A new cephalopod find in the Eocene beds near Grdoselo in Istria, Croatia

The article presents a cast fossil of a cephalopod found in sandy limestones (packstones) of Eocene age near Grdoselo in Central Istria. The phragmocone remains most probably belong to the nautilid species Eutrephoceras (Simplicioceras) centrale (Sowerby, 1812). We have determined the age of the fossil with calcareous nannoplankton. The nannofossil assemblage is typical of the Middle Eocene and allows the assignment to the biozone NP15 (Nannotethrina fulgens) of the Middle Eocene.


Introduction
Grdoselo is situated approximately 10 km north of Pazin (Fig. 1). The outcrop extends along a cart track leading through the forest. We visited this locality several times and made another tho rough inspection of the surface layer of the outcrop in September 2010. The time of the field work was selected with respect to the fact that a certain period of time had passed since our last visit and that there had been some substantial rainfall after which interesting macrofossils are commonly found. This time as well we found some cast fossils of molluscs and sea urchins. Finally, we detected a smooth surface of a few square centimetres and after carefully removing the overlying weathered rocks it became clear that this was not another fossil sea urchin. After its excavation we realised that this was a relatively well preserved cast fossil of a cephalopod (Fig. 2), which was not similar to the Aturia specimen found a few years earlier and described in Mikuž (2009). Well preserved nautilid fossils are rare in the Eocene of Istria. The Middle to Late Lutetian age of the rocks outcropping in Grdoselo has been determined by Mik{a et al. (2005: 101) on the basis of orthophragmid foraminifera. They determined assemblages that correspond to biozones SBZ 14/15 or P11-P12. A bit further towards the south, in the vicinity of Pi}an, BaBi} et al. (2007: 96) studied the flysch beds and established that the sediments of the Pazin Basin in that locality can be assigned to P12 to P13 planktonic foraminiferal biozones, while nannoplankton assemblages were consi stent with the age of NP16 to NP17. These biozones in fact correspond better to the Late Lutetian and Early Bartonian.
To pinpoint the age of the beds outcropping in the Grdoselo locality we studied them for cal care ous nannoplankton. We collected samples from the umbilici of the two nautilid cast fossils from this site (a specimen presented in Plate 1 and another, poorly preserved specimen which is not documented) and another sample from the umbili cus of a fossil gastropod belonging to the genus Pleurotomaria, a relatively rare find in this site.
The list of fossil species of nautilids mentioned in the sources listed above includes "Nautilus" sp., Eutrephoceras imperialis (Sowerby), E. dis culus (Deshayes), Hercoglossa aff. harrisi Miller & Thompson, Aturia ziczac (Sowerby), Aturia sp. and Aturia cf. ziczac (Sowerby), all found in proximity of Grdoselo. Material: A relatively well preserved cast fossil with the basic morphophological characteristics of a nautilid (Pl. 1, figs. 1a-1d). The fossil was found on 3 September 2010 by the first author of this paper. Locality: The Eocene sandy limestones within the megasequence in proximity of Grdoselo (Fig. 2). The beds contain an abundance of various macrofossils. The larger specimens include echinoids, mostly belonging to the Echinolampas genus, less commonly to the largest Eocene species Conoclypus conoideus.

Systematic palaeontology
Description: A relatively large cast fossil represents a phragmocone of a nautilid shell (Pl. 1, figs. 1a-1d). The remains of the living chamber are not preserved. The cast fossil displays the characteristic involute coiling and growth with younger and wider chambers overgrowing the older and narrower parts of the shell. The left hand side of the phragmocone is in a better state of preservation than the right (Pl. 1, fig. 1b), the older part of which is rather heavily corroded (Pl. 1, fig. 1c). 15 to 16 chambers can be distinguished on the surface of the phragmocone, which are separated by relatively simple suture lines. The exact number of the chambers is impossible to determine. The visible older part of the phragmocone consists of short (�10 mm) living chambers, while in the younger part they become progressively longer (�30 mm). The siphuncle is round and somewhat deepened, it lies in the middle of the concave oval septal wall (Pl. 1, fig. 1d) of the last preserved septum, which is relatively wide and low and slightly narrowed in its lower or ventral part.
Note: SowerBy (1812: [11][12] mentions that the species is named after the position of the siphuncle in the middle of the septa dividing the living chambers in the phragmocone.

