Edoardo Martinetto

Edoardo Martinetto

Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso, 35, I-10136 Torino; e-mail: edoardo.martinetto@unito.it

Abstract
Compressed leaves and carpological assemblages were newly studied in a few layers of the Piànico-Sèllere lacustrine succession, and the early 20^th century leaf collection by Rytz was preliminarily revised.The macrofloral record of the BVC member includes the locally extinct species Acer cappadocicum Gleditsch, Pinus peuce Griseb., Prunus lusitanica L., Pyracantha coccinea M. J. Roemer, and Rhododendron ponticum L. var. sebinense (Sordelli) Sordelli. Other three locally extinct species (Picea omorika (Pancic) Purkine, Rhamnus alaternus L., Tilia caucasica Rupr.) should be better documented. From the biostratigraphic point of view the Piànico fossil flora does not display taxa typical for the Early Pleistocene (e.g. Carya, Liquidambar, Eucommia), and a single fruit remain may putatively represent the extinct species Potamogeton marginatus Dorofeev, which occurs from the Holsteinian to the Weichselian in East Europe. The new plant macrofossil analyses confirm that aquatic plants are absent from the leaf assemblages of BVC member) and very rare in the carpological ones (Najas marina L. and Potamogeton). Occurrence of lake-margin species suggest that patches of sedge and reed marsh bordered the lake at the beginning of BVC deposition, and later decreased or disappeared. On the low mountain slopes, steeply dipping into the lake, a closed, prevalently deciduous, woody vegetation was growing during BVC deposition. Evergreen shrubs to small trees might well have grown as understorey in the deciduous woodlands (Buxus, Ilex) or in more open, dryer rocky places (especially Pyracantha). The macrofossil evidence suggests that, unlike Picea abies (L.) Karsten, Pinus peuce and Abies cf. alba were not restricted to higher altitudinal belts. The occurrence of Picea abies (L.) Karsten cones in the basal layers of the MLP member most probably indicates the settlement of spruce close to the lake shore, in agreement with the cooling-related contraction of the broadleaved forest and descent of vegetation belts downslope, shown by the pollen diagram.

1. Introduction
The Piànico-Sèllere lacustrine deposits (Fig. 1) are known since long time for the rich palaeobotanical content, and their compressed leaf assemblages have been described in several publications (e.g.: Fischer in Baltzer, 1896; Sordelli, 1896; Amsler, 1900; Patrini, 1921; Maffei, 1924; Rytz, 1925, 1953; Emmert-Straubinger, 1991). Geological and palaeobotanical studies of the 20^th century led to general consensus on a “Riss-Würm” age (i.e. around 130 ka) for the plant-rich layers of this succession (e.g. Lona and Venzo, 1956), but recently the dating of two tephra layers provided definitely older, though controversial, ages: around 800 ka (Pinti et al., 2001, 2007) and around 400 ka (Brauer et al., 2007). Recently studied fossil vertebrates (Sala, B., pers. comm.) exclude an attribution to the last interglacial too. Pollen analysis allowed the identification of three major temperate periods (namely “Piànico-Sèllere Interglacial, Clusone I interstadial, Clusone II interstadial”) alternating with stadial phases (Rossi, 2003). The forest dynamics during the temperate phases are marked by the abundance of “Quercetum elements” and Abies, while Pinus, Picea and steppe elements expand during stadial episodes.

The 19^th and 20^th century palaeobotanical works were published before any accurate assessment of the stratigraphic frame of the Piànico-Sèllere lacustrine deposits (Moscariello et al., 2000), so that the related collections cannot be assigned to a precise litho- and chronostratigraphic interval. Lithologic descriptions allow to assume that most of the published palaeobotanical data have to be referred to the BVC member (Fig. 2) or, to a lesser extent, to the MLP member of the Piànico Formation. A list of taxa which have been cited in the foregoing studies is reported in Table 1. This information can be still useful at present in order to assess the local occurrence and the frequency of particular taxa. However, the need for new macrofossil collections, precisely located in the updated stratigraphic framework and correlated to definite pollen zones, is self-apparent. For this reason new plant macro/mesofossil analyses have been started since the year 2000; these already provided some interesting results, even if they are still in a preliminary state up to date. The aim of this contribution is to compare such results with the previously available palaeobotanical records and to draw some palaeoenvironmental conclusions in the frame of the palynologic and stratigraphic record.

