Ruprechtia in the Miocene El Cien Formation, Baja California Sur, Mexico

430 IAWAIAWA Journal Journal 35 (4), 35 2014: (4), 2014 430–443

Sergio R. S. Cevallos-Ferriz1,*, Hugo I. Martínez-Cabrera2 and Laura Calvillo-Canadell1 1Instituto de Geología, UNAM, Ciudad Universitaria, Circuito de la Investigación Científica, Copilco El Alto, Coyoacan, 04510 Mexico, D.F. 2Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Av. Luis Donaldo Colosio s/n y Madrid, campus Universidad de Sonora, 83000 Hermosillo, Sonora, Mexico *Corresponding author; e-mail: [email protected]

ABSTRACT Fossil woods from the El Cien Formation have yielded important information on the taxonomic composition and climate of a flora established in the west coast of Mexico during the Miocene. This report of a new genus and species, Ruprechti- oxylon multiseptatus Cevallos-Ferriz, Martínez Cabrera et Calvillo-Canadell, is based on woods with the following combination of features: vessels solitary and in radial multiples of 2–3; vestured, alternate, oval to polygonal intervessel pits; vessel-ray and vessel-parenchyma pits similar in size to intervessel pits, but with slightly reduced to reduced borders; 2–5 septa per fibre; scanty paratracheal, unilateral and vasicentric axial parenchyma; uniseriate homocellular rays, oc- casionally locally biseriate; crystals in fibres. The presence ofRuprechtioxylon (Polygonaceae) in the El Cien Formation confirms that plants of lineages growing today under contrasting climates lived together in the past. This record adds a new species to the growing list of Neotropical taxa that were present in Mexico prior to the great Plio-Pleistocene exchange of biota in the Americas. Keywords: Ruprechtia, Miocene, Baja California Sur, Mexico, Polygonaceae.

INTRODUCTION In the last two decades the study of fossil plants from Mexico has improved our understanding of the historical processes leading to the extraordinarily high biodiversity of the region (e.g., Cevallos-Ferriz et al. 2012; Pérez-García et al. 2012). Cenozoic plant diversity of Mexico was previously restricted to a few isolated records, but today the number of recognized taxa based either on macro (Martínez-Cabrera & Cevallos- Ferriz 2006; Martínez-Cabrera et al. 2006) or micro fossils has grown significantly, and whole plant reconstructions are possible (e.g., Calvillo-Canadell & Cevallos-Ferriz 2005, 2007; Calvillo-Canadell et al. 2010; Hernández-Castillo & Cevallos-Ferriz 2009; Hernández-Damián 2010). Both approaches, the identification of isolated organs and whole plant reconstructions, not only have provided valuable information on biologi- cal and ecological aspects of Mexican paleocommunities (e.g. Martínez-Cabrera &

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Cevallos-Ferriz 2008; Martínez-Cabrera et al. 2012), but have also contributed to a better understanding of the historical processes involved in the assembly of the low latitude North America flora (e.g., Cevallos-Ferriz et al. 2012; Pérez-García et al. 2012). Disentangling these historical processes, however, still presents a challenge (e. g., Atwater 1970; Cevallos-Ferriz & González-Torres 2005; Eguiluz et al. 2005; Ferrari et al. 2005; Padilla & Sánchez 2007). Recently, plants like Bursera (Burseraceae), Rhus, Pistacia, Haplorhus (Anacardia- ceae), Inga, Pithecellobium, Lonchocarpus (Leguminosae), Brosimum, Ficus (Mora- ceae), that today grow in dry tropical vegetation have been reported from Neogene localities in Mexico (e.g., Hernández-Castillo & Cevallos-Ferriz 1999; Ramírez & Cevallos-Ferriz 2000, 2002; Calvillo-Canadell & Cevallos-Ferriz 2005). Some can be dated back to the Miocene and Oligocene (e.g., Coatzingo Formation, Puebla and Ixtapa Formation, Chiapas; Calvillo-Canadell & Cevallos-Ferriz 2005) and even Eocene (La Popa Formation, Nuevo León; Calvillo-Canadell & Cevallos-Ferriz 2005). Other genera, collected in the aforementioned localities, suggest an interesting biogeographic- temporal pattern, where their first occurrence has been recorded at higher latitudes and subsequently their distribution expanded southwards. However, there is the alternate explanation that these genera once had a broader area of distribution and through time their ranges were reduced. Unfortunately, older Paleogene outcrops from Mexico are marine and lack evidence of plants that might have grown in Mexico at that time. Fur- thermore, since the Cretaceous Mexico was covered by seas that retreated southwards during the Paleogene exposing continental areas that were colonized by plants most probably expanding their distribution from nearby northern areas. The genera mentioned above disappeared from areas in the north of the continent where they were first reported (Martínez-Cabrera & Cevallos-Ferriz 2004; Calvillo-Canadell & Cevallos- Ferriz 2005). These include Tapirira (Eocene of Oregon, USA; Oligocene-Miocene of Baja California Sur; and Miocene of Chiapas, Mexico) which today grows from southern Mexico to South America; Loxopterygium (Oligocene of Puebla, Mexico; Miocene of Ecuador) today growing in South America; Haplorhus (Oligocene of Puebla, Mexico) currently growing in xeric vegetation in Peru and Bolivia; Inga (Eocene of Nuevo Léon, Oligocene of Puebla, Miocene of Mexico and Ecuador) today distributed in the Neotropics, and Prioria (Oligocene of Puebla, Mexico) today occupying areas from Central America to South America (Martínez-Cabrera & Cevallos-Ferriz 2004; Calvillo-Canadell & Cevallos-Ferriz 2005; Sainz-Reséndiz 2011). Here we describe a new species with a similar spatio-temporal pattern: Ruprechti- oxylon multiseptatus Cevallos-Ferriz, Martínez Cabrera, et Calvillo-Canadell. Today species of Ruprechtia (Polygonaceae) occur in dry tropical vegetation of the west coast of Mexico to South America (Pendry 2004; Sanchez & Kron 2011). This Baja California fossil wood is the first occurrence of Ruprechtia in the fossil record and suggests the genus may have expanded its distribution through time. This record not only adds information to further support the above mentioned biogeographical pattern, which has been used to explain Cenozoic diversity in Mexico (e.g., Cevallos-Ferriz et al. 2012; Pérez-García et al. 2012), but also confirms the emerging concept of a complex vegetational history in the area as suggested by recent palaeobotanical work (e.g., Cevallos-Ferriz et al. 2012; Pérez-García et al. 2012).

