Southern beech (Nothofagaceae) fossil leaves from the Río Turbio Formation (Eocene–? Oligocene), Santa Cruz Province, Argentina

Nothofagus sp. cf. N. obliqua, Nothofagus sp. aff. Nothofagus alessandri and Nothofagus magelhaenica are reported for the first time for the Paleogene Río Turbio Formation along with another five fossilspecies: Nothofagus simplicidens, Nothofagus variabilis, Nothofagus lanceolata, Nothofagus subferruginea and Nothofagus dicksonii. Around one hundred specimens of fossil leaves were recorded and analysed throughout the unit, but they are fairly more abundant in the upper member of the Río Turbio Formation where they reach 68% of the overall assemblage. The observed increasing trend in Nothofagus abundance throughout the unit coincides with the beginning of the flora turnover that characterized Patagonian ecosystems from the Late Eocene onwards and it is in agreement with the marked global cooling trend of the terminal Paleogene.

In this paper 107 specimens referable to 8 taxonomic units of Nothofagus are described. Some of them are reported for the first time for the Río Turbio Formation. Distribution of fossil Nothofagacean leaves throughout the unit is also analysed with the aim of quantifying the dispersion of the southern beach forests in southern Patagonia during the Eocene/Oligocene.

MATERIALS AND METHODS
The leaf imprints described in this paper were collected by the author in different outcrops where both, lower and upper members are exposed (Fig. 2). Because leaves imprints lack organic preservation, comparisons and identification of the fossil impressions were made by use of autopomorphies at species level and then at the generic level by default (Hill, 2001). The specimens with well-preserved venation were studied using modern leaf architectural analysis. Terminology and systematic descriptions follow Ellis et al. (2009) andWolfe (1975). For detailed illustration and detailed analysis, specimens were photographed under unilateral lowangle light with a Nikon DS.Fi1-U2 digital camera attached to a Nikon SMZ800 stereomicroscope. Draws of the venation patterns and scaling of images were accomplished using Corel Draw X7. Suprageneric nomenclature follows APG IV (2016). Plant fossils are held in the Paleobotanical Collection of the Museo Regional Padre Manuel Jesús Molina (MPM PB) located in Río Gallegos, Santa Cruz province, Argentina.