2. Material and methods
The stratigraphic framework of the Piànico-Sèllere succession, as well as the name and location of the studied sections (Fig. 4X) and units are based on Moscariello (2000) and Rossi (2003). Field observations indicated that compressed plant remains were abundant within thinly laminated lacustrine chalk, even if the highest concentrations were found within a few turbidite layers. The traditional way employed to collect plant macrofossils from the Piànico Formation (Sordelli, 1896; Emmert-Straubinger, 1991) consisted in splitting blocks of sediments along the bedding planes. With such a method leaves and large winged fruits/seeds can be easily sampled, even today, in almost every portion and every outcrop of the BVC unit as well as in limited portion of the MLP one. However, as far as a better framework of the Piànico succession has been available (Moscariello et al., 2000), only two samplings of this type have been organized, the first one has been carried out by the “Caffi” Natural Science Museum of Bergamo in a portion (ca. 1 m thick) of the “Beehive” section (Confortini et al., 2003), and yielded about 300 much fragmentary leaf specimens (“L”-assemblage) studied by Leidi (2004); the second one has been carried out, during the preparation of the field guide for the INQUA-SEQS 2006 excursion, in a 50 cm-thick potrion of the “Sergio” section, about 2 m above the base of BVC (“S”-assemblage: Fig. 4). Field observations allowed to detect, whitin this interval, a surface densely covered by oriented Taxus baccata L. needle-leaves (Fig. 4(4)), associated to Buxus leaves, which is certainly the result of a selective depositional mechanism.

In the remaining portion of the Piànico-Sèllere succession, the stratigraphic position (Fig. 2) of relevant and easily identified plant macrofossils (cones, Buxus leaves) has been marked under the survey of C. Ravazzi, and will be exploited to draw palaeoenvironmental conclusions.

Furthermore, a preliminary revision of about 300 leaf compressions or impressions, forming the “Rytz collection”, has been carried out. These fossils, most probably sampled within the BVC member (abundant Buxus leaves, see below) in the period 1921-1934 (Rytz, 1925, 1953), are preserved on pieces of white laminated chalk from the Piànico-Sèllere basin, which were formerly housed at the Bern Botanical Institute (Switzerland). Original hand-written labels with names of plant taxa were found in several boxes, even if in some cases they obviously did not correspond to the content. This permitted to check the determination of most of the taxa formerly reported by Rytz (1953).

Leaves of the Piànico Fm. have so-far been identified by comparison to modern material, mostly on the basis of a gross-morphological approach. Only Emmert-Straubinger (1991) reported some details about the cuticular features of Hedera leaves from Piànico, which would suggest a major affinity to H. helix L. rather than to H. colchica (K.Koch) K.Koch. More extensive cuticular investigations would be needed in order to verify the occurrence of problematic taxa (e.g. Laurus), in particular the locally extinct ones.

A second, most useful, method to collect plant macro/mesofossils, that has been poorly practised in this site till now, consists in the analysis of the plant remains obtained from bulk sediment samples. Emmert-Straubinger (1991) affirmed having used such a method, but did not report detailed results. This method allows to get original palaeofloristic data based, for example, on small-sized seeds and fruits of herbaceous plants, which do not have a leaf or pollen record (see below). This type of carpological material has been identified by comparison with the extensive reference collections of both recent and Pleistocene material of the Museum fuer Naturkunde in Berlin, obviously with the help of relevant literature, e. g. Velichkevich and Mamakova (2003), Velichkevich and Zastawniak (2006). Recently, field work has been carried out in several sections of the Piànico-Sèllere basin in order to locate layers with suitable concentrations of terrestrial plant macrofossils. Two assemblages of this type (Fig. 3) have been detected in turbidite layers, and the following bulk samples have been collected:

• 
  B4N: Sergio section, turbiditic layer below the t0 one in the BVC member, size 3 dm³;
  
• 
  B4A: Main section, at the top of slump nr. 3 in the BVC member; size 0.8 dm³.
  

The sediment has been placed in a basin, completely dry and reduced to blocks not bigger than 3x3x3 cm. A volume of 5% H²O² comparable to the sediment’s one has been added and left to react for 2 hours. The fluidified sediment has been filtered with a mesh size of 0.3 mm, and the residue has been left to dry for several days (heating avoided). All the cycle has been repeated for three times. At the end of the last filtering operation the residue has been dropped again in 5% H²O² and, after a few minutes, the floating fraction (“A”), which is richer in fruits and seeds, has been filtered separately from the sinking one (fraction “B”), always with a final mesh size of 0.3 mm. The abundant plant remains deposited at the bottom of the basin have been collected with a battery of sieves whose final mesh size was 0.8 mm (fraction “C”). Most mesofossils sorted from the residue were still covered by a white carbonatic “dust” which hampered identification and photography; this has been removed by immersion in acetic acid for several days.

