt-Riegraf, 1995; Klug, 2001; Riegraf and Moosleitner, 2010) and belong to nautilid genera such as Germanonautilus von Mojsisovics, 1902 (Riegraf and Schmitt-Riegraf, 1995; Klug, 2001). Some of these findings are isolated, but many were found in situ in the body chambers of nautilids (see Klug, 2001). The oldest records of isolated rhyncholites, presumably assignable to ammonoids, are from the Pliensbachian (Lower Jurassic) of northwest Europe (Riegraf and Moosleitner, 2010), while the earliest examples to date of complete ammonoid jaws, with conchorhynchs, originate from the Middle Jurassic (Bathonian/Bajocian) of Dagestan, Russia (Mironenko and Gulyaev, 2018).
Rhyncholites and conchorhynchs are particularly common in Cretaceous strata (see Mironenko and Rogov, 2018). This holds true also for the Maastrichtian type area where numerous calcified jaw elements are found in fossil hash levels at the base of several lithostratigraphical units, often associated with omission surfaces (Jagt and Jagt-Yazykova, 2019). In correlative ‘white chalk’ strata in northeast Germany and Denmark, they are much rarer (Reich and Frenzel, 2002; Rasmussen et al., 2011; Gravesen and Jakobsen, 2013). Ammonoid jaw elements disappear from the fossil record shortly after the Cretaceous–Paleogene (K/Pg), as a result of the extirpation of that cephalopod order, while those of nautiloids can be found in considerable numbers in Cenozoic strata (see e.g., Pacaud, 2010; Weaver et al., 2012).
Although we can be certain that all Mesozoic types of rhyncholites and conchorhynchs belonged to ammonoids or nautilids, it is virtually impossible to assign most of these to specific shell-based genera or species when they occur isolated. For this reason, a kind of parataxonomy is used in the description and naming of these calcified jaw elements. Well-preserved conchorhynchs and rhyncholites are known from various localities across Europe, including the type area of the Maastrichtian Stage (Fig. 1; see Van der Tuuk, 1980a, b, 1982, 1985; Jagt, 2012; Jagt and Jagt-Yazykova, 2019). In fact, with the exception of the Aken Formation, all units of early Campanian to early Paleocene age recognised in this area have yielded such jaw elements over recent decades (Table 1). In the early 1990s, the upper Meerssen Member was well exposed in the southwesterly corner at the former ENCI-HeidelbergCement Group quarry, south of Maastricht, allowing the collection from the coarse-grained, loose biocalcarenites of large quantities of macrofossils by wet sieving on several mesh widths. Assemblages recovered included echinoids, crinoids, ophiuroids, asteroids, brachiopods, serpulids and bryozoans (see Jagt, 1999, 2000). The present conchorhynch specimen originates from such residues; it was briefly described by Van der Tuuk and Jagt (1999), but not named. Here we re-examine it by using a scanning electron microscope, discuss its microstructure and taxonomic affiliation and name it formally.
Section snippets
Conchorhynch structure and terminology
For calcitic upper jaw elements, or rhyncholites, a widely accepted terminology is available. The pointed anterior end, which protruded from the jaw in the live animal, is called the hood, whereas the elongated part that was embedded in the organic tissues of the jaw, is referred to as the shaft (see Riegraf and Schmitt-Riegraf, 1995). For now, there is no equivalent terminology for conchorhynchs, which explains why different names occur in the literature. We are of the opinion that it would be
Systematic palaeontology
Class Cephalopoda Cuvier, 1798
Order Nautilida Agassiz, 1847
Family incertae sedis
Form genus Conchorhynchus de Blainville, 1827
Type species: Conchorhynchus ornatus de Blainville, 1827, by monotypy.
Conchorhynchus illustris sp. nov.
urn:lsid:zoobank.org:act: 1B6123F1-B08E-4A31-8B44-F3576F90808B
Fig. 3, Fig. 4
Type. Holotype, and single specimen known to date, is NHMM JJ 11128.
Derivation of name. From Latin illustrare (adjective illustris), in the sense of ‘remarkable’.
Type locality and stratigraphical
Characteristic features for differentiating between ammonoid and nautilid conchorhynchs
As mentioned above, calcareous conchorhynchs were present in the jaws of only three groups of cephalopods: the nautiloid order Nautilida and the ammonoid suborders Phylloceratina and Lytoceratina. However, it is still unclear how to determine if a certain morphotype of conchorhynch belonged to ammonoids or to nautilids. Triassic conchorhynchs, assigned to Conchorhynchus avirostris, have been found in situ in the shells of Germanonautilus (Klug, 2001). On this evidence, all Triassic
Conclusions
Amongst calcified parts of the jaw apparatus from upper Maastrichtian levels in the type area of the Maastrichtian Stage (the Netherlands, Belgium), there is an atypical conchorhynch that is here described as a new parataxon, Conchorhynchus illustris sp. nov. Until now, criteria for assigning isolated finds of Jurassic and Cretaceous conchorhynchs to any specific group of cephalopods have not been determined. However, an overview of the characteristics of ammonoid and nautilid conchorhynchs
Acknowledgements
We are very grateful to two anonymous reviewers and the handling editor, Eduardo Koutsoukos, for pertinent comments on an earlier version of the typescript. In addition, we thank Larisa Doguzhaeva (then Stockholm), Neil H. Landman (New York) and Kazushige Tanabe (Tokyo) for sharing their views on the present conchorhynch with us, admittedly years ago.
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