Pleistocene molluscs from research boreholes in the Heidelberg Basin

Cores cut in the research boreholes at Viernheim and Parkinsel P34 and P35 in Ludwigshafen were analysed to investigate their fossil content, and particularly the remains of molluscs. The selected material was suitable for reconstructing the palaeoclimatic conditions and simplifi es the chronostratigraphic classifi cation of individual beds. Two mollusc species and one rodent species from the Lower Pleistocene (Lower Biharium) were identifi ed in the northern Upper Rhine Graben for the fi rst time (in the Viernheim borehole). The fossils from the Lower Pleistocene sections of the Viernheim borehole are clearly related to the Uhlenberg fauna from Bavarian Swabia dated as Upper Villanium/Tegelen. [Pleistozäne Mollusken aus Forschungsbohrungen im Heidelberger Becken] Kurzfassung: Bohrkerne der Forschungsbohrungen Viernheim und Parkinsel P34 und P35 aus Ludwigshafen wurden auf ihren fossilen Inhalt, besonders auf Molluskenreste, untersucht. Das ausgelesene Material ist geeignet die paläoklimatischen Verhältnisse zu rekonstruieren und erleichtert die chronostratigraphische Einstufung einzelner Schichten. Zwei Molluskenarten und eine Nagetierart wurden erstmalig aus dem Altpleistozän (Altbiharium) der Bohrung Viernheim für den nördlichen Oberrheingraben nachgewiesen. Die aus den altpleistozänen Abschnitten der Bohrung Viernheim vorliegenden Fossilien weisen deutliche Beziehungen zu der in das Obere Villanium/Tegelen datierten Uhlenberg-Fauna aus Bayerisch-Schwaben auf.


Introduction
The mollusc fauna in the Upper Rhine Graben is very important for palaeoclimatology because most of the mollusc species are still present today in the fl ood plains of large valleys, and their habitats and lifestyles are known.They provide information on hydrological and climatic change, and landscape history, and can therefore provide "valuable pieces in the dating puzzle" (ENGESSER & MÜNZING 1991).Most endemic mollusc species already colonised our landscapes in the Upper Pliocene.In addition, the northern Upper Rhine Graben in the Pleistocene lay in an optimum development zone according to the description by LOŽEK (1969) of the climatically favourable zones of the Czech Republic and Southwest Germany.During the interglacials, the climate here meant that the mollusc fauna was much more habitat sensitive than that of northern Central Europe.During the periods of glaciation, the glacial ice did not extend into the Upper Rhine Graben area, which meant that the area had favourable conditions for the survival of animals, as well as molluscs.Loess steppes exist in the Upper Rhine area during the high glacial periods.

Research boreholes in the
Heidelberg basin

Fossil material and depositional conditions
The thanatocoenosis of molluscs from a core only refl ects a small piece of the overall landscape complex.It is therefore also important to look at the sediments in more detail.Fossils are often found in the argillaceous layers of the "Zwischenhorizonte" (intermediate horizons) as well as in the fi ne and medium-grained sands of the "Kieslager" (gravel beds).The grey sand horizons of the "Rhenish Facies" (HOSELMANN 2008) in the Lower Quaternary may contain open steppe fauna of the glacials and interglacials.The coarser the sediment, the poorer the preservation of the mollusc shells.The coarser gravels also contain few species because they are formed during glacial conditions.An exception are the loess horizons which are, however, usually only found at the boundary to slope deposits.Loess horizons contain mollusc fauna which although species poor, contain characteristic high glacial species.These horizons are not found in the cores from the fl uviatile Rhine Graben.However, molluscs do occur in some sandy beds, which indicate high glacial conditions.The work of BARTZ (1959) is recommended for a short, though usually still valid subdivision of the Pleistocene in the northern Upper Rhine Graben, and for the description of the most important associated fossil localitiesalthough the middle sandy sequence described in this paper is today classifi ed as Middle Pleistocene (ENGESSER & MÜNZING 1991).