In sample B4A the three fractions ready for analysis (A-C) were examined separately in order to understand the differences in their content. First of all the volume of fraction C was nearly 50 times the one of fraction B, and fraction A was nearly 1/3 of B. However, the most concentrated and diverse assemblage of small-sized fruits and seeds was present in A. Fraction B contained many coniferous needles and a few seeds, and carpological remains of three species have been exclusively found in B: Carex gr. caespitosa, Thalictrum cf. flavum. and Brassicaceae indet. The complete analysis of C has been carried out only for the material larger than 3 mm. Sorting the material smaller than 3 mm is extremely time-consuming and poorly productive: only an immature Tilia fruit and a Buxus endocarp have been found in 1/10 of it, apart from the common needles.

The experience gained from extensive sampling in some Pliocene localities (Martinetto 1994; Basilici et al., 1997) would suggest that the studied sediment samples, which are comparably small, only provided the commonest components of the Piànico macro/mesofossil assemblages. Future work should be addressed to the collection of larger samples, even from the same layers (up to several hundred litres) in order to gather remains of rarer species.

3. Results
In the “S” compression assemblage 17 taxa have been identified on the basis of leaf remains (Tab. 2), and relatively few fruits and seeds have been detected (a few fruits of Acer spp., 2 seeds and 1 cone scale of Abies cf. alba Miller, 1 fruit of Buxus sempervirens L. and Carpinus betulus L.). Within the “L” leaf assemblage not more that 40 out of 300 specimens can be identified on a macromorphological basis, and they are assigned to the 9 taxa reported in Tab. 3.

The palaeobotanical analysis of samples B4N and B4A permitted to identify respectively 26 and 23 taxa, mainly represented by fruit/seed remains. The assemblage of sample B4N (Tab. 4) testify to the occurrence of 12 species of herbaceous angiosperms and 13 of woody plants. Some non-arboreal forms represent emergent freshwater macrophytes or wetland plants (Alisma, Carex, Cladium, Phragmites-type), and just a few species belong to submerged forms (Najas, Potamogeton). Ajuga cf. reptans L., Micromeria thymifolia (Scop.) Fritsch, Hypericum cf. androsaeum L., H. perforatum L. and Origanum vulagare L. can be interpreted as herbs growing in better-drained conditions, while the remaining taxa have broad ecological requirements. The commonest arboreal forms are Abies cf. alba Miller, Acer campestre L., Acer cf. opalus Miller, Acer ex sect. Platanoidea, Carpinus betulus L., Pinus, and Tilia spp. The commonest shrub is Pyracantha coccinea M. J. Roemer.

In sample B4A 11 species represent herbaceous angiosperms and 10 are woody plants. The main differences in comparison to B4N are the scarcity of Acer and Pyracantha remains, compensed by the abundant occurrence of Buxus ones. Herbs requiring well-drained conditions include two species not represented in B4N: Fragaria vesca L. and Moheringia cf. trinervia (L.) Clairv.. Finally, two interesting woody forms occur with a few carpological remains in both samples: Sambucus nigra L. (Fig. 3(9-13)) and Vitis vinifera L.ssp. sylvestris Gmelin (Fig. 3(4)).

Among the cones which were sampled in definite stratigraphic position, and later identified, three speciemens beloged to Picea abies (L.) Karsten s. l. (50, 60 and 130 cm above the MLP base, Oblique and Wall sections)., and five specimens to Pinus peuce Griseb. (BVC, about 6 m from the base in the Sergio section, and MLP, respectively 30, 42, 146 and 167 cm above the base in the Wall section).

3.1 Notes on some locally extinct species

The earliest works on the Piànico fossil flora, in particular by Sordelli (1878, 1896), even reported the occurrence of exclusively fossil species: Neckera ossulana Sordelli, Acer sismondae Gaudin, Castanea latifolia Sordelli, Rhododendron sebinense Sordelli. Such species were not confirmed by Emmert-Straubinger (1991), who only listed modern European entities. However, also this author confirmed, and even increased, the record of locally extinct plants in the Piànico fossil flora: Acer cappadocicum Gleditsch, Picea omorika (Pancic) Purkine, Pinus peuce Griseb., Pyracantha coccinea M. J. Roemer, Rhamnus alaternus L., Rhododendron ponticum L., Tilia caucasica Rupr.

This species is well documented by cones with unmistakable characters, and even the large ovoid seeds permit a reliable distinction from other European pines. A well-preserved cone specimen was found directly inside the B4N sample (Fig. 3 (5)); in B4A a seed occurred, and other 4 cones have been collected in the field (see above).

