Cenoceras rumelangense, Weis & Schweigert & Wittische, 2023

Weis, Robert, Schweigert, GÜnter & Wittische, Julian, 2023, A new giant nautilid species from the Middle Jurassic of Luxembourg and Southwest Germany, Swiss Journal of Palaeontology (24) 142 (1), pp. 1-11 : 5-9

Cenoceras rumelangense n. sp. [Weis & Schweigert]

Figures 5 , 6 , 7 , 8 , 9

1995 Cenoceras sp. : Dietl, p. 357, fig. 1.

2005 Cenoceras sp. : Gross, p. 58.

? 2008 Cenoceras gr. obesum : Rulleau, pl. 22, fig. 2 [only specimen EPF Baj74].

2013 Cenoceras sp. : Dietl, p. 20, fig. 24.

? 2016 Metacenoceras sp. : Grulke, p. 139, figs. 1&2.

2021 Cenoceras obesum : Schweigert, p. 9, fig. 14.

Derivation of name: Te species is named after the town of Rumelange, a municipality in the southernmost part of Luxembourg that is widely known for its rich fossil findings from Toarcian to Bajocian rocks, including the herein described nautilids.

Holotype: Holotype is specimen MNHNL BM790

( Fig. 5 ).

Paratypes: 17 specimens from the Humphriesianum Zone of Rumelange ( MNHNL: BM232 , BM237 , BM278 , BM280 , BM370 , BM372 , BM383 , BM641 , BM783 , BM784 , BM791 , BM792 , BM793 ; MEP1; MNM1; Geolor 1 & 2) and one specimen from the Middle Jurassic Ostreenkalk Formation (Humphriesianum Zone) of Gruibingen , Baden-Württemberg ( SMNS 62245 ) .

Type locality: Rumelange, abandoned quarry site ʽ Berenskaul-Waisskaul’ on the Luxembourgish–French border (Coordinates 49°27 ′ 12 ʺ N and 6°00 ′ 41 ʺ E; Bintz et al., 1973, p. 159) ( Fig. 3 ).

Type horizon: ‘Marnes sableuses d’Audun-le-Tiche’ (dom4), lower Bajocian, Humphriesianum Zone.

Diagnosis: Giant-sized, narrow-umbilicate species of Cenoceras with strongly inflated subtrapezoidal whorl section, generally higher than wide; longitudinal striation on the flattened venter and on the outer third of the flank.

Description: Te holotype ( Fig. 5 ) is a steinkern with a few preserved remnants of the shell. Te body-chamber occupies half of the last whorl. Te last septa are more closely spaced, indicating that the specimen reached the adult age. Te venter is distinctly flat and is ornamented with dense longitudinal striae that are particularly well developed on the body-chamber and which extend on the outer third of the flanks. Te section of the body-chamber is markedly subtrapezoidal, compressed (compression index is 0.81 on the body-chamber and 0.77 on the phragmocone); the compression is more marked in younger whorls and less strong on the body-chamber but does not affect the lateral dimensions. Te flanks are slightly rounded on the inner third (located towards the umbilicus) and distinctly flattened to slightly concave on the outer two thirds. Te flanks are covered with fine falciform striae, and the outer third shows a reticulate ornamentation pattern. Te phragmocone shows rather distant septa: the number of septa on the last whorl is n = 10. Te lateral lobe is distinctly large and shallow (depth of the lateral lobe = 10 mm), and is followed by an even less profound ventral lobe. Te latero-ventral saddle is rather distinctive. Traces of the siphuncle are not visible in this specimen.

Umbilicus: Troughout all ontogenetic stages the overhanging umbilicus remains open, with a well-rounded umbilical edge.

Flanks: Lower part gradually rising from the steep and overhanging umbilicus forming a well-rounded umbilical edge. Middle part of flanks form a trapezoidal section; upper part become slightly concave in the latest part of the adult body-chamber.

Venter: Venter is rounded only in the juvenile stage and soon becomes flattened by developing a prominent ventrolateral edge.

