Middle Miocene Nautilids Aturia from the Southwestern Margin of the Central Paratethys, Croatia Marija Bošnjak, Jasenka Sremac, Tomislav Malvić & Valentina Gašo

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Middle Miocene nautilids Aturia from the southwestern margin of the Central Paratethys, Croatia Marija Bošnjak, Jasenka Sremac, Tomislav Malvić & Valentina Gašo

INTRODUCTION Nautilids of the extinct Cenozoic genus Aturia Bronn, 1838 are recorded all over the world, including the Miocene deposits of the Central Paratethys (e.g., Fritz and Hiden, 1994/95; Lukeneder and Harzhauser, 2002; Schlögl et al., 2011). This study presents new results on the Middle Miocene Aturia fossils housed at the Croatian Natural History Museum (CNHM). Specimens, determined as Aturia aturi (De Basterot, 1825), were collected in the vicinity of Čučerje near Zagreb, Croatia (Fig. 1) by Gorjanović-Kramberger (1908) and Kochansky-Devidé (1944).

MATERIAL AND METHODS CNHM collection comprises 36 specimens, inventory numbers 1740, 1741, 557, 559, 560 and 561. They are in most cases preserved as phragmocone casts, lacking living chambers. We measured their length-A, width-B, and thickness-C (Fig. 2, Table 1), and chose the best preserved specimens from the Trstenik locality (Inv.No. 1740) for reconstructions Figure 1. Location of the studied area. A) Geographical location (after and biometrical studies. Study included: (a) the correlation between the measured values and (b) F test, obtained from Google Earth). B) Assumed paleogeographic location, marked with circle the Microsoft Excel Program, as well as the (c) reconstruction of the whole shell in the CorelDRAW program. (after Kováč et al., 2018).

RESULTS A Correlation between the measured values of length, width and thickness points to the dual distribution of data, more expressed in adult specimens (Fig. 3). F-test shows similarity of variances of data taken from axis A and B, implicating mutual variability of data A and B (Fig. 4). Within the group of the completely preserved 8 specimens, R value is almost 1, therefore the reconstruction presented in Fig. 5 can be applied to any other well preserved sample in the collection. A/B axis ratio in the best preserved sample (Fig. 5) is 1,651. B C DISCUSSION AND CONCLUSIONS Fossil nautiloid shells are often found damaged, with preserved phragmocones, while the living-chambers are destroyed due to the post-mortal drift. Measured incomplete Aturia samples from Croatia point to the Golden ratio spiral growth mode (Fibonacci sequence), with A/B ratio close to the “phi” number: 1.618. Regular growth enables us: a) to apply the obtained relations to the whole shells and b) to reconstruct the whole specimen from the phragmocone. Dual correlation patterns of measured axes, with differences more expressed in adult specimens, indicate the sexual dimorphism in Aturia aturi species, rather than the existence of Figure 2. Aturia aturi specimen, inv.no. 1740.8, from the CNHM collection. A, B and C marking measured elements two Aturia species. Similar pattern is described in extant nautiloid populations, with generally smaller female shells (e.g., Dunstan et al., 2011). Further analyses should include larger of the shell. number of specimens from more localities, in order to confirm these preliminary results.

Table 1. Meaurements of the analyzed specimens. y = -0,1211x + 22,593 25 R² = 0,1168

15 y = -0,0156x + 16,503 10 R² = 0,0031

0 1 6 11 16 21 26 31 36

A (mm) B (mm) Linear (A (mm)) Linear (B (mm)) Figure 3. Correlation between measured values of length, width and thickness F-Test Two-Sample for Variances F-Test Two-Sample for Variances

References Variable Variable Variable Variable 1 2 1 2 Chirat, R. 2000: The so-called ‘cosmopolitan palaeobiogeographic distribution’ of Tertiary Nautilida of the genus Aturia Bronn 1838: Mean 17,59 15,065 Mean 22,9 16,87727 the result of post-mortem transport by oceanic palaeocurrents. Variance 7,8956 3,104486 Variance 8,0319 5,613342 Palaeogeography, Palaeoclimatology,Palaeoecology,157, 59–77. Dunstan, A.J., Ward, P.D. and Marshall, N.J. 2011: Nautilus Observations 8 8 Observations 11 11 pompilius Life History and Demographics at the Osprey Reef df 7 7 df 10 10 Seamount,Coral Sea,Australia.PLoS ONE 6(2): e16312. Fritz, I. and Hiden. H.R.,1994/95: Funde von Aturia (Nautiloidea) F 2,543288 F 1,430859 aus dem miozänen Kalksteinbruch Retznei (Südweststeiermark). P(F<=f) one- P(F<=f) one- Mitt. Abt. Geol. undPaläont. Landesm. Joanneum, 52/53, 13–18. tail 0,290786 Gorjanović-Kramberger D. 1908: Geologische Übersichtskarte des tail 0,120636 Königreiches Kroatien-Slavonien. Erläuterungen zur geologischen F Critical one- F Critical one- Karte von Agram,Zone 22, Col XIV. Nakl.Kralj. zemalj. vlade,1–75. tail 3,787044 tail 2,978237 Kochansky V. 1944: Miozäne marine Fauna des südlichen Abhanges der Medvednica, Zagreber Gebirge. Geol. vjesnik Hrv. drž. geol. zav. Figure 4. F-test Two-samples for variances Hrv. drž. geol. muz. 2/3, 171−280. Kováč, M., Halásová, E., Hudáčková, N., Holcová, K., Hyžný, M., Jamrich, M. and Ruman, A. 2018: Towards better correlation of the Figure 5. Reconstruction of the complete shell (modified after Chirat, 2000) Central Paratethys regional time scale with the standard geological time scale of the Miocene Epoch. Geol. Carpathica,69/3, 283-300. ACKNOWLEDGMENTS Lukeneder, A. and Harzhazser, M. 2002: Shell Accumulations of the Authors are grateful to the Croatian Science Foundation Project IP-2019-04-7042 Nautilidae Aturia (Aturia) aturi (BAST.) in the Lower Miocene (Sedimentary paleobasins, water corridors and biota migrations, PI: Professor Paratethys (Lower Austria). Abh. Geol. B.-A.,57, 459-466. Schlögl, J., Chirat, R., Balter, V., Joachimski, M., Hudáčková, N. and Marijan Kovačić, PhD) and to the University Support „Mathematical methods in Quillévéré, F. 2011: Aturia from the Miocene Paratethys: An geology V“ (PI: Professor Tomislav Malvić, PhD) for their financial support. exceptional window on nautilid habitat and lifestyle. Palaeogeography, Palaeoclimatology,Palaeoecology,308, 330-338.