Nitellopsis (Tectochara) merianii (Al. Braun ex Unger, 1852) Grambast & Soulie-Marsche, 1972

Sanjuan, Josep & Alqudah, Mohammad, 2018, Charophyte flora from the Miocene of Zahle (Beeka Valley, Lebanon). Biostratigraphic, palaeoenvironmental and palaeobiogeographical implications, Geodiversitas 40 (10), pp. 195-209 : 203-208

Nitellopsis (Tectochara) merianii

(Al. Braun ex Unger, 1852) Grambast & Soulié-Märsche, 1972 ( Figs 7 ; 8 )

Chara merianii Unger, 1852: 82 , pl. 25, figs 10-12.

Tectochara merianii – Grambast & Grambast-Fessard 1954: 668.

Nitellopsis (Tectochara) merianii – Grambast & Soulié-Märsche 1972: 11.

MATERIAL EXAMINED. — Dozens of well-preserved gyrogonites have been recovered from sample Zahle 1. Hundreds of well-preserved gyrogonites from locality Zahle 2 and Zahle 3. One hundred wellpreserved specimens have been measured from sample Zahle 2.

DESCRIPTION

Gyrogonites very large and variable in size, ovoid in general shape 1071-1440 µm high (mean 1265 µm) and 828- 1222 µm wide (mean 1035 µm), with isopolarity index ranging between 107 and 147 (mean 123) ( Figs 7 ; 8 ). Spiral cells concave to convex. Eight to ten (frequently nine) convolutions are visible in lateral view (154 µm high in mean value) ( Fig. 7 D-F). Apex nitellopsidoid according the terminology of Horn af Rantzien (1959), flat to slightly convex or sub-rounded, with thinning and narrowing of the spiral cells in the periapical zone ( Fig. 7 A-C). Apical nodules present in the 30% of the population. Apical nodules are variable in height from few µm to 97 µm (50 µm high in mean value). The basal pole is rounded or slightly conical with large basal pore, 80 µm across, and located within a characteristic pentagonal funnel ( Fig. 7 G-I). The basal plate shows a characteristic discoidal shape and it is very thin ( Fig. 7J, K ).

BIOSTRATIGRAPHY

This species is known all over the Oligocene, Miocene and Pliocene age sediments from Europe and Asia (Soulié- Märsche 1989).

PALAEOECOLOGY

Inferred through comparison with the ecological requirements of its single living representative Nitellopsis obtusa (check the chapter charophyte palaeoecology).

DISTRIBUTION

Fossil Nitellopsis (Tectochara) merianii represents the ancestor of the extant species Nitellopsis obtusa (Soulié-Märsche et al. 2002) . These authors linked both species in an evolutionary lineage that ranges from the Uppermost Eocene (Upper Priabonian) to Quaternary. This species followed four distributional phases (Soulié-Märsche et al. 1997; Soulié- Märsche et al. 2002; Sanjuan & Martín-Closas 2015): 1) Nitellopsis (Tectochara) merianii appears to be an exclusive Europe species during the uppermost Eocene (Upper Priabonian) and Early Oligocene (Rupelian); 2) During the uppermost Oligocene-Early Miocene, it expanded from Western Europe across the Paratethys realm reaching NE China and SE Asia. Fossil populations covered the entire Eurasian landmass during this time span comprising a range of latitudes from 18°N to 50°N; 3) During the upper Miocene-Pliocene Nitellopsis (T.) merianii-N. obtusa lineage maintained its Eurasian distribution; and 4) The last episode in the biogeographic history of this lineage is represented by the living species Nitellopsis obtusa that has been recovered from lacustrine deposits ranging in age from the Early Quaternary to the present. Nitellopsis obtusa is considered a boreal lake species, displaying since the Quaternary to the present, similar Eurasian distribution to N. (T.) merianii which during the Neogene, is recorded from the west coast of Europe to Japan. In recent years Sleith et al. (2015) found N. obtusa in North America ( USA) but at the moment is considered the product of introduction by humans and not a native taxon.