Condition of preservation of fossil molluscs
Complete gastropods and molluscs are only rarely preserved.This is possible in some of the clay lenses of the intermediate horizons as well as in a few hard "Terra Rossa beds" from Cromerian horizons.This also applies to small specimens in sand, fi ner gravel beds and fl ood sediments.In other cases, identifi cation of species is based on the characteristic properties of the aperture armatures and the surface structures.The shells are sometimes corroded, polished or seriously fragmented by transport and the infl uence of water.We now know that the thickness of the shells does not provide any information on the climatic conditions: thickshelled gastropods were also present during interglacials and are therefore not necessarily an indicator of a colder climate (GEISSERT 1967a).

The Oberer Zwischenhorizont (OZH)
The Oberer Zwischenhorizont (Upper Intermediate Horizon) is undoubtedly the most interesting level in all of the investigated boreholes because of the numerous mollusc remains and also because it contains sensitive interglacial species whose presence can be correlated with palynological analysis.Exceptions are the ar-gillaceous peat beds from the low peat bogs of the Upper Pleistocene which mostly originated in periglacial periods.They are present in thin beds, e.g. in boreholes from around Darmstadt and Riedstadt (SCHWEISS 1988).At the west edge of the Rhine Graben, the minor thickness of the Obere Kieslager (Upper Gravel Horizon) of the OZH means that the OZH is already encountered at a depth of around 30 m.In addition, this OZH originated in the Middle Pleistocene as deduced from pollen analysis (KNIPPING 2004) and from the molluscs.The further to the east one looks at the Quaternary sequences, the thicker and more detailed they become.

The Pliocene clays
Because of the absence of molluscs, the Pliocene samples provided no results in the Ludwigshafen boreholes or in the Viernheim borehole.Only a few rare shell remains were found which could not be identifi ed because of the frequent strong corrosion of the fragments.Plant residues were however frequently found.

The Ludwigshafen boreholes (Rheinland-Pfalz)
The fi lling of the Rhine Graben with Quaternary sediments increases considerably to the east towards the edge of the Odenwald, so that the    Zwischenhorizont in P34 analysed by KNIPPING (2004) confi rmed its attribution to the Cromerian Complex (interglacial III or IV).This also matches the results reported by RÄHLE (2005) and the author for the boreholes in Mannheim and Ludwigshafen described below.

Borehole P35
The Quaternary is divided up into sequences of terraces built up by gravel horizons and intermediate horizons, whereby the uppermost "Oberer Zwischenhorizont" of borehole P35 begins at 19 m and does not end until 34 m. 5 samples were taken from this zone.It is followed by an 8-metre-thick "Unteres Kieslager".This is underlain by sandy sequences containing organic material (peat) at six positions.Mollusc samples were extracted from 50-50.3 m and at 88.7 m.
A "Unterer Zwischenhorizont" lies between 98 and 109 m.The "Unterer sandig-schluffi gen Ab-folgen" (Lower sandy-silty sequences) continue on to the Pliocene boundary.A sample from 178.9 m was selected from this zone.Another sample was collected in the Pliocene clays at 236.1 m in P35.Two additional samples from the Pliocene at 276.2 and 287.5-287.9m were investigated from borehole P35a which was drilled directly adjacent to P35.
The most concentrated mollusc remains in borehole P35 are also found in the uppermost layer in the OZH at 21.9 m, and in the "Oberer sandige Folge" (Upper Sandy Suite) at 50 m.Both horizons contain gastropod fauna primarily belonging to interglacial species.The samples differ in the spectrum of species they contain.The sample at 21.9 m primarily contains terrestrial species pointing to the presence of a fl ood plain forest (Perforatella bidentata).
The fauna from the sample at 50 m is aquatic consisting of stagnant plant-rich waters (Gyraulus crista, Lymnaea stagnalis, Bathyom-    MEULEN, 1975) (Fig. 8).The transition to the Pliocene contains plant remains but no molluscs, as in all of the samples looked at.The drill site was selected to penetrate the most undisturbed possible sequence of Pleistocene sediments (HOSELMANN 2008).

