Geologia Croatica A new occurrence of a classic “ Árpád-type ” mollusc fauna from the Upper Miocene of Kozármisleny , southern Hungary

Much of the sedimentary fi ll of the Neogene Pannonian Basin System was deposited in the Late Miocene – Early Pliocene Lake Pannon, a large, long-lived, brackish lake populated by a highly endemic “Ponto-Caspian-type” biota. In particular, cardiid bivalves refl ect the extraordinary diversity with more than 200 species in the lake (MÜLLER et al., 1999; GEARY et al., 2000). Whereas high diversity in some other groups, such as the gastropods, was achieved by a combined effect of in situ evolution and inheritance from Early – Middle Miocene lakes (HARZHAUSER & MANDIC, 2008), Lake Pannon cardiids probably all originated from ancestors living in the restricted marine environment of the Middle Miocene Sarmatian sea (e.g. VRSALJKO, 1999). Historically, the fi rst signifi cant endemic cockle assemblage from the lake was described by HÖRNES (1862) from the village of Árpád (today part of Pécs), southern Hungary. Later these cardiids played an outstanding role in the classifi cation of Miocene and Pliocene brackish cockles of the entire Paratethyan region. Although similar faunas are well-known from the southern regions of the Pannonian Basin, especially in Croatia (e.g. BRUSINA, 1884; BASCH, 1990), some of the Árpád species are very rare elsewhere. For example, the type species of the genus Lymnocardium, L. haueri, has not been known to occur in any other locality (reports on L. haueri from Serbia (STEVANOVIC, 1951) and Croatia (BASCH, 1990) refer to a morphologically clearly distinct form, probably another species). AB STRA CT


INTRODUCTION
Much of the sedimentary fi ll of the Neogene Pannonian Basin System was deposited in the Late Miocene -Early Pliocene Lake Pannon, a large, long-lived, brackish lake populated by a highly endemic "Ponto-Caspian-type" biota.In particular, cardiid bivalves refl ect the extraordinary diversity with more than 200 species in the lake (MÜLLER et al., 1999;GEARY et al., 2000).Whereas high diversity in some other groups, such as the gastropods, was achieved by a combined effect of in situ evolution and inheritance from Early -Middle Miocene lakes (HARZHAUSER & MAN-DIC, 2008), Lake Pannon cardiids probably all originated from ancestors living in the restricted marine environment of the Middle Miocene Sarmatian sea (e.g.VRSALJKO, 1999).Historically, the fi rst signifi cant endemic cockle assemblage from the lake was described by HÖRNES (1862) from the village of Árpád (today part of Pécs), southern Hungary.Later these cardiids played an outstanding role in the classifi cation of Miocene and Pliocene brackish cockles of the entire Paratethyan region.Although similar faunas are well-known from the southern regions of the Pannonian Basin, especially in Croatia (e.g.BRUSINA, 1884;BASCH, 1990), some of the Árpád species are very rare elsewhere.For example, the type species of the genus Lymnocardium, L. haueri, has not been known to occur in any other locality (reports on L. haueri from Serbia (STEVANOVIC, 1951) and Croatia (BASCH, 1990) refer to a morphologically clearly distinct form, probably another species).
The original descriptions by HÖRNES (1862) were based on donated specimens, and the exact locality where the shells had been collected remained unknown.Recently SZÓNOKY et al., (1999) measured a fossiliferous outcrop located immediately south of Árpád.Some 2.5 km NE of their locality, in the area of the village of Kozármisleny, a road cut was deepened in 2008 as part of the construction of the M6 highway system, exposing fossiliferous silt layers deposited in Lake Pannon.When visiting the outcrop, we were allowed only a restricted time for scanning the sequence and collecting samples.The mollusc fauna of the outcrop, however, proved to be very remarkable.The 10 bivalve (including 8 cardiid) and 4 gastropod species correspond to the classic Árpád fauna, containing the same morphological types and including some of the rarities (L.haueri, L. hungaricum, L. arpadense, Pteradacna pterophora).This paper gives a brief account of the outcrop and its mollusc fauna, complete with the results of palynological investigations.

THE KOZÁRMISLENY OUTCROP
The excavation site (46°05'N and 18°27'E) is located in the Pécs Basin, between the villages of Kozármisleny and Árpád (Nagyárpád, today part of Pécs) in southern Hungary (Fig. 1), at an elevation of 160 m above sea level.The Pécs Basin is fi lled with lacustrine, fl uvio-lacustrine, and aeolian deposits of Miocene to Pleistocene age.In the southern margin of the basin, the layers dip approximately 1-3° to the southsoutheast (SEBE et al., 2008).
The north to south oriented road cut exposed the deposits of Lake Pannon in an 8-10 m thick sequence below a thin Pleistocene loess cover.The section is composed primarily of yellowish-gray, medium-to coarse-grained silt, with 20-30 cm thick intercalations of fi ne-grained silt.Layers of both the eastern (A) and western (B) walls were numbered from bottom to top.
Most of the layers seemed to be barren of molluscan fossils or contained only shell fragments, but silt layers A1 to A3 and a very-fi ne, yellow sand layer at the bottom of wall B (B1) contained abundant and well-preserved shells.