Suture line: On the flank, the suture line forms a single, relatively shallow lateral lobe, with its deepest part located in the outer third of the flank. From the saddle at the ventrolateral edge, it turns towards mid-venter in a right angle without forming an extra ventral lobe.

Ontogeny: Growth pattern lacks principal changes of coiling or other modifications during ontogeny, except the inflated whorl section gradually changing from being rounded in the juvenile stage to subtrapezoidal in the medium and adult stages. In the latest stage, the chambers and suture lines are narrowing thus indicating adult size.

Ornamentation: Te almost uncompressed specimen from Swabia ( Fig. 9 ) is the only one with a completely preserved calcitic replacement shell. Te adult shell is ornamented with dense longitudinal striae both on the venter and on the outer third of the flank. Te aperture does not exhibit any apertural modifications and crosses the venter in a straight way. Due to the shell preservation, the beginning of the body-chamber and the suture lines of the phragmocone cannot be traced.

Measurements and morphometrics: Various measurements and parameters of C. rumelangense specimens are reported in Table 1. We compared the data with measurements and parameters published for two coeval large-sized nautilid taxa: Cenoceras obesum (sensu Rulleau, 2008) and Metacenoceras clausum .

PCA biplot for absolute morphometric descriptor values (scaling 1)

Although the three groups seem separated in the biplot

( Fig. 1 ), the eigenvalues showed that the first axis was explaining much more than the following axes for the absolute values PCA. Tis means that the clausum and obesum groups are closer to each other than they are to the rumelangense group. Tis is also supported by the clustering analysis ( Fig. 1 ). Assuming that there are indeed three groups, the clustering analysis puts specimen EPF Baj74 (in Rulleau, 2008, p. 52) with the rumelangense group, not with the obesum group in which it was originally classified ( Fig. 1 ). Also, the specimens attributed to the rumelangense group, MNHNL BM 370 (sample 19) and MNHNL BM 783 (sample 25), were clustered together with the obesum group.

PCA biplot for ratios of morphometric descriptor values (scaling 1)

Tree groups are separated in the biplot ( Fig. 2 ), the eigenvalues showed that the three first axes are relevant (see inset Fig. 1 ) for the ratio values PCA. Clustering results overlay well with PCA results and original grouping, aside from specimen EPF Baj74 (sample 13), which is associated with the rumelangense group again ( Fig. 2 ).

For the PCA on absolute values, the proportion explained by the principal components 1 and 2 were 94% and 4%, respectively. For the PCA on absolute values, the proportion explained by the principal components 1 and 2 were 65% and 21%, respectively.

Remarks. In first view, the herein described Cenoceras seems to fit well with the description and illustration of Nautilus obesus Sowerby, 1816 (p. 124). However, the provided illustration is very schematic and, more important, the original of the illustrated specimen could not be traced ( Foord & Crick, 1890). Tus, Foord and Crick (1890) re-interpreted this species based on further material. Interestingly, they noted that in one of the studied specimens they had assigned to Sowerby ʼ s species, the shell is preserved and it is said to have a shell showing only growth lines. In contrast, in our material with preserved shell, as mentioned above, a strong longitudinal striation is present at least in the ventral and ventrolateral part of the conch, most likely expanding to the entire conch in the juvenile stage as in other Cenoceras (s.str.) species. We therefore suspect that at least some of the material of Nautilus obesus sensu Foord & Crick does not represent Cenoceras s.str. but Metacenoceras Tintant, 1984 ( Metacenoceras , previously considered a subgenus of Cenoceras was given full genus rank in Dietze et al., 2021. p. 48). Tis is corroborated by the fact that Foord and Crick (1890) included Orbigny ʼ s Nautilus lineatus in the synonymy of N. obesus Sowerby. For Nautilus lineatus Sowerby in Orbigny, Tintant (in Enay et al., 1994) introduced the new species Cenoceras (Metacenoceras) moutierense pointing out the differences with the incomplete specimen illustrated by Foord & Crick (1890, fig. 11) as Nautilus obesus . Although the determination of the latter was said to go back to the times of J. Sowerby, it is unknown whether it belonged to the type series. Moreover, this specimen has not preserved its shell and the entire body-chamber is missing. Although a conspecificity of our specimens with Sowerby ʼ s original Nautilus obesus cannot be ruled out, this is unprovable due to the apparent loss of the type, and consequently, we here regard Nautilus obesus J. Sowerby, 1816 as a nomen dubium. Rulleau (2008) illustrated several large-sized nautilids from the Humphriesianum Zone of France as Cenoceras ex gr. obesum . His material cannot be assigned to Sowerby ʼ s species for the same reasons. Comparing Rulleau ʼ s measurements with those of the herein described material, however, clearly indicates that his material differs significantly from ours and must represent another species, with the exception of specimen EPF Baj74, whose measurements fall into the variation of C. rumelangense as shown by our cluster analysis. Pending a verification of the original specimen in the Espace Pierres Folles collection, we tentatively include this specimen in our new species, with a question mark. We furthermore consider it likely that the specimens illustrated by Grulke (2016, p. 139) from the upper Bajocian of Frogden Quarry, Sherborne represent large-sized specimen of C. rumelangense . If confirmed, these records would represent the largest specimens known, with 700 resp. 750 mm of diameter ( Grulke, 2016, p. 139).