Systematic Palaeontology
Order FAGALES Engler, 1892Family NOTHOFAGACEAE Kuprian, 1962 Genus Nothofagus Blume, 1851 Type species. Nothofagus antarctica (G. Forster) Oersted, 1871 The genus Nothofagus is divided into four monophyletic subgenera Brassospora Nothofagus, Lophozonia and Fuscospora, based on pollen, leaf and reproductive morphology (Dettman et al., 1990;Hill & Read, 1991). All recently inferred phylogenies (Hill & Jordan, 1993;Manos, 1997; (Romero 1980, Gandolfo & Romero 1992, Premoli 1996. Fossil leaves of Nothofagus vary from very small (less than 1 cm in length) to medium-sized forms (about 9 cm) with rounded to acute marginal teeth, and a few to many secondary veins (Tanai, 1986). All the leaves here described are simple, with variable shape and symmetry. The margin can be serrate or crenate with simple or compound teeth. The venation is pinnate and simple craspedodromous with secondaries veins variable in number (7 to at least 10 pairs) and straight or curved towards the margin. The secondary veins emerge at acute angles from the midvein (40º-60º). Tertiary veins percurrent, straight or sinuous and obtuse to the midvein. Four and fifth order veins are reticular and some have fimbrial and none, one or two agrophic veins.
Following the strict consensus tree obtained by Vento & Agraín (2018) and based on the comparison of the fossil studied with the extant species, the fossils described where grouped according to its possible subgenus categories.   marginal. Margin toothed, serrate and crenate. Teeth simple, convex/convex, regular spaced and angular sinuses. Secondary veins ending in the simple teeth, entering along the basal side of the tooth. Venation pinnate and simple craspedodromous, the primary vein moderately thick and nearly straight. At least 7 pairs of secondary veins emerging at acute divergence angle of less than or equal to 41º, straight in course curving up near the margin. Tertiary order venation percurrent, straight to sinuous, with obtuse angle in relation to the midvein. Quaternary veins poorly preserved, seems to be reticulate.
Remarks. The leaf shape along with the venation pattern and the convex/convex simple tooth shape are characters that match the description of Nothofagus simplicidens. This species was formerly described by Dusén (1899) as petiolate leaves of variable size and shape, mainly oblong to oval (sometimes lanceolate) with eight to ten straight secondary veins each supplying the tooth apex. Later, Romero & Dibbern (1985) in their revision of Nothofagus species described by Dusén add the characterization of the leaf margin formed by convex/convex simple tooth. Tanai (1986) revised the species of Nothofagus or Nothofagus-like fossil leaves from South America and West Antarctica. This author considered N. simplicidens distinguishable by its single-serrate margin with obtuse teeth and regularly percurrent tertiary veins with the secondary vein entering straight the main teeth along the basal side of tooth. Vento & Prámparo (2018) mentioned the presence of Nothofagus simplicidens for the Río Turbio Formation. Their specimens are comparable to the ones here described in the leaf size and shape, the simple serrate margin, number of secondary veins and venation pattern. According to Tanai (1986), five living South American Nothofagus species (Nothofagus alessandri, N. alpina, N. glauca, N. obliqua and N. procera Remarks. These specimens are characterized by the double serrate margin, well developed fimbrial vein along the margin and the secondary veins entering centrally the principal teeth. These characters match the description of Nothofagus variabilis Dusén (1899, p. 96, pl. IX, figs. 8-13). These leaves are variable in their shape and size reason why Dusén defined three different forms: oblonga, subrotundata and microphylla but according to Tanai (1986) these three forms are difficult to distinguish taxonomically as independent varieties due to gradation of leaf forms that its fossil record possesses. Romero & Dibbern (1985) also abolish these variations and they are no longer considered. Among the extant species, Tanai (1986) related Nothofagus variabilis with N. fusca (Hook. f.) Oerst. and N. truncata (Colenso) Cockayne mainly due to "the presence of the marginal vein and the secondary veins that enter centrally the main teeth which have glandular tip" but instead, both species have simple teeth. Among the species of South America, N. variabilis shares the compound serrate margin and the presence of a fimbrial vein with N. dombeyi (Mirb.) Oerst. and N. betuloides (Mirb.) Oerst., but differs from both in the termination of the secondary veins which forks near the margin and the apical branch reaches the sinus bottom while the basal branch enters the glandular tooth apex centrally. Remarks. These specimens fit the description given by Tanai (1986) for N. magelhaenica, represented by elliptic leaves with acute to obtuse base, craspedodromous secondary veins diverging from the pinnate midvein at acute angles and margin serrate with two orders of teeth. Tanai (1986) accepts this combination despite she considered that all the specimens illustrated by Dusén (1899) are single-toothed in margin and not belong to N. magelhaenica. Although none of the specimens described by Dusén as N. magelhaenica seem to have a complete margin, in some of these it is possible to infer the compound teeth by the bifurcation of the secondary vein near the margin. Both authors, Dusén and Ta Description. Leaves ovate in shape, petiolate with convex base and acute apex, asymmetrical at the base, about 1.2 cm long by 0.6 cm wide (L:W ratio 2:1), nanophyll. Margin serrate with compound teeth, sinus shape angular, primary tooth acute whose sides are convex in the apical side and convex to straight in the basal side. The secondary veins end in the main tooth apex which is accompanied with a small subsidiary tooth in their basal side. A branch from the secondary vein curves upward and supplies the apex of the subsidiary tooth. Venation pinnate, primary vein curved, major secondary veins craspedodromous (at least 5 pairs), with regular spacing, emerging at acute angle and decurrent attachment to midvein. Basal secondary with agrophic veins developed on one side only. Intercostal tertiary veins percurrent. Plicate vernation. Remarks. The two specimens are comparable with the extant species Nothofagus obliqua (Mirb.) Oerst and N. glauca (Phill.) Krasser but the last lacks of agrophic veins (Gandolfo & Romero 1992). Nothofagus obliqua and the studied specimensshow unequal laminas, more than 6 secondary veins, basal secondary veins with outer secondary upon one half of the lamina, tertiary mainly alternate and the principal sinus, which separates two composite teeth, is equidistant from the two adjacent secondary veins (Gandolfo & Romero 1992 fig. 13). Nothofagus lanceolata and the studied fossil leaves are quite similar in the lamina shape, venation type and toothed margin however, the specimen described by Dusén seems to be slightly asymmetrical towards the base (Fig. 4G).
In their revision of the Fagaceae and Nothofagaceae leaves described by Dusén(1899), Romero & Dibbern (1985) propose that there are no differences between N. lanceolata and N. elongata. Dusén defined these two species upon the differences in their margin, base shape and number of secondary veins. According to Romero & Dibbern (1985) both fossil-species are composed by leaves with composite teeth and similar number of secondary veins ranging from 9 to 12 representing a single species. Tanai (1986) considered N. lanceolata and N. elongata as questionable species referable to Nothofagus but still recog-nized as two different taxa which differ mainly by the presence of a compound serrate margin in N. lanceolata and a single serrate margin in N. elongata.
Among the extant species N. lanceolata was compared by Dusén with Nothofagus dombeyi but they differ in the number (higher in the fossil-species) and terminations of secondary veins (Tanai, 1986). Romero & Dibbern (1985) mentioned Nothofagus variabilis like the most similar species being more comparable in the number of secondary veins and the presence of tertiary teeth occasionally.
Remarks. Among the fossil-species, the studied specimens match the description of Nothofagus subferruginea (Dusén) Tanai in their size, shape, number of secondary veins and margin. According to Tanai (1986) leaves of N. subferruginea are sometimes simple toothed on the upper margin of the blade as do in the extant N. alessandri. This is in agreement with the specimen illustrated in the Fig. 4K. This fossil-species, formerly described as Fagus subferruginea, is a common element found in the fossil associations recovered from the Paleogene of Patagonia (see Tanai, 1986, p. 532) and Antarctica (Barton, 1964, Zastawniak, 1981, Zastawniak et al., 1985, Torres, 1990. It was emended by Tanai (1986) due to the presence of secondary veins directly entering to the teeth with one or three branching veinlets and synonymized with other "Fagus" species also described by Dusén (eg. Fagus obscura and Fagus intergifoglia). As noted by Tanai (1986) the fossils described as N. subferruginea are closely similar to the extant Nothofagus alessandri Espinosa in their lamina shape, venation pattern and tooth. Also, she found resemblances between N. alessandri and Fagus novae-zealandica Oliver and F. australis Oliver from the upper Miocene of New Zealand (Oliver, 1936) and pointed out that N. subferruginea and both New Zeland fossils may be closely related.
According to Tanai (1986) there are no extant leaves which closely match these fossil leaves in all characters. Among the Nothofagaceae he related Nothofagus dicksoni to N. fusca (Hook. f) Oerst. and N. truncata (Colenso) Cock. despite the notorious differences in size, number of secondary veins and less conspicuous teeth.
Nothofagus sp. aff. N. alessandri Espinosa. Fig. 5 fig. 3G) seem to be similar to the specimen here described. Both share the lamina shape, margin type and the general venation pattern. Unfortunately, these specimens were not described in detail, precluding more precise comparisons. Following the key of leaves of living species of Nothofagus developed by Gandolfo & Romero (1992), among the species that have outer secon-dary veins the studied specimen seems to be similar to Nothofagus alessandri. They share the composite teeth, the rounded base, the presence of outer secondary veins developed only on one side of the leaf and tertiary veins percurrent. Also, both show a principal sinus, which separates two composite teeth, located just above each secondary.

