First record of the enigmatic coleoid genus Longibelus from Sakhalin (Far East Russia): a contribution to our understanding of Cretaceous coleoid habitats in the Pacific Realm

A newly collected specimen of the enigmatic coleoid genus Longibelus is recorded from lower Turonian strata along the River Shadrinka in Sakhalin (Russian Far East). To date, this is the first record of Late Cretaceous coleoid cephalopods from the island and, in fact, from the entire Pacific coast of the Russian Federation. Lithological characteristics, coupled with published geochemical analyses (δ13C and Corg content), suggest the habitat of this coleoid taxon to have been the middle to outer (i.e. distal) shelf. Its provenance from the stratigraphical level that is known as the Scaphites Event, characterised by a mass occurrence of Scaphites and Yesoites, may be indicative of occasional or marginal overlap in ranges, rather than life in similar habitats. On the basis of lithological features and in view of the extremely rare occurrence of Longibelus in rich ammonite assemblages with clear ecological/bathymetric preferences, the natural habitat of Longibelus may have comprised neritic to mesopelagic zones over distal shelves and slopes.


Introduction
In the Pacific Realm, coleoid cephalopods rank amongst the rarer macrofossils in Upper Cretaceous strata, the exception being stratigraphical levels assigned to the Yezo Group which were deposited in the forearc basin of the northwest Pacific (Fuchs et al., 2013). From Hokkaido (northern Japan) in particular, numerous specimens of the enigmatic orthoconic genus Longibelus Fuchs et al., 2013 have been recorded. This genus, which was erected on the basis of significant differences from members of Naefia Wetzel, 1930 includes forms that had formerly been referred to that genus (Fuchs & Tanabe, 2010;Fuchs et al., 2013;Hayakawa & Takahashi, 1993;Hewitt et al., 1991;Hirano et al., 1991). To date, Longibelus, which ranges from the Cenomanian to the Maastrichtian in the Pacific Realm, comprises two species, i.e. the type species, L. matsumotoi (Hirano et al., 1991), and L. kabanovi (Doguzhaeva, 1996). Fuchs et. al. (2013) and Fuchs (2019) interpreted Longibelus as a link between the superorders Decabrachia and Belemnoidea; however, its systematic position at the order or family level is not yet clear. Some similarities to decabrachians are based on initial segments of the siphuncle, but it is not yet clear if the closing membrane was lost at the evolutionary stage illustrated by Longibelus (see Fuchs, 2019).
For now, the original natural habitats of 'longibelid' coleoids are poorly known (Fuchs et al., 2013). The present record from Sakhalin originates from fine-grained sedimentary rocks that illustrate deposition in the middle to outer shelf. In view of the fact that this specimen constitutes a disarticulated phragmocone in such a fine-grained matrix, a longer period of post-mortem floating in the water column may be excluded.

Geological setting
Sakhalin Island (Fig. 1) is part of the North Pacific island arc positioned along the continental margin; this includes the Japanese Islands. Cretaceous deposits are widely distributed here; an Albian-Maastrichtian sequence crops out in the West-Sakhalin Mountains (i.e. the Main Cretaceous Field), with a continuous (uninterrupted) reference section in the valley of the River Naiba (Matsumoto, 1942;Poyarkova, 1987;Kodama et al., 2002;Yazykova, Fig. 1 a Distribution of Cretaceous deposits in Sikhote Alin, Primorye and the islands of Sakhalin and Shikotan; b the Naiba River reference section near the village of Bykov; c sketch map of that part of the Naiba section where Longibelus sp. was found and of the distribution of outcrops of Cenomanian-Turonian strata. 30/10 is the number of the locality that yielded the present coleoid specimen First record of the belemnoid genus Longibelus from Sakhalin 2004), that corresponds to the Yezo Group (Albianlower Campanian) and Hacobuchi Group (upper Campanian-Maastrichtian) in Hokkaido (Matsumoto, 1959b). The lower Turonian portion in this sequence starts with sandy siltstone beds (Member III, sensu Zonova et al., 1993;Zonova & Yazykova, 1998), overlain by thin-bedded clayey siltstones. Up section, this grades into siltstones of middle Turonian age.
The present coleoid was recovered during field work at locality 30 on the left bank of the River Shadrinka which is an easterly tributary of the River Naiba (Figs. 1, 2) in 1987. The sequence outcropping is assigned to Member IV of the Bykov Formation; biostratigraphically it belongs to the Scaphites planus ammonite Zone and to the uppermost part of the Mytiloides aff. labiatus and the lower part of Inoceramus hobetsensis-I. iburiensis inoceramid Zones (Fig. 3). Generally, the Bykov Formation corresponds to the upper part of the Yezo Group in Hokkaido (northern Japan) on the evidence of ammonite and inoceramid bivalve faunas (Jagt- Yazykova, 2012;Matsumoto, 1959aMatsumoto, , 1959bTanabe, 1979). The Scaphites planus ammonite Zone and Inoceramus hobetsensis-I. iburiensis inoceramid Zone in Sakhalin can be well correlated with the Fagesia thevestensis-Mammites aff. nodosoides ammonite Zone and Mytiloides subhercinicus and Inoceramus costatus inoceramid zones in Hokkaido, respectively (Toshimitsu et al., 1995).

