A Dactylioceras – Meleagrinella (Clathrolima) assemblage from the Agardhbukta (eastern

a first in situ

Generally, the thickness and completeness of the Lower Jurassic succession in Svalbard increase eastwards.At the western coast of Spitsbergen, the Lower Jurassic is represented by the Brentskardhaugen Bed which is underlain by a poorly fossiliferous Teistberget Member (usually 5-26 m).
Successions of the Agardhbukta, Kapp Mühry and Wilhelmøya show much more complete sections with a total thickness up to ~60-65 m (Pchelina, 1980), and farther eastwards attaining ~200 m at Svenskøya (Olaussen et al., 2018).Sections exposed at Kapp Mühry and on Wilhelmøya are crucial for our understanding of the Lower Jurassic history of Svalbard due to the significant thickness of the Lower Jurassic here.Klubov (1965) presented the first relatively detailed description of the Lower Jurassic succession on Wilhelmøya.There, he distinguished Pliensbachian-Toarcian deposits, which in the lower part (member 28 in Klubov, 1965, represented by light-yellow weathered mudstones) yields a foraminiferal assemblage dated by Basov as Pliensbachian (see also Basov et al., 2009).Worsley (1973) erected here the Wilhelmøya Formation which was elevated to subgroup rank by Mørk et al. (1999).
The Tumlingodden Member, upper unit of this subgroup, belongs to the Lower Jurassic based on foraminifer dating (Klubov, 1965).Additional evidence for an Early Jurassic age of the upper part of the Wilhelmøya Formation came from an analysis of dinoflagellate cyst assemblages derived from the study of the succession in Marhøgda, Sassenfjorden (Bjaerke & Dypvik, 1977).A Pliensbachian to Aalenian age of this part of the succession has recently been supported by the analysis of dinocysts from DH2 and DH4 wells (western central Spitsbergen) performed by Rismyhr et al. (2018).Pchelina (1980) summarised all available data about the uppermost Triassic and Lower Jurassic of Spitsbergen, including results of geological mapping by Soviet geologists.She provided a description of key sections and erected a set of new formations and members.Among the new paleontological findings, important for establishing the age of the Lower Jurassic part of the succession, she mentioned the occurrence of Modiolus tiungensis Petrova.14 m above the base of the Tumlingodden Member at Kapp Mühry, i.e., from the level a little below Pliensbachian foraminifers of the same member.This bivalve species ranges from upper Pliensbachian to lower Toarcian in Northern Siberia (Kirina, 1976a, b).Thus, the age of its occurrence at Kapp Mühry is late Pliensbachian.Until now, no other in situ macrofossil records were known from the Lower Jurassic of Spitsbergen.