Occurrence: Cenoceras rumelangense n. sp. occurs in the Humphriesianum Zone (lower Bajocian, Middle Jurassic) of Luxembourg and Southwest Germany (Baden-Württemberg). Unpublished data suggest a possible occurrence in coeval strata of North Switzerland (personal observation by GS). Clustering analyses performed in this study furthermore hints at the occurrence of the taxon in the Rhône Valley, France (specimen EPF Baj 74 in Rulleau, 2008, p. 52). Unstudied specimens from Sherborne, UK, that were illustrated by Grulke (2016) might also belong to C. rumelangense and thus potentially extend the geographic occurrence towards the Northwest of Europe.

On the occurrence of ‘giant’ nautilids in the Jurassic ‘Gigantism’ is an ecologically important trait associated with competitive superiority; although it has been studied in specific cases, the general conditions at the origin of a very large body size remain still obscure ( Vermeij,

2016). Te term ‘giant’ or ‘gigantism’ is arbitrary, as any definition is comparative in itself; according to Klug et al. (2015), a giant species must fulfil the following three criteria: (1) species of the respective size should not have occurred all the time. Tere should be a low number of giant species; (2) adults of giant species have to be much larger than the average of their group; (3) the largest specimen(s) of the respective species should not show strong signs of pathology or abnormal growth.

Pathological gigantism ( Manger et al., 1999) can be ruled out in the case of C. rumelangense , as none of the studied specimens shows any abnormalities in growth that would hint at a pathological development. In addition, there are several specimens that reach 500 mm diameter or more ( Table 1), making the species outstanding by size not only for the genus Cenoceras but also in general for Mesozoic nautilids.

Living nautilids, namely Nautilus pompilius , N. vitiensis , N. samoaensis , N. vanuatuensis only rarely reach diameters larger than 200 m (Barord et al., 2023), N. belauensis and the questionable taxon N. repertus being credited with the largest shell diameters reaching up to 240 mm ( Saunders, 1987). Compared to living nautilids, Mesozoic taxa reach considerably larger dimensions. Large-sized nautilids (> 300 mm) occasionally occur throughout the Jurassic. Already Zieten (1831) reported large-sized specimens from the Lower and Middle Jurassic of Swabia with diameters of “anderthalb Schuh” (c. 430 mm) and named them Nautilites giganteus Schübler. Te collection of the National Museum of Natural History Luxembourg contains a large nautilid (330 mm; MNHNL n° SI340), ascribed to the genus Cenoceras from the ‘Marnes et calcaires de Strassen’ unit (Sinemurian, Lower Jurassic). Te Stuttgart Natural History Museum houses a specimen of Hemicenoceras arare ( Dumortier, 1869) from the early Pliensbachian of Swabia with a diameter of 350 mm ( SMNS 70673). In the Middle Jurassic, Branger (2004) reported a large specimen (440 mm) of Cenoceras mazardrikense from the Callovian of western France. Te specimens of C. rumelangense reported herein are thus considerably larger than these previously reported ‘giant’ nautilids.