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Materials and Methods Geological setting In the southern part of the Baja California Peninsula, approximately 100 km north- west of La Paz, Baja California Sur (Fig. 1), the clastic sedimentary rocks of the El Cien Formation are exposed (Applegate 1986). The El Cien Formation is a sedimentary sequence deposited during the Late Oligocene–Early Miocene; it rests unconformably on the sandstone of the Tepetate Formation and is overlain by the volcanic rocks of the Comondú Formation (Applegate 1986; Fischer et al. 1995). Applegate (1986) proposed the El Cien Formation as a stratigraphic unit and divided it into three Members, the Cerro Tierra Blanca, San Hilario and Cerro Colorado. Most authors agree that the two basal members are a single unit. The El Cien Formation, in the sense of the present paper, corresponds to the (1) Monterrey Formation (Darton 1921; Heim 1922; Beal 1948; Mina 1957; Ojeda 1959; Alatorre 1988), (2) San Gregorio Formation (Hausback 1984; Kim & Barron 1986), and (3) the basal member of the El Cien Formation (San Juan Member) sensu Schwennicke (1994) and Fischer et al. (1995).

112° W 111° 110°

110°W 105° 100° 95° 90°

Gulf of California 30° N Cd. Constitución 25°N

El Cien Gulf of California 25° Gulf of Mexico

La Paz 24° 20°

Peninsula of Baja California

Rancho Matanzas Pacific Ocean 18° Transpeninsular highway

El Cien 23° Cañada El Canelo Cabo San Lucas 10 km

Figure 1. Maps showing the location of the El Cien Formation and the fossil wood localities.

The morphospecies described here is based on silica permineralizations that were collected in the upper Member (Cerro Colorado) of the El Cien Formation near Rancho Matanzas and Cañada El Canelo, which is located 5 km northeast and 3.5 km southwest from El Cien town, respectively (Fig. 1). The sediments of this Member are composed mainly of fine to coarse-grained sandstones, tuffaceous sandstones and conglomerates (Applegate 1986; Fischer et al. 1995) and represent a progradational sequence from offshore to non-marine environments (Gidde 1992). The change in depositional environ- ment is evidenced by the presence of primary sedimentary structures and ichnofossils at the base – indicative of near shore environments – and by root casts of fossil plants and fossil caliche deposits towards the top, indicating the change from lagoon to ter- restrial environments.

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There has been no radiometric dating of the Cerro Colorado Member itself. How- ever, K– Ar radiometric dates for the underlying member gives an age of 25.5 ± 0.4 Ma, whilst the overlying volcanic rocks have been dated at 21±0.4 Ma (Hausback 1984). Therefore, we assume the Cerro Colorado Member is approximately 25–20 Ma old.

Fossil preparation and anatomical measurements The fossil woods were thin-sectioned in transverse (TS), tangential (TLS) and radial (RLS) sections using standard techniques for petrified woods. The description of the morphospecies is based on the observations of wood of two different plants. The group of slides representing each tree is given its own IGM-LPB series number. With the exception of vessel frequency, which was calculated based on counts of 15 fields of view, quantitative characteristics are the result of at least 25 measurements with the mean (x) and standard deviation (s) provided. Presence of the vestured pits was con- firmed by their observation with a SEM, Jeol model C35AD-4 (Instituto de Biología, UNAM). Measurement techniques and terminology generally follow those described by an IAWA Committee (1989). Familial affinity was determined by consulting references such as Metcalfe and Chalk (1950), Détienne and Jacquet (1983), Ilic (1987), Terrazas (1994), Barajas-Morales and León (1989), Carlquist (2003) and searches of InsideWood (InsideWood 2004-onwards; Wheeler 2011). Comparisons were then made with extant wood samples housed in the National Xylarium of the Institute of Biology, UNAM, and the National Herbarium of The Netherlands, Utrecht University Branch (sample numbers prefixed by Uw). All fossil specimens are deposited in the National Palaeontological Collection, Institute of Geology, Universidad Nacional Autónoma de Mexico (UNAM), Mexico.