Material
During fieldwork in the Agardhbukta area in 2018, interesting Toarcian macrofossils were collected from a coastal section located south of Myklegardfjellet mountain (Fig. 1; coordinates 78.03945 ο N, 18.70837 ο E).A rich fossil assemblage was recorded from the topmost part of the Svenskøya Formation.In this section, a light-yellow sandstone (bed 1), containing numerous pebbles and fossil wood, is overlain by a ~2 m-thick siltstone (bed 2, Figs. 2 & 3) with numerous hard phosphorite concretions, especially common between 0.5 and 1.5 m above the base of this unit.These concretions occurred in laterally traced levels located at 0.5-1, 1.1, 1.2 and 1.5 m above the base of bed 2. Phosphorite concretions frequently contain ammonite moulds and belemnite phragmocones, while partially dissolved belemnite guards and bivalves are less common.Small pebbles are numerous in the lower part of the bed (0.5-1 m above the base) but disappear above.The ammonite assemblage consists of micro-and macroconchs of late Dactylioceras and shows no features of re-deposition or condensation.The ammonites are generally well-preserved moulds of complete  1. Van Mijenfjorden;2. Hyrnefjellet;3. Sørkappland;4. Dunérbukta;5. Agardhbukta;6. Kapp Mühry;7. Svenskøya;8. Wilhelmøya;9. Marhøgda;10. DH2,DH4 wells;(B) Map of Agardhbukta.adult specimens and their fragments, and usually rest horizontally to subhorizontally (Fig. 2B).Ammonite moulds are uncrushed (except for their body chambers in some cases) suggesting quick cementation of phosphorite concretions during the early diagenesis.Bivalves mainly belong to Meleagrinella (Clathrolima), as well as a single specimen of Entolium.
It should be noted that Lower Jurassic fossils from this area are well known, and have been described or mentioned in a few publications (Frebold, 1929a;Pchelina, 1980;Ershova & Repin, 1983), but in contrast to the studied collection all these occurrences came from the Brentskardhaugen Bed, and include only younger ammonite taxa, such as diverse Pseudolioceras, Porpoceras and Coeloceras species.(1968) and Guex (1971Guex ( , 1973) ) recognised micro-and macroconchs in this ammonite group.In spite of the lack of clear mature aperture modifications in dactylioceratid ammonites, dimorphs are more or less clearly recognised.Two different approaches were in use for the description of dimorphs in this group: corresponding micro-and macroconchs were either referred to different (sub)genera (e.g., Guex, 1971;Kovacz, 2014) or described under the same species name accompanied by [m] or [M] symbols (e.g., Bardin et al., 2015).Here, corresponding micro-and macroconchs are ascribed to different Derivation of name.-in the memoir of Dr. Janusz Kopik (1930Kopik ( -2021)).
Description.-Semievolute coiling, with rib furcation point covered by overlying whorl.Umbilicus wide and shallow, suboval whorl-section with little flattened flanks and venter (Table 1).The peristome is simple, and in some cases accompanied by a shallow constriction and a significant decrease of rib ratio towards the aperture.Sharp and rectiradiate ribs develop a little above umbilical seam; they curved forwards in the upper third of flacks, where they split into two ribs, with secondaries which sometimes are not connected with primaries, or usually remain undivided.Ribs cross the venter either with a forward bend or remain straight.The secondary/primary rib ratio in the outer whorl is between 1.25 and 1.7.Near the terminal aperture, ribbing becomes irregular.The suture-line is unknown due to poor preservation of inner whorls.) kopiki and could be ascribed to this species.The same is true for specimens from South Barrow test well 3 (northern Alaska) figured by Imlay (1955, pl. 10, fig. 10).These ammonites are associated with possible microconchs showing the presence of small nodes in the rib furcation points (Imlay, 1955 Material.-7 specimens (ESM 146/6,11,12,15,17,21,26) from 0.7 to 1.2 m above the base of bed 2, Agardhbukta section; one specimen from the Brentskardhaugen Bed of (ESM 146/35, collected by E.S. Ershova in 1977), Spitsbergen.
Description.-Evolute to semievolute coiling, umbilicus wide and shallow, suboval whorl-section (Table 2).Rib pattern is highly variable.Inner whorls (in some cases up to 1 cm diameter) are covered by straight primaries, which terminate with small but prominent nodes near to ventrolateral shoulder.
Later, 1 or 2 thin secondaries appear here, and nodes fade gradually.
Remarks.-The present material strongly resembles Dactylioceras sp. 2 (Guex, 1973, pl. VIII, fig. 5), which also shows the occurrence of ventrolateral nodes in its inner whorls, but studied ammonites are characterised by much more distant ribs.Attribution of the described specimens to subgenus Microdactylites remains questionable, as the presence of ventrolateral nodes in inner whorls was previously unknown for this taxon (Guex, 1971).Microconchs with such a rib pattern were supposed Importantly, mass immigration of Arctic dinoflagellates to Europe began during the Bifrons Chron, i.e., synchronous with the appearance of Meleagrinella (Clathrolima) substriata in the same area.
However, the migration pattern of dactylioceratid ammonites appears to be different.In Europe they have a continuous record, whereas in Siberia and NE Russia there is a significant hiatus in dactylioceratid lineage (Kutygin & Knyazev, 2000;Knyazev et al., 2003;Fig. 6).This suggests repetitive invasions of this ammonite group to the Boreal seas.On the other hand, dactylioceratid successions of Arctic Canada are still insufficiently known (Imlay, 1955;Frebold, 1975) and the presence of the 'Dactylioceras gap' is possible here.In the southern part of Canada, such as in Alberta, the dactylioceratid succession is complete (see Them et al., 2017).
Figs. 3A-B, 3D-F, 3H-I; 4D-F, and 4H Remarks.-The type of ribbing and shell outline of this species strongly resembles those of Zugodactylites, except for lacking tubercles in the rib furcation point throughout the ontogenesis.One doubtful specimen, though, does actually have possible tubercles (fig.4.F).Only in some specimens, ribs in the terminal body chamber may show weak thickenings in the rib furcation point.Corresponding microconchs (see below) show a clear tuberculate stage up to a diameter of c. 1 cm.Such a mix of Dactylioceras-Zugodactylites ribbing suggests the stratigraphic position of the discussed species near to a transition between these genera, i.e., in the top of the Communis zone.The described species strongly resembles Dactylioceras (D.) toxophorum (Buckman) (the type specimen figured in Buckman, 1928, pl.DCCCLXXVI), but differ by a little more involute coiling, lower position of rib furcation point, and more prominent ribs.D. (D.) athleticum (Simpson) also shows a close resemblance to the new species, but is characterised by more evolute coiling and a higher rib furcation point.Variability of D. (D.) kopiki sp.nov. is mainly expressed in a variation of primary rib frequency, ranging in inner whorls from ~40 to 76 per whorl, and a rib ratio ranging from 1.25 to 1.7.The position of the rib fucation point is also variable, but it usually covered by outer whorls.In some specimens (fig.4.D) ribbing became smoother on the terminal body chamber.A closely related specimen from the Astronomicheskaya river (NE Russia) (fig.4.G) can be referred to this species as cf.-form.A single specimen intermediate in size between D. (D.) kopiki sp.nov.and D. (Microdactylites) sp., has a rib pattern intermediate between these two taxa, with small prominent nodes in inner whorls and alternating biplicate and single ribs in the outer whorls (fig.4.F).This ammonite specimen can be tentatively referred to as D. (D).kopiki.Ammonites from Prince Patrick Island (Arctic Canada) referred to by Imlay (1955) as Dactylioceras cf.D. directum (Buckman) show the same rib pattern and variability as D. (D.

Figure 6 .
Figure6.Stratigraphic position of the studied assemblage and its correlation with NW European ammonite zonal scale.Succession of Subboreal Province afterPage (2004); succession of Spitsbergen afterShurygin et al. (2011) with corrections; succession of Northern Siberia and Northeast Asia afterShurygin et al. (2011).
The described material is stored in the collections of the Earth Science Museum, Moscow State University, Moscow, Russia (abbreviated as ESM).Ammonites were studied by M.A. Rogov, and bivalves by O.A. Lutikov.Dimorphism in dactylioceratids has been distinguished since the late 60s, when Lehmann
Schootbrugge et al., 2020)e, and possibly through the Arctic Ocean in the direction of North America.A similar migrational pathway has recently been proposed for dinoflagellates, as some taxa first appear in Siberia and later in Europe (van deSchootbrugge et al., 2020).