Te genus Paracenoceras Spath, 1927 is also known for some giant species, such as Paracenoceras giganteum ( Orbigny, 1843) and P. ingens Tintant et al., 2002 from the Kimmeridgian (Upper Jurassic). Large-sized specimens of Paracenoceras spp. with diameters up to 500 mm are cited from southern Germany, Pomerania and France ( Loesch, 1914; Kuhn, 1936; Tintant in Enay et al., 1994; Schweigert, 2021; Schlampp, 2022). Turmann and Étallon (1861) noticed a diameter of 7 dm for Nautilus giganteus . Tintant et al. (2002) reported that some paracenoceratids from Switzerland could reach 600 mm or even more. However, the largest specimen measured by Tintant et al. (2002) was a Paracenoceras ingens of 522 mm diameter. To our knowledge, nautilids from the Cretaceous and younger ones did not reach diameters over 450 mm. Tus, the specimens of C. rumelangense n. sp. reported herein ( Tab. 1) appear to be the largest post-Triassic nautilid documented so far in the scientific literature.

Taphonomical remarks

All of the herein studied specimens of Cenoceras rumelangense n. sp. show an extensive overgrowth of epifauna. Amongst the most common organism are serpulids ( Dorsoserpula lumbricalis ), small oyster shells ( Liostrea ) and bryozoans. Other bivalves, such as Eopecten abjectus (Phillips, 1829) are also present in some specimens. Typically, the epifauna is present on both flanks of the shell. An overview about benthic fauna from the ‘Marnes sableuses d’Audun-le-Tiche’ unit in Rumelange is given by Fayard et al. (2005).

In the specimen from Swabia ( SMNS 62245), epizoans are represented by serpulids ( Dorsoserpula lumbricalis [ Schlotheim, 1820]) and bryozoans ( Berenicea , Kololophos ), whereas oysters are missing. Only the venter of the adult body-chamber is free of epifauna. Tis strongly points to a post-mortem overgrowth, when the empty shell was in an upright position at the seafloor forming a benthic island in a muddy substrate, otherwise the ventral area of the shell would have been overgrown as well. Similar observations were made in Lower Jurassic representatives of Cenoceras ( Evans & King, 2019) .

Figures

Fig. 5 Cenoceras rumelangense n.sp., holoytpe (specimen MNHNL BM790),‘Marnes sableuses d’Audun-le-Tiche’, Humphriesianum Zone, Berenskaul,Rumelange, Luxembourg

Fig. 6 Cenoceras rumelangense n.sp., paratype (specimen MNHNL BM383),‘Marnes sableuses d’Audun-le-Tiche’, Humphriesianum Zone, Rumelange,Luxembourg

Fig. 7 Cenoceras rumelangense n.sp., paratype, inner whorl of a bigger specimen (specimen MNHNL BM783),‘Marnes sableuses d’Audun-le-Tiche’,Humphriesianum Zone,Berenskaul, Rumelange, Luxembourg

Fig. 8 Cenoceras rumelangense n.sp., paratype,largest measured specimen (specimen MNHNL BM791),‘Marnes sableuses d’Audun-le-Tiche’, Humphriesianum Zone, Berenskaul, Rumelange, Luxembourg

Fig. 9 Cenoceras rumelangense n.sp., paratype (specimen SMNS 62245), Ostreenkalk Formation, Humphriesianum Zone, Gruibingen, Southwest Germany

Fig. 3 Stratigraphic position (marked by a black arrow) and geographic location of the type locality (marked by a red circle) nearby Rumelange. Map of Southwest Luxembourg and stratigraphic log: Service géologique du Luxembourg (www.geology.lu)

Fig. 1 PCA biplot for absolute morphometric descriptor values (scaling 1). Clustering results are overlaid and represented by grey lines. Colour/shapes represent a priori grouping to species level

Fig. 2 PCA biplot for ratios of morphometric descriptor values (scaling 1). Clustering results are overlaid and represented by grey lines.Colour/shapes represent a priori grouping to species level