Materials and methods
The single, incomplete specimen (field number 30/10) of Longibelus sp. is stored in the collections of the Chlupáč Museum of Earth History, Faculty of Science, Charles University (Prague) under registration number CHMHZ-MK-01_21. Remains of the original shell, where present, have been diagenetically altered; there are no microstructures observable. This specimen has been studied in great detail using a digital microcamera. To bring out morphological details, it was coated with an ammonium chloride sublimate. The graphical program Corel Draw X7 has been used for the reconstruction of phragmocone parameters prior to deformation (Fig. 4). Our descriptive terminology follows Fuchs et al. (2013).

Discussion and remarks
In addition to other features, the existence of a marginal siphuncle, i.e. a markedly deep notch and the dorsal surface with longitudinal lines, allow placement of this phragmocone in the genus Longibelus. The diameter of chamber length fully corresponds to that of L. matsumotoi (Hirano et al., 1991). However, the apical angle (16 degrees) is greater than in typical specimens (10-13 degrees) of that species. Chamber length ratio has been calculated from the reconstructed (undeformed) crosssection (Corel Draw X7; see Fig. 4b, c) and the value 0.36-0.42 fits the range for Longibelus (i.e. 0.37-0.50; compare Fuchs et al., 2013). The ventral suture line forms a weak lobe only, in contrast to the distinct lobe seen in L.
matsumotoi. In this respect, Longibelus sp. from Sakhalin differs from its closest congener L. matsumotoi, and is, therefore, left in open nomenclature. The present fragment represents approximately the middle part of the phragmocone (Fig. 4d). This assumption should be supported by the fact that our specimen is significantly larger than a single Turonian specimen of L. matsumotoi from the Tappu area (Hokkaido, Japan) recorded by Hayakawa and Takahashi (1993). However, the above-mentioned differences from L. matsumotoi may indicate the presence of a new and unknown species within this genus.

Stratigraphical range
The present specimen stems from strata assigned to the Scaphites planus ammonite Zone and the lower part of the Inoceramus hobetsensis-I. iburiensis inoceramid Zone (Zonova & Yazykova, 1998). In lithostratigraphical terms, this is Member IV of the Bykov Formation (Zonova & Yazykova, 1998;Zonova et al., 1993). The closely related Longibelus matsumotoi ranges from the Cenomanian to the Maastrichtian in northern Japan (Fuchs et al., 2013).