MOLLUSCS
Apart from the fossiliferous layers A1 to A3 and B1, mollusc shells were also collected from debris at the foot of the walls.The following forms were identifi ed: Congeria rhomboidea rhomboidea HÖRNES, 1862 (Fig. 4d).Well-preserved specimens, 5-7 cm in length and 4-5 cm wide, occurred in layers A1 to A3.Several dozen specimens were collected, some with articulated valves, some as single valves.
Lymnocardium majeri (HÖRNES, 1862) (Fig. 4k).A large number of excellently preserved specimens were collected from layers A2 and B1, with an average size of 3 cm length and 2.5 cm width.

PALYNOLOGICAL INVESTIGATIONS
Three samples from layer A1 were collected for palynological analysis.The objective was to better assess the biodiversity in this environment, and to provide data for biostratigraphic and environmental interpretation.
Palynological samples were prepared using standard processing techniques at the Laboratory of the Hungarian Geological Institute.The samples were treated with HCl (36%) and HF (40%) to remove the mineral fraction, and a heavy liquid (ZnCl 2 , density 2.2 g/cm 3 ) was used to separate the organic matter from the undissolved inorganic components.The organic residue was not sieved, and additional oxidation was unnecessary.Organic residue was mounted in glycerine jelly and three slides were investigated.Two dinocyst taxa, three fresh water algae and 18 sporomorph taxa were identifi ed.
The Batiacasphaera-like dinocyst of layer A1 is poorly preserved.It is similar to those forms that occur elsewhere in the Galeacysta etrusca Zone, (younger than 8 Ma) in having an intercalary archeopyle and punctated ornamentation, but differs from them in its habit and glassy appearance.Gonyaulacaceae sp.indet. is well-preserved.A reworked origin for this specimen can be excluded.It resembles some wellknown dinofl agellates from the Galeacysta etrusca Zone (Gonyaulax spinifera -G.digitalis), but its tabulation is not identical with them.
Neither form is suitable for a more precise biostratigraphic evaluation.Their presence indicates a probable brackish environment.

Chlorophyta
The samples contained Botryococcus braunii KÜTZING 1849, a green alga common in both brackish and freshwater ecosystems worldwide.Mougeotia laetevirens (A.Braun) WITTROCK 1877 and a freshwater Spirogyra sp.indicate the presence or proximity of a freshwater environment.
This pollen spectrum can be readily compared to those of other Late Miocene and Early Pliocene samples from the Transdanubian region of Hungary, such as those in drill cores from Lake Balaton (NAGY-BODOR, 1988) and from the borehole Pula-3 (NAGY, 2005), respectively.In spite of all the similarities, the Kozármisleny samples contain less broadleaved species and fewer grasses than the two other spectra, and indicate a relatively humid, warm temperate climate.

DISCUSSION AND CONCLUSIONS
As indicated by grain size and sedimentary structures, the silt layers of the Kozármisleny outcrop were deposited in the shallow sublittoral zone of Lake Pannon.The ecological demand of molluscs supports this interpretation.L. majeri, L. rogenhoferi, Pteradacna pterophora, and Valenciennius are known to have been sublittoral dwellers.L. arpadense, similarly to its ancestor L. diprosopum, however, usually occurs in littoral sands (GEARY et al., 2010).At the time of deposition, the lake shore was situated a couple of kilometres to the northwest, possibly in the Mecsek Mountains.Recently SEBE et al., (2013) described the occurrence of a wave-cut platform, together with the mollusc Dreissenomya unioides, a burrowing dreissenid (also present in the Kozármisleny fauna), from the southern slope of the Mecsek Mountains, at 380 m above sea level.
The diverse cardiid assemblage seems to indicate brackish water, whereas the algae and sporomorphs argue for a brackish to freshwater environment.Spores and pollen indicate a warm temperate climate.
The palynological investigations failed to identify agediagnostic dinofl agellates.The great similarity of the mollusc fauna to the classic Árpád one however, allows inference of the Prosodacnomya vutskitsi Zone (7-5 Ma).Although the Árpád outcrop displays more sand layers, the close proximity and identical fauna suggests that the two outcrops, Árpád and Kozármisleny, expose the same sedimentary rock body.

Figure 1 :
Figure 1: Geographic position of the Kozármisleny profi le.

Figure 2 :
Figure 2: The Kozármisleny road cut (wall A) with grain-size distribution curves of the Upper Miocene layers.

Figure 3 :
Figure 3: Grain-size spectra of samples from diff erent layers in wall A.