Systematic description

Order: Caryophyllales Family: Polygonaceae Genus: Ruprechtioxylon Cevallos-Ferriz, Martínez Cabrera et Calvillo-Canadell Species: Ruprechtioxylon multiseptatus Cevallos-Ferriz, Martínez Cabrera et Calvillo- Canadell gen. et sp. nov. (Fig. 2) Etymology: The generic name recognizes the similarity of this fossil wood with the wood of the genus Ruprechtia. The specific epithet refers to the presence of multiple septa in the fibres Holotype: LPB 4172-4188 Additional material: LPB 4316-4330 Age: Early Miocene Diagnosis: Growth rings marked by latewood fibres; vessels solitary and in radial multiples of 2–3; intervessel pits alternate, circular to oval, vestured; vessel-ray and vessel-axial parenchyma pits opposite to slightly horizontally elongated, with borders slightly reduced to reduced and similar in size to intervessel pits; septate fibres, 2–5 septa per fibre; axial parenchyma scanty paratracheal, vasicentric, aliform, and unilateral; rays exclusively uniseriate, some locally biseriate, homocellular, composed of all procumbent cells; crystals in fibres.

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Figure 2. Ruprechtioxylon multiseptatus (LPB 4172-4188). – A: Vessel and parenchyma distribution. The arrow on the left side points at a growth ring marked by flattened latewood fibres; the arrow on the right side is pointing at unilateral parenchyma. TS. – B: Unilateral (left arrow) and scanty paratracheal axial parenchyma (right arrow). TS. – C: Scanty paratracheal parenchyma. TS – D: Septate fibers (top arrow pointing left), fibers with crystals (lower arrow pointing right) and uniseriate rays. TLS. – E: Vessel-parenchyma pits horizontally elongated with slightly reduced borders. – F: Vessel element with simple perforation plate and polygonal, alternate intervessel pits. – G: Fiber with crystals. – H: Scanning electron micrograph showing a vessel element with alternate intervessel pits. – I: Close-up of apertures with small projections along the pit aperture interpreted as vestures. – J: Rays composed of procumbent cells. RLS.

Description: Growth rings marked by about three rows of radially flattened latewood fibres. Diffuse porous wood. Vessels solitary (80%) and in radial multiples of 2–3 vessels, mean number of vessels per group 1.27 (Fig. 2A, B). Vessel elements are round to oval in outline (Fig. 2A–C), with tangential diameters of 70–155 µm (X = 110, s = 20 and lengths of 170–380 µm (X = 271, S = 61); vessel walls in transverse sections are 4–8 µm (X = 5.38, s = 1.06) thick; simple perforation plates inclined c. 20°; 4–11 (X = 6.9, s = 2.3) vessels per mm2; dark contents in vessel elements; intervessel pits

Downloaded from Brill.com10/04/2021 03:56:43AM via free access Cevallos-Ferriz et al. – Miocene Ruprechtioxylon (Polygonaceae) 435 alternate, circular to oval in outline (Fig. 2F), 3–5.5 µm (X = 4.3, s = 0.7) in diameter and vestured (Fig. 2H, I); vessel-ray and vessel-axial parenchyma pits are opposite, slightly horizontally elongated, with slightly reduced edges and similar in size to intervessel pits (Fig. 2E). The fibres are septate, 2–5 septa per fibre (Fig. 2D) 2–13 µm (X = 7.7, s = 2.8) in diameter, fibre walls are 2–6µ m (X = 3.2, s = 0.9) thick (thin- to thick-walled in IAWA classification). Axial parenchyma scanty paratracheal, often unilateral, vasicentric and aliform (Fig. 2A–C), 5 or 6 cells per parenchyma strand. Rays mostly uniseriate, sometimes locally biseriate, homocellular (Fig. 2D, J), composed exclusively of procumbent cells; 9–14 (X = 11.7, s = 1.5) rays per mm (Fig. 2A), 2–21 (X = 10.7, s = 5.1) cells high. Long (up to 15) chains of prismatic crystals in fibres, one crystal per chamber (Fig. 2G).

Affinities Comparison with extant taxa The combination of septate fibres, vasicentric axial parenchyma, and homocellular uniseriate rays in Ruprechtioxylon multiseptatus resembles the wood structure of several genera in Anacardiaceae, Burseraceae, Sapindaceae, Leguminosae, Combretaceae and Polygonaceae. However, the absence of vestured intervessel pits in the first three families precludes relationship to any of them. Ruprechtioxylon multiseptatus is similar to Cono- carpus erectus, Pteleopsis myrtifolia and some Terminalia species, e.g. T. myriocarpa and T. triflorain Combretaceae. A key difference between these Combretaceae species and the El Cien fossil is the occurrence of chambered or non-chambered crystals in axial parenchyma cells (Tortