The first gladius-bearing coleoid cephalopods from the lower Toarcian “Schistes Cartons” Formation of the Causses Basin (southeastern France)

The fossil record of gladius-bearing coleoids is scarce and based only on a few localities with geological horizons particularly favourable to their preservation (the so-called Konservat-Lagerstätten), which naturally leads to strongly limited data on geographical distributions. This emphasizes the importance of every new locality providing gladius-bearing coleoids. Here, we assess for the first time the gladius-bearing coleoid taxonomic diversity within the lower Toarcian “Schistes Cartons” of the Causses Basin (southeastern France). The material includes two fragmentary gladii, identified as Paraplesioteuthis sagittata and ?Loligosepia sp. indet. Just with these two specimens, two (Prototeuthina and Loligosepiina) of the three (Prototeuthina, Loligosepiina and Teudopseina) suborders of Mesozoic gladius-bearing coleoids are represented. Thus, our results hint at a rich early Toarcian gladius-bearing coleoid diversity in the Causses Basin and point out the need for further field investigations in the lower Toarcian “Schistes Cartons” in this area. This new record of Paraplesioteuthis sagittata is only the second one in Europe and the third in the world (western Canada, Germany and now France). Based on these occurrences, we tentatively suggest that P. sagittata originated in the Mediterranean domain and moved to the Arctic realm through the Viking Corridor to eventually move even farther to North America.

However, as these Lagerstätten are not continuous in time and space, the record of Mesozoic gladius-bearing coleoids is intermittent.As noted by Fuchs et al. (2016), our knowledge on geographical distributions of species still needs to be greatly improved, which confers great importance upon every new locality yielding gladius-bearing coleoids.
Here, we describe fossils of gladius-bearing coleoids from the lower Toarcian ''Schistes Cartons'' Formation (contemporaneous with the famous Posidonia Shales of Holzmaden; see references above) of the Causses Basin in southeastern France.The material described here represents the first gladius-bearing coleoid remains documented from this area.Thus, this work is the first attempt to assess the taxonomic diversity of gladius-bearing coleoids from the lower Toarcian ''Schistes Cartons'' of the Causses Basin.These findings also allow to provide new insights on the paleobiogeography of Toarcian gladius-bearing coleoids and are a first step towards discussions on hypothetical migration corridors.
The Toarcian sediments of the Causses Basin were deposited in a shallow epicontinental sea located at a paleolatitude of 25 to 30 • N. At the base of the ''Schistes Cartons'' Formation, Bomou et al. (2021) documented a negative carbon isotope excursion and higher mercury fluxes that were linked with the Toarcian Oceanic Anoxic Event (T-OAE).This event, originating from the intense volcanic activity of the Karoo Ferrar igneous province, is characterized by a widespread deposition of organic-rich shales concomitant with the onset to an episode of global warming.Bomou et al. (2021) showed that the deposition of the ''Schistes Cartons'' Formation took place during a prolongated period of widespread oxygen-deficiency and elevated carbon burial.
Description.Although the single specimen (M486_2024.1.1)is only partially preserved, morphological description and taxonomic identification at the species level remain possible.
The specimen consists of a long and slender gladius (interpreted here as in dorsal view) with a triangular, anteriorly diverging median field (Figs.4A-4C).The preserved gladius length (= median field length) is 203 mm.Although the anterior end of the gladius is not preserved, we hypothesize that the original gladius length did not exceed 220 mm (according to Klug et al., 2021a, gladii of Paraplesioteuthis sagittata rarely reach 200 mm).
Of note, we suspect that the anterior part of the gladius, which is poorly preserved, was affected by slight disruptions and distortions (Figs.4A, Fig. 4B).In our opinion, this impedes providing a reliable measurement of the anteriormost gladius width.Partially preserved lateral reinforcements (most conspicuous in the posterior part of the gladius, Figs.4A-4C, 5) diverge from posterior to anterior extremities.Based on the estimated median field width hypz to hyperbolar zone length (see Fig. 3 and Appendix S1A), we estimate that the lateral reinforcements form an opening angle of ∼11 • (see Appendix S1A).The lateral fields are relatively slender.Although the outline of the hyperbolar zone is hardly discernible, it appears relatively long (estimated hyperbolar zone length to median field length ratio is 0.44).Finally, it cannot be determined whether the median ridge is bipartite as commonly described for Paraplesioteuthis representatives.
Remarks.In our opinion, the gladius is too poorly preserved to provide an accurate measurement of the original gladius width max .Nevertheless, based on the general shape of the gladius, we consider that the original gladius width max to gladius length ratio likely fell within the range of values mentioned by Fuchs (2020, p. 10) for Paraplesioteuthis (i.e., 0.15-0.25).Of note, the opening angle of ∼11 • framed by the lateral reinforcements in the present specimen is lower than the range of values given by Fuchs (2020, p. 10) for Paraplesioteuthis (i.e., 14 • −20 • ).On the other hand, it is comparable to the opening angle of 10 • mentioned by Hall (1985) for the P. hastata (Münster 1843) specimen from western Canada (P. hastata is regarded as conspecific with P. sagittata by Fuchs & Larson, 2011a).Thus, we suggest that values of opening angle for Paraplesioteuthis should be redefined as ranging from about 10 • to 20 • .
According to Fuchs & Larson (2011b), the Middle Jurassic genus Romaniteuthis differs from Paraplesioteuthis by having a reduced median field width (i.e., gladius width max to gladius length ratio 0.05-0.15;Fuchs, 2020) and lateral reinforcements as keels.Although the specimen described herein is too poorly preserved to provide a reliable measurement of the preserved gladius width max , its original gladius width max to gladius length ratio was probably not less than 0.15.Furthermore, it does not exhibit prominent lateral keels.
Paraplesioteuthis lateral fields are relatively short (Fuchs & Larson, 2011a; see Fig. 4D, Fig. 4E).In this regard, the lateral fields of the present specimen seem more similar to that of Romaniteuthis, since they appear slightly elongated, in oval shape (compare Figs. 4A-4C, 5 with fig 7.2 in Fuchs & Larson, 2011a).This may however be due to slight intraspecific variability.Finally, the estimated hyperbolar zone length to median field length ratio for the present specimen is 0.44, which nearly falls within the range of values given by Fuchs (2020) for Paraplesioteuthis (i.e., 0.45-0.55).
In sum, despite a possible slight difference in shape of lateral fields, we consider that other features of the present gladius support its attribution to the species Paraplesioteuthis sagittata.
Order Vampyromorpha Robson, 1929Suborder Loligosepiina Jeletzky, 1965Diagnosis (after Fuchs, 2020).Small-to large-sized octobrachiates with bullet-shaped body; gladius length (= median field length) equals mantle length; gladius with triangular median field and cup-shaped conus; gladius slender to wide, maximum gladius width always exceeds maximum median field width; median field width very slender to moderately wide without pronounced median reinforcements, anterior median field margin concave, straight or convex; median field area small to large; hyperbolar zone mostly well-arcuated, rarely indistinct, long to very long; lateral fields usually moderately wide.
Description.The preserved gladius length (= median field length) of this specimen (M486_2024.1.2) is 76 mm and the maximum preserved gladius width is approximatively 33 mm.Although a significant part of the original gladius is most certainly missing, we tentatively hypothesize that the original gladius did not exceed 150 mm.Unfortunately, few features can be described on this specimen.On the posterior part of the specimen, inconspicuous lines are interpreted as a disrupted median line and inner asymptotes (Figs.6A-6C, 7).The black structure preserved anteriorly is interpreted as the ink sac (Figs.6A-6C).The specimen outline appears weakly constricted posteriorly, although it cannot be determined whether it is a genuine feature, or if it is due to slight taphonomic disruptions.The median field width max of the original gladius cannot be assessed.
Remarks.The overall shape of the gladius combined with the presence of lines interpreted as a median line and inner asymptotes hint at the possibility that this specimen belongs to the suborder Loligosepiina.Mesozoic gladius-bearing coleoids belonging to the suborder Teudopseina exhibit a characteristically constricted median field anteriorly, which does not seem to be the case in specimen M486_2024.1.2.Within the suborder Loligosepiina, the assignment of this specimen to the genus Loligosepia is unsettled.It is only based on a conjectural original size of the specimen (estimated around 150 mm) that is comparable with that of representatives of the Loligosepia species L. bucklandi (Voltz, 1840) and L. aaalensis (Schübler in Zieten, 1832).In our opinion, Jeletzkyteuthis species differ mostly by their larger gladius size.For example, J. coriaceus (Quenstedt, 1849) gladiuses regularly exceed 200 mm in length, according to Klug et al. (2021a).Besides, the poor preservation of both specimens prevents a robust comparison between the present specimen and the J. coriaceus specimen from the coeval locality of Langres (Guérin-Franiatte & Gouspy,