CHAROPHYTE BIOSTRATIGRAPHY

The species Nitellopsis (T.) merianii and Lychnothamnus barbatus var. antiquus are of limited biostratigraphic value. Nitellopsis (T.) merianii has a broad stratigraphic and geographic distribution in all Eurasia and it has been recovered in deposits ranging in age from the Uppermost Eocene (Upper Priabonian) to the Pliocene (Soulié-Märsche et al. 1997; Sanjuan et al. 2014; Sanjuan & Martín-Closas 2015 and references herein). Lychnothamnus barbatus var. antiquus is known from Miocene, Pliocene and Holocene sediments in Europe (Soulié-Märsche 1989; Bahtia et al. 1998; Mazzini et al. 2013). The biostratigraphic distribution of Chara aff. microcera in Europe is quite wide ranging in age from the Lower Oligocene (middle Rupelian) to Lower Miocene (Burdigalian). This species has a biostratigraphic interest since it represents the key species for the homonymous biozone of the Oligocene. This biozone was defined by Riveline et al. (1996) as the interval from the first occurrence of Chara aff. microcera to the first occurrence of Lychnothamnus ungeri . Sanjuan et al. (2014) correlated the lower limit of this biozone, and hence the first occurrence of Chara aff. microcera in the Ebro Basin ( Spain) with the MP23 European mammal reference level and calibrated it with the reversed magnetozone of chron C12 (C12r), providing an age of c. 31 Ma. The last occurrence of this species in Europe is recorded in lower Miocene deposits (Burdigalian) from the Aquitaine basin, France ( Feist & Ringeade 1977; Riveline 1986). Hence the presence of Ch. microcera together with N. (T.) merianii and L. barbatus var. antiquus suggests that the base of the lacustrine deposits of Zahle may be Early Miocene in age ( Fig. 9 ). However, it is difficult to provide a precise age for the base of this lacustrine sequence based solely on a reduced number of charophyte species.

CHAROPHYTE PALAEOECOLOGY

Gyrogonites provide valuable information of the palaeoecology of lacustrine environments. The charophyte assemblage composed of 4 well-preserved species clearly refers to freshwater environments.The presence of the species Nitellopsis (T.) merianii and Lychnothamnus barbatus var. antiquus is of special interest from the palaeoecological viewpoint since both species have living representatives i.e., Nitellopsis obtusa and Lychnothamnus barbatus respectively.Hence, palaeolimnological characteristics of the lake area from Zahle can be tentatively reconstructed by comparison with the ecological requirements of their extant counterparts.The typical depth range of Lychnothamnus barbatus in Europe is from 2 to 8 m where it forms dense meadows of up to 1m high plants ( Krause 1986). This endangered species represents a characteristic feature for strictly cold and oligotrophic freshwaters usually associated to phreatic origin from northern Europe ( Krause 1997;Soulié-Märsche &Martín-Closas 2003). Previous palaeoecological studies in Miocene lacustrine deposits of Catalonia ( Spain) demonstrate that fossil L. barbatus had similar ecological requirements to its living representative (Martín-Closas et al. 2006). These authors suggested that this species grew under mesotrophic-oligotrophic conditions by the comparison with other groups of fossil aquatic plants and diatoms. Lychnothamnus barbatus has never been reported from brackish water. However, recent botanical studies in the Lake Kuźnickie (central-western Poland) demonstrated that this species is able to grow and overwinter under eutrophic conditions (Pelechaty et al. 2017). On the other hand Nitellopsis obtusa has an optimal growth in permanent, cold, alkaline freshwater lakes at a depth range of 4-12 m ( Krause 1985; Soulié-Märsche et al. 2002). In these conditions plants can reach a length of 2 m. Nitellopsis obtusa can thrive in mesotrophic waters but collapses at salinities higher than 5‰ ( Katsuhara & Tazawa 1986). Nitellopsis obtusa is currently classified as a boreal species since it is exclusive distributed in deep and shallow large lakes of northern Europe (Scandinavia, Poland and Russia), Asia and in North America, Great Lake area, ( Corillion 1972; Soulié-Märsche et al. 2002). In these permanent lacustrine habitats N. obtusa forms large and dense meadows covering the lake ground and tends to reproduce vegetatively instead of sexually (Soulié-Märsche et al. 2002). Thus, sexual propagules, i.e., oogonia or gyrogonites, are very unusual.Rey-Boissezon & Auderset (2015) recently noticed the presence of a large number of gyrogonites of N. obtusa in a very shallow and small pond from Switzerland. These authors suggested that this unusual gyrogonite production could represent an adaptive ability of this species to change its reproductive strategy in response to high luminosity and high temperature, parameters that can be related to shallower waters.