The species is represented in both the “S” and “L” leaf assemblages by 3 to 5-palmate leaves with entire margin and attenuate lobe apex. The distinction from A. lobelii Ten., endemic to southern Italy, has not been adequately discussed in the literature on Piànico-Sèllere fossil leaves. Both species belong to section Platanoidea, and fruits with a morphology typical for this section occur frequently in sample B4N, but do not provide definite support to the specific identification.

Leaves of this species had already been reported by Amsler (1900) and Rytz (1953), and carpological remains found in B4N and B4A samples, definitely confirm the identification. In fact Pyracantha is distinguished by Cotoneaster for the “berry” with 5 stones (instead of 3), with woody and persistent style, attached to the apical part of the ventral side of the stone (in Cotoneaster attached to the median part). The B4N sample also yielded a few fruits (see Fig. 3(3)) resembling Pyracantha clactonensis (Reid and Chandler) Field, which has been recently confirmed as an own extinct species by Bidgland et al. (2001) on the basis of biometric analyses of Middle Pleistocene fruit-stones from Barling, England.

Potamogeton marginatus Dorofeev vel P. lucens L. (Fig. 3(8))

The low but distinct carina in the middle of the dorsal valve and the permanent style are differential characters from the similar Potamogeton x zizii W.D.J.Koch ex Roth and P. perfoliatus L.. Unfortunaterly, a single specimen is unsuitable for a definite identification, so that more abundant material should be detected in the future.

Rhododendron ponticum L. var. sebinense (Sordelli) Sordelli (Fig. 4(9-11); Fig. 3(6))

Denk (2006) discussed the opportunity to use the name Rhododendron ponticum L. var. sebinense (Sordelli) Sordelli for this fossil taxon, and also stated that “is more similar to the modern eastern subsp. ponticum (Turkish & Caucasian populations) than to the western subsp. baeticum (Boissier and Reuter) Handel-Mazzetti” (Iberian populations).

A single fragmentary fruit (capsule) of Rhododendron has been found in sample B4A, its morphology with thin receptacle and short calyx lobes permits to exclude assignment to either R. ferrugineum L. (capanulate recptacle) or R. hirsutum L. (long, ciliate calyx lobes), whereas R. ponticum L. produces comparable fruits. Further characters, which agree with the R. ponticum-type (general shape, oblique axis), are shown by a complete capsule (external impression) still preserved in the Rytz collection, which is very similar to the specimen figured by Sordelli (1896, pl. 43, fig. 9). Therefore, we may assume with a good degree of confidence that these fruits were produced by the same plant taxon as the leaves (R. ponticum var. sebinense).

4. Comparison with similar macrofloral assemblages

As for the “exotic” elements, there are no sites in Europe which share all the taxa occurring in the Piànico-Sèllere lacustrine deposits. A combination of Celtis australis L., Buxus sempervirens L. and Pyracantha coccinea M.J. Roemer has been described for the Bilzingsleben II site^1footnote (Germany), and, according to Mai (1983), in central Europe would suggest assignment to a warm interglacial phase in the Elsterian-Saalain interval. This argument cannot be applied to the fossil assemblages found to the south of the Alps, where the pollen floras show several peculiarities in the Middle Pleistocene.

In Italy there are just a few leaf assemblages which could serve as reference points:

Oriolo, in the Emilia-Romagna region (Martinetto and Sami, 2001) - The site is dated around 800.000 BP by combination of magnetostratigraphy and Mammal biochronology. The palaeoflora has a larger number of “exotic” elements (Acer aff. Palmatum L., Carya, Gleditschia, Parrotia, Pterocarya, Tsuga, Zelkova) in comparison to the Piànico-Sèllere one, but shares with it Acer cf. cappadocicum Gleditsch, Pinus cf. peuce Grieseb. and Pyracantha coccinea M.J. Roemer.

Riano Romano near Rome (Follieri, 1958; Mastrorilli, 1965) According to Bonadonna and Bigazzi (1969) the site is radiometrically dated around 300.000 BP. The macroflora, rather similar to the Piànico-Sèllere one, mainly differs for the occurrence of the “exotic” Pterocarya and Zelkova. The single “exotic” taxon found in both assemblages is Acer cf. cappadocicum.

Re in the Vigezzo valley (northernmost Piedmont). The site is not reliably dated, but yielded abundant leaf remains of arboreal plants still living in northern Italy (Gianotti, 1949; Sordelli, 1896). The occurrence of Buxus sempervirens L., “Castanea latifolia Sordelli” and Rhododendron ponticum L. is remarkable for suggesting an affinity to Piànico-Sèllere.