DISCUSSION: PALEOBIOGEOGRAPHY OF PARAPLESIO-TEUTHIS SAGITTATA
The present record of Paraplesioteuthis sagittata is only the second one in Europe and the third in the world (western Canada, Germany and now France; Fig. 8).During the Early Jurassic, the NW Tethyan and Artic realms (which consisted of two epicontinental seas) were linked by a narrow seaway named the Viking Corridor (Ziegler, 1988;Fig. 8).In this time interval, the Serpentinum Zone (from where the gladius-bearing coleoids described herein come) marks the onset of the disruption of a previous ammonite provincialism, with a strong homogenization of all Tethyan and Arctic ammonite species (Dera et al., 2011).This event is linked with the origination of numerous cosmopolitan ammonite taxa in the Mediterranean domain (Macchioni & Cecca, 2002).In this context, the Viking Corridor probably regulated the mixing between Artic and Euro-Boreal ammonites (Smith, Tipper & Ham, 2001;Dera et al., 2011).
Based on this, it can be hypothesized that some Early Jurassic gladius-bearing coleoids (such as P. sagittata), similarly to ammonites, broadly originated in the NW Tethyan realm and moved to the Arctic realm through the Viking Corridor, to eventually move even farther to North America (in the context of the Early Toarcian global warming event; see, e.g., Dera & Donnadieu, 2012).Other routes cannot be excluded, such as the Hispanic Corridor (Fig. 8), a narrow epicontinental seaway that was sporadically active since the Late Sinemurian −Early Pliensbachian time interval (Aberhan, 2001;Aberhan, 2002;Venturi, Bilotta & Ricci, 2006;Dera et al., 2009).However, according to Dera et al. (2011, p. 100), the Hispanic Corridor ''. . .was certainly too shallow for allowing massive movements of hemipelagic organisms such as ammonites''.Since gladius-bearing coleoids are also hemipelagic organisms, we presume that gladius-bearing coleoids, similarly to ammonites, were not able to go through the Hispanic corridor.In the hypothesis that gladius-bearing coleoids moved from the Mediterranean domain to North America through the Viking Corridor, we would expect to find P. sagittata specimens in localities from the Artic Realm, provided that the geological time interval is represented and that there are geological horizons peculiarly favourable to the preservation of gladius-bearing coleoids.

CONCLUSIONS
Two of the three suborders of Mesozoic gladius-bearing coleoids are present in the studied material, which comprises so far only two specimens.This implies a previously unrecognized early Toarcian gladius-bearing coleoid diversity in the Causses Basin and points out the need for further field investigations in the lower Toarcian black shales in this area.Finally, based on the known worldwide occurrences of P. sagittata, we tentatively suggest that this species originated in the Mediterranean domain and moved to the Arctic realm through the Viking Corridor to eventually move even farther to North America.

Figure 1
Figure 1 Geographical location of the Causses Basin.The black star indicates the Saint Bauzile site, where the studied material was retrieved.Image source credit: Moreau et al. (2021) ©Cambridge University Press, reproduced with permission.Full-size DOI: 10.7717/peerj.16894/fig-1