Upper Middle Pleistocene
The  1999).The species can be easily identifi ed on the basis of the two teeth (basal and palatal) in the lower aperture area.This is a pure fl ood plain snail (alder marsh) which requires a wet habitat.It is found in both interglacial as well as glacial contexts.It still lives in eastern Central Europe (Thuringia, Bavaria, Saxony, Austria, Czech Republic, Slovakia, Poland and Hungary) and at scattered localities in south-eastern Scandinavia.The species was found far to the west in the Pleistocene.This is confi rmed by specimens collected near Paris by GEISSERT (pers.comm.) The related species Perforatella dibothrion is a type fossil for the interglacial habitats of the Rhine valley in the Lower Pleistocene and Lower Middle Pleistocene.GEISSERT (1970) found a complete example in the Mosbacher Sande 2 and reports several discoveries in gravel pits in the Upper Rhine (GEISSERT 1969).Two fragments were found in the sample from 167.35 -167.45 m in the Viernheim borehole, which may belong to this species.RÄHLE (2005) also mentions Perforatella remains which he assigns to this species, from the Oberer Zwischenhorizont in borehole P18 Mannheim-Lindenhof.The main differences to P. bidentata are a much higher aperture and two stronger teeth, although the space between the two teeth is narrower than in P. bidentata.The shell has clear ribs, and the fi rst whorls have a surface structure with small JOACHIM WEDEL scales.This species is only found alive today in eastern Slovakia and in the Carpathians where it lives in the sub-mountainous zone in moderately wet deciduous forests (KERNEY et al. 1979).Its distribution in the Late Pleistocene in Europe was continental-Atlantic, i.e. much broader than today.

Gastrocopta (Vertigopsis) moravica oligodonta (KROLOPP 1979) (Fig. 2)
This is the fi rst confi rmation of this species in the Upper Rhine Graben, and comes from the Viernheim research borehole.The shell is less conical than the nominate form G. moravica (PETRBOK, 1959).The intraparietal tooth of the aperture is reduced to a thickening at one point.KROLOPP (1979) found it together with Gastrocopta serotina (LOŽEK, 1964)  Gastrocopta (Vertigopsis) sp.(Fig. 3) The small conical shell is 1.9 mm high and 1.0 mm wide.The fi ve arched whorls increase uniformly in size up to the penultimate whorl -the last whorl becomes smaller towards the aperture.The apertural border with the lip is well formed and slightly folded over towards the spindle.The shell is shiny, slightly stripy and has a weakly developed palatine torus.It has three strong teeth, one columellar, one palatal and one parietal, although the adjacent palatine wall of the specimen is damaged and partially missing.
The teeth are almost the same distance apart and their tips almost point towards one another.A characteristic feature is the complete absence of a basal tooth.This form has not previously been identifi ed in the Pleistocene.

Conclusions
Fossil
Terrestrial snails typical of warmer conditions, as still found today under leaf mould in shady fl ood plain forests, are the waxy glass snails, e.g.Aegopinella nitens(MICHAUD, 1831).The two completely preserved valves of the pea clam Pisidium amnicum (O.F.MÜLLER, 1774) from 27.8 -28.0 m are very interesting.This species occurs again at 178 m although it is primarily associated in the sample with species which tended to live in glacial, open landscapes.However, this mollusc is generally more frequent in interglacial deposits.The molluscs in both boreholes indicate interglacial conditions.There is still some doubt, however, whether we are dealing here with the Cromerian or the Holsteinian interglacial because important type fossils are absent.The situation in the other boreholes described below is different because the molluscs in these boreholes can be confi dently assigned to the Cromerian Complex.2.4 Viernheim research borehole (Hessen)The conclusions drawn from the Viernheim borehole are that the three intermediate horizons encountered in the well refl ect differentiated climatic conditions.Unlike the Ludwigs- Forest species are absent at 194 m.Open landscape species such as Pupilla muscorum are dominant.These molluscs may indicate the earliest glacial period of the Pre-Tegelen in the Lower Quaternary.There is now a signifi cant general decline downwards in the number of species and individuals.The last mollusc remains are found at 223 m comprising only three freshwater species and one terrestrial species.In contrast, the samples now contain large quantities of plant remains.