The predominance of this species in samples Zahle 1, 2 and 3, suggests that hydrological conditions were especially favourable for Nitellopsis (T.) merianii . Hence, through comparison with the ecology of N. obtusa and Lychnothamnus barbatus as modern analogues and in agreement with facies analysis, rocks from the base of the lacustrine sequence of Zahle were formed in a large lake, with permanent, cold and oligotrophic water. Moreover, the presence of a large number of gyrogonites of the species N (T.) merianii recovered from Zahle 2 could suggest periods with an increase of light irradiance and temperature probably due to the fluctuation of the water table of the lake related to seasonality. However, the large number of gyrogonites of this species also could be explained for the latitudinal position of the Bekaa Valley (37°N) which implies a higher solar irradiance compared to north European localities where N. obtusa grows nowadays. Large number of gyrogonites of this species were also recovered from Holocene lacustrine deposits in North Africa (Kröpelin & Soulié-Märsche 1991; Soulié- Märsche 1993). This large amount of gyrogonites in lacustrine deposits from low latitude basins reinforce the idea that high light irradiances could favour the productivity of gyrogonites in the Nitellopsis (T.) merianii-obtusa lineage.

CHAROPHYTE PALAEOBIOGEOGRAPHY

Because of its geographic position between Europe and Asia, where fossil charophytes have been more intensely studied, Lebanon and the whole Middle East region represents a very interesting area for the biogeography of extant and fossil charophyte taxa. The fossil charophyte assemblage recovered from Miocene deposit of Zahle is composed of well-known European species that have been recognized for the first time in Lebanon and in the Middle East. The Zahle section represents one of the southernmost localities providing a complete fossil assemblage of the boreal species Nitellopsis merianii-obtusa and Lychnothamnus barbatus . Moreover, the occurrence of Chara aff. microcera in the Middle East region is significant since it is a characteristic species, of Oligocene/Early Miocene European basins that has been now found outside Europe for the first time. The presence of this significant species in the Middle East may lay the basis for future biostratigraphic correlations between non-marine distant Paleogene/Neogene basins of Europe and the Middle East.

CONCLUSIONS

The lacustrine deposits of Zahle has yielded four charophyte species identified as Nitellopsis (Tectochara) merianii, Lychnothamnus barbatus var. antiquus , Chara aff. microcera and Chara sp. which provide valuable palaeoecological, palaeobiogeographical and biostratigraphical data. The biostratigraphic distribution in Europe of the assemblage here described and illustrated suggests that the lower part of the lacustrine sequence of Zahle is Miocene in age. The presence of the species Chara aff. microcera in sample Zahle 1 supports that the lowermost part of this lacustrine interval may be Lower Miocene in age, which is in agreement with the maximum age of the overlying basalts located northwards of the studied area (Upper Miocene, based on isotopic data). However, this age assignation is not consistent with recent studies based on fossil micromammals which supported that these lacustrine deposits are Upper Miocene in age. The occurrence of the fossil species Nitellopsis (Tectochara) merianii and Lychnothamnus barbatus var. antiquus is significant from the palaeoecological (paleolimnological) and evolutive point of view since they represent the ancestors of the extant species Nitellopsis obtusa and Lychnothamnus barbatus respectively. In agreement with the ecological requirements of the extant species and the facies analysis, we can infer that the lake that stood in the Bekaa Valley during the Miocene was relatively deep (2-8 m deep), permanent and containing cold and oligotrophic waters that may had experienced periods of fluctuation of the water table. This study provides new data about the palaeogeographic distribution of Neogene charophyte species, representing at the same time the first study of Neogene fossil charophytes from Lebanon and the Middle East.

Figures

FIG. 7. — Charophytes from the lacustrine deposits of Zahle, sample Zahle 2: A-H, Nitellopsis (T.) merianii (Al. Braun ex Unger, 1852) Grambast & Soulié-Märsche, 1972; A, apical view,no.2017019 AUBGM;B, apical view,no.2017020 AUBGM;C, apical view,no.2017021 AUBGM;D, lateral view,no.2017022 AUBGM;E, lateral view,no.2017023 AUBGM;F, lateral view,no.2017024 AUBGM;G, basal view,no.2017025 AUBGM;H, basal view,no.2017026 AUBGM;I, basal view,no.2017027 AUBGM;J, K, basal plates of Nitellopsis (T.) merianii; J, basal plate,no.2017028 AUBGM;K, basal plate,no.2017029 AUBMG.Scale bars:A-H,400 µm;J-K,100 μm.

FIG. 8. — Biometric values of Nitellopsis (T.) merianii (Al. Braun ex Unger, 1852) Grambast & Soulié-Märsche, 1972 from sample Zahle 2 (n = 100 gyrogonites): A, gyrogonite height (µm); B, gyrogonite width (µm); C, number of convolutions visible in lateral view; D, isopolarity index (ISI = h/w × 100).

FIG. 9. — Neogene chronostratigraphy. Suggested age of the base of the lacustrine deposits of Zahle according to the charophyte assemblage. European charophyte biozonations and distribution of species are after Riveline et al. (1996).