DISCUSSION AND CONCLUSIONS
One hundred and seven specimens of Nothofagaceae were studied and assigned to two taxonomic units similar to extant species (Nothofagus sp. cf. N. obliqua and Nothofagus sp. aff. N. alessandri), and to six fossil-species. These are: Nothofagus simplicidens, Nothofagus variabilis, Nothofagus lanceolata, Nothofagus subferruginea, Nothofagus dicksonii and Nothofagus magelhaenica, being the last species reported for the first time for the unit. With few exceptions (Nothofagus dicksonii and Nothofagus sp. cf. N. obliqua) most of the Nothofagus species are present in both members of the Río Turbio Formation (Tab. 2). However, if we analysed the distribution of the specimens through the entire formation, we observed significant differences in their abundance (Fig. 6). The upper member of the RTF concentrates the 68 % of the Nothofagacean specimens while in the lower member the specimens barely reaches the 17% (Fig. 7). This represents a four-time increase in the Nothofagus abundance toward the upper levels of the Río Turbio Formation.
The deposition of the Río Turbio Formation took place during a particular time-interval characterized by a progressive cooling trend estimated by climate proxy data (Zachos et al., 2001). The associations of leaves recovered from the Río Turbio Formation show a mixture of species with subtropical or tropical affinities (e.g. Lauraceae, Sapindaceae, Malpighiaceae) with others developed in cold temperate climate (see Appendix 1), mainly represented by Nothofagus (Romero, 1978(Romero, , 1986aPanti, 2014b).
The observed increase in the abundance of cold temperate species such as Nothofagus along with the decrease in richness and abundance of the megathermal taxa towards the upper member of the Río Turbio Formation (Menéndez, 1971;Panti, 2014bPanti, , 2018 would be in line with the global cooling trend. This turnover may have marked the beginning of a new floristic scenario, with a widespread cool-adapted flora across these southern latitudes. Table 2. Nothofagus species described and specimens number discriminated by member. Fig. 7. Number of specimens per species discriminated by member. Dark gray, upper member; light grey, lower member.