In conclusion, the presently available macrofloral documentation only allows to state that the Piànico-Sèllere comprehensive palaeoflora differs from the few analogous assemblages (Martinetto, 1999) known in the Early Pleistocene (incl. possible transition to the Middle Pleistocene at Oriolo: Martinetto and Sami, 2001) for the absence of really extinct taxa (possible exception: Potamogeton marginatus Dorofeev) and the scarce number of locally extinct ones (e.g. lack of Carya, Liquidambar, Eucommia), which is also characteristic for the Middle Pleistocene flora of Riano Romano.

The absence of extinct aquatic and wetland plants in a Middle Pleistocene carpofloral assemblage could be regarded as surprising in comparison to the central European record (Velichkevich and Mamakova, 2003), however an identical situation has already been reported, at comparable latitude, for an interglacial deposit (Holsteinian?) of southern France (Field et al., 2000). Furthermore, the carpological analyses carried out till now in the Piànico Formation are not sufficient for definitive considerations, so that further sampling would be needed to confirm the real absence of extinct plant taxa.

5. Palaeoenvironmental conclusions

The new analyses confirm the conclusion by Emmert-Straubinger (1991) that aquatic plants are absent from the leaf assemblages of the Piànico laminated chalks (now placed in the BVC member) and very rare in the carpological ones (Najas marina L. and Potamogeton in B4N). Lake-margin species are represented in B4N by abundant Cladium mariscus L. and rare Alisma sp. fruits, as well as culms of cf. Phragmites. Such records suggest that patches of sedge and reed marsh occurred around the lake at the beginning of BVC deposition, and later (layer T19: Fig. 2) decreased or disappeared. In fact, the B4A sample did not provide aquatic plants, and the wetland plants were only represented by a few fruits of Lycopus europaeus L. and Thalictrum minus L..

The macrofossil assemblages studied in the BVC member do not provide contrasting evidence for the altitudinal development of vegetation belts proposed on a palynological basis by Moscariello et al. (2000) and Rossi (2003). Yet, on the basis of actuopalaeontological observations (Spicer & Wolfe, 1987), the occurrence of a large and well-preserved Pinus peuce Grieseb. cone (sample B4N) and diversified Abies cf. alba Miller remains (“S” assemblage), would suggest that these conifers were not restricted to higher altitudinal belts, but occurred, at least with a few fertile specimens, not far from the lake-shore. On the other hand, Picea abies (L.) Karsten, which lacks in the whole BVC macrofossil record, would be likely restricted to the upper altitudinal belts.

The single stratigraphically ordered macrofossil assemblage from the MLP member (“L”) does not differ significantly from the BVC ones, since it still contains thermophilous (Pyracantha coccinea) and locally extinct elements (Acer cappadocicum, Rhododendron ponticum L.var. sebinense (Sordelli) Sordelli). The main difference is represented by the absence of Buxus, in fact isolated leaves have been observed till to the upper part of the BVC, but never in MLP, including the L-assemblage (in agreement with pollen records). Thus, it may be suggested that the vegetation of the Clusone I interstadial was much alike the one of the Piànico-Sèllere interglacial phase, and only more detailed investigations could confirm if the absence of Buxus and the rare occurrence of Picea macrofossils (Tab. 4) would reflect cooler palaeoclimatic conditions.

In the basal layers of the MLP member, field observations permitted to detect several cones of Picea abies (L.) Karsten (Fig. 3(14)), which have not been found in the BVC member, and probably indicate the settlement of spruce close to the lake shore, in agreement with the suggested contraction of the broadleaved forest and descent of vegetation belts downslope (Moscariello et al., 2000; Rossi, 2003), as a consequence of the cooling shown by the pollen diagram at the base of MLP (Presolana I stadial).

The data so-far obtained clearly indicate that the intensification of plant macrofossil investigations would be most useful to complete the picture of plant community change around the Piànico-Sèllere lake at the transition from the Piànico-Sèllere interglacial phase (Rossi, 2003) to the stadial and interstadial ones, furthermore the analysis on new palaeocarpological samples would most probably permit the detection of diagnostic biostratigraphic indicators (Velichkevich and Mamakova, 2003; Velichkevich and Zastawniak, 2006), which could contribute to the resolution of the controversy about the chronostratigraphic position of the Piànico-Sèllere interglacial.

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Table 1 – List of plant macrofossil taxa so-far reported for the Pianico-Sèllere lacustrine deposits, updated from Moscariello et al., 2000.

Figure 1. Synthetic stratigraphic section of the Piànico Fm., showing the position of the sections which yielded the plant macrofossils studied in this paper. B: Beehive section; M: Main section; O: Oblique section; S : Sergio section ; W: Wall section. Modified after Moscariello et al. (2000).

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