Discussion
In general, there are only very few records of Late Cretaceous coleoids from the Far East of the Russian Federation. Zakharov et al. (2010) described rostra referred to as Belemnitella? sp. and Dimitobelus? sp. from Campanian-Maastrichtian deposits of Pacific guyots (Magelland Rise). In the northern Pacific region, the true Belemnitida became extinct during the late Albian (Iba et al., 2011). The present phragmocone from the lower Turonian of Sakhalin thus is the first non-belemnite record of a coleoid cephalopod from this vast territory. The widely distributed genus Longibelus has an extensive stratigraphical range from the Aptian to the Maastrichtian, with distinct species having been recorded, albeit under a different generic name, from India (Doyle, 1986;Vartak et al., 2010), the Caucasus (Doguzhaeva, 1996), Chile (Stinnesbeck, 1986;Bandel and Stinnesbeck, 2006), Mexico (Ifrim et al., 2004) and the northern Pacific region (Alaska, Hokkaido, Sakhalin; Fuchs et al., 2013 and references therein; present paper). Thus, the genus was able to survive major Cretaceous crises. Oceanic anoxic events (OAE) had an impact on the biodiversity in shelf ecosystems and resulted in preferred extinction of shallower-water organisms. Survival of perturbations during ?OAE1, OAE2 and OAE3 suggests that the habitats of these squid comprised distal neritic to oceanic settings. This assumption is in accordance with the palaeoenvironmental implications outlined by Fuchs et. al. (2013).
Lithological comparisons of Turonian strata in Hokkaido and Sakhalin have provided important palaeoenvironmental background for our understanding of habitats. It is of note that both single records of Longibelus from these areas contrast markedly with the abundance of the genus in post-Turonian times. The lower Turonian strata of the Yezo Group in Hokkaido are characterised by bioturbated mudstones with intercalations of sandstone beds of probable turbiditic origin (Takashima et al., 2010). A similar lithology is seen in the Bykov Formation (Member III; see Fig. 3). The similarity in lithological characteristics is indicative of middle to outer shelf conditions (see below). Both Turonian specimens, however, differ in taphonomic aspects. Longibelus matsumotoi from Hokkaido shows a better preservation, is significantly smaller and more complete, comprising numerous chambers. This kind of preservation would rule out a longer period of transport (floating).
The single, incomplete 'longibelid' from Sakhalin originates from mass accumulations of scaphitid ammonites (Fig. 6) in the Bykov Formation. The rarity of this coleoid might be indicative of a different, i.e. deeper, habitat.
Scaphitids were inhabitants of inner to middle shelf zones, occurring at depths of less than 100 m (Landman et al., 2012 and references therein). This matches our assumption that was based on the lithology of the section in the River Naiba area (Fig. 7). Arkhipkin (2014) hypothesised that scaphitids were permanently attached to algae or branch-bearing invertebrates; in other words, nearsessile benthic cephalopods. This hypothesis was later rejected by Landman et. al. (2016), who argued that scaphitids were able to move about by swimming. According to these authors, scaphitids were adapted to feed on smaller organisms in the water column on the basis of features of their aptychi and radula. However, these heteromorph ammonites lived close to the sea floor and had limited vertical and horizontal migratory capacities (Landman et al., 2012(Landman et al., , 2016Tanabe, 1979).
Well-oxygenated shelf conditions are substantiated by faunal recovery following the Cenomanian-Turonian Boundary Event (CTBE) in the Pacific Realm, inclusive of a rise in ammonite diversity, of which the so-called Scaphites Event in the upper lower to middle Turonian is an expression (Jagt- Yazykova, 2012). Stable isotope analyses for the Bykov Formation, as published by Hasegawa et al. (2003), reveal a negative shift in δ 13 C TOM (terrestrial organic matter) from − 23 to − 24.5‰, and a positive trend in Organic Carbon Content (C org , from 1.1 to 0.5 wt%). However, during the lower-middle Turonian interval, limiting conditions for benthic organisms have been suggested for the underlying strata only [i.e. Member III, as correctly pointed out by Yazykova et. al. (2004)]. Important geochemical data that reflected global changes (δ 13 C TOM ) have been published in particular for the stratigraphical Fig. 6 Two examples of Scaphites planus (Yabe, 1910) from the lower Turonian Scaphites Event in Sakhalin. Locality: River Naiba (collected by E.A. Jagt-Yazykova), field numbers 031/2 and 030/1, respectively. Specimens will be stored in the collections of CNIGR Museum VSEGEI (Sankt Peterburg, Russia). Scale bar equals 10 mm equivalent of Member III, i.e. the Yezo Group, in Hokkaido (Takashima et al., 2010;Uramoto et al., 2013). These are in agreement with data from Sakhalin in documenting a greenhouse environment with the highest sea level and temperatures reaching c. 16-17.5 °C in sublittoral basins of Sakhalin during the middle Turonian (Zakharov et al., 1999).
Here we can, in part, corroborate the assumption made by Fuchs et. al. (2013) that 'longibelid' coleoids were inhabitants of rather distal neritic to oceanic settings, but this hypothesis is in need of more rigorous testing. The incomplete nature of the present Longibelus phragmocone, preserved in fine-grained strata (laminated siltstones), may rule out mechanical damage that is typical of higher water energy in inner shelf/ nearshore environments. The fracturing might rather have resulted from predator or scavenger activity and probably does not reflect post-mortem drift of the shell.

Conclusions
We here document a new record of the coleoid genus Longibelus from the northern Pacific Realm and the first mention from Sakhalin Island (Russian Far East). The single specimen available is well constrained stratigraphically within the lower Turonian sequence on the basis of co-occurring ammonites and inoceramid bivalves. This novel record significantly contributes to the palaeobiogeographical distribution of the genus during the early Turonian and its presence at higher latitudes during that interval can be linked to greenhouse conditions. Survivor strategies at the CTBE, which expresses anoxic conditions on a global scale, suggest the original habitat of Longibelus to have been the outer shelf and open ocean. This assumption is here adopted; lithological, palaeoecological and taphonomic data have documented middle to outer shelf settings in the study area. Fig. 7 Lower Turonian facies distribution in relation to the Scaphites event and the occurrence of Longibelus sp., suggesting a 'longibelid' distal neritic to oceanic habitat; 1 clastic nearshore sediments (sandstones) of the inner shelf; 2, 3 siltstones to mudstones (based on Jagt- Yazykova, 2012;Zonova & Yazykova, 1998;this paper). Not to scale