Size of the nautilid cast -phragmocone:
Size of specimen = 145 x 104 x 90 mm Maximum radius of phragmocone = 93 mm Length of septa = 10 to 30 mm Radius = 50 mm Width of cast fossil = 90 mm Hight of last whorl = 74 mm Distance from outer edge to the center of siphuncle = 40 mm Diameter of siphuncle = 6 mm Diameter of umbilicus = 9 mm Remarks: Hagn et al. (1992: Taf. 50) report of a specimen of Nautilus centralis Sowerby from the Lutetian strata of the Kressenberg locality in Germany. The size and the form of the suture line are comparable to the specimen from Grdoselo. Certain similarities are also observable with the specimens described by ScHafHäutl (1863). Plate 54, fig. 2a-c illustrates a specimen of Nautilus macrocephalus Schafhäutl, the differences between the specimens include size, shape of the last turn of the coil and position of the siphuncle. The specimen from Istria is more comparable to the Nautilus ellipticus illustrated by ScHafHäutl (1863) in Plates 57 and 58. They are similar in size, shape of the shell and the suture line but not in the position of the siphuncle as this is not shown in the illustrations of ScHafHäutl (1863). edwardS (1849) describes seven different nautilids from the Eocene of England. Plate 8 illustrates five species with the shape of the last turn of the coil and the position of the siphuncle. The shape of the last turn in the coil of the specimen from Istria is most similar to that of Nautilus so werbyi Wetherell figured by edwardS (Pl. 8, fig. 3), while the position of the siphuncle is most similar to N. centralis (edwardS, 1849: Pl. 8, fig. 2).
calzada in Viader (1983: 406-407) present two Illerdian nautilids from northeastern Spain. The first is a specimen of Deltoidnautilus rollandi (Leymerie, 1845) and the other of Aturoidea par kinsoni (Edwards, 1849). These specimens are not comparable to the one found in Istria.
The shape of the phragmocone and the suture line of the Istrian specimen is similar to the specimen of Angulithes (Cimomia) imperialis (Sowerby) from the Cuisian beds in Austria, figured by ScHultz (1998: 36-37, Taf. 10, Fig. 10-11). The shape and width of the septa, size and depth of the umbilicus bear some resemblance to the species Euciphoceras regale (Sowerby) and Cimomia elliptica (Schafhäutl), figured by galácz (2004: Text-figs. 2 and 4). The position of the siphuncle and partially the shape of the coil section are rather similar to Euciphoceras regale from the London clay and Euciphoceras sp. from Fayum in Egypt, figured in dzik & gaz ª dzicki (2001: 304, fig. 6).

Stratigraphic and geographic range:
The presence of the species Nautilus centralis in the Eocene London clay is mentioned by SowerBy (1834: 248). According to ScHultz (1976: 51), the specimens of the species Eutrephoceras (Simpli cioceras) centrale (Sowerby, 1812) are characteristic of Lutetian beds. lloMPart (1993: 46) asserts that in Spain this species is present in the Lower Eocene from Ilerdian to Cuisian. The same author writes that the specimens of the species have been found in the Thanetian of Austria, Ypresian of England, Switzerland, Germany and Italy, and in the Upper Eocene of Romania, Austria and Hungary. MooSleitner (2004: 34) presents the nautilid species Angulithes (Cimomia) imperialis from the Eocene beds of Helveticum, that is, from the St. Pankraz locality in the area of Salzburg in Austria.

Biostratigraphic dating of the fossil with calcareous nannoplankton
We have sampled the sediment matrix in the umbilici of two cast fossils of nautilids and a cast fossil of a gastropod belonging to the genus Pleurotomaria, all from Grdoselo. We studied the samples for calcareous nannoplankton. The nannofossil assemblage is consistent with the biozone NP15 (Nannotethrina fulgens) of the Middle Lutetian.
The nannofossils in samples Nautilid 1 and 2 and Pleurotomaria are moderatly to well preserved. The assemblages are diverse: in total 43 species of nannoplankton were determined with an average of 28 species per sample. The assemblages are characteristic of the Middle Eocene with species like Chiasmolithus solitus, C. gran dis, Sphenolithus spiniger, S. furcatholithioides, Helicosphaera heezenii, H. seminulum, Discoaster wemmelensis, Pseudotriquetrorhabdulus inver sus. In the sample Nautilid-2 a broken, but easily recognisable specimen of Chiasmolithus gigas was found (Pl. 2, fig. 9). This stratigraphic range of this species is very short -it occurs only in the CP13b biozone of okada & Bukry, 1980, which can be correlated with the middle part of NP15 of Martini, 1971. No zonal markers of the biozone NP16 (Reticulofenestra umbilicus, R. reticulata, Discoaster bifax) were found in any of the three samples. The above allows the assignment of the age of all three samples to the NP15 Nannotethrina fulgens zone of Martini, 1971. Some of the biostratigraphically significant species of calcareous nannoplankton are shown in Plate 2.

Conclusions
The cast of a nautilid phragmocone from Istria has been found on the 3 rd of September 2010 in Middle Eocene -Lutetian sandy limestone in Grdoselo. The same fossil bearing beds contain abundant echinoid remains, among them several specimens of the species Conoclypus conoideus. The fossils of these large sea urchins generally occur in their living position. The cast fossil of the nautilid species Aturia cf. ziczac (Sowbery, 1812) (Mikuž 2009) has also been found in the same locality. The nautilid phragmocone presented here (Pl. 1, fig. 1a-1d) has been determined as Eutrep hoceras (Simplicioceras) centrale (Sowerby, 1812) on the grounds of its relatively simple suture, the position of the siphuncle and some other relevant characteristics.
Several similar forms are mentioned in relevant literature, but they are very difficult to compare, mostly due to the poor state of preservation they are usually in. Furthermore, the reference figures in literature display the nautilid fossils in various ways: their shells, shells combined with cast fossils or cast fossils exclusively, median longitudinal sections of the phragmocone, usually without living chambers etc. It is impossible to observe all the characteristics of an individual Eocene species in deficiently preserved specimens presented in a variety of different ways.
The calcareous nannoplankton assemblages found in the matrix attached to some macrofossil finds allow the assignment of the age of these beds to the NP15 Nannotethrina fulgens zone of Martini, 1971 in the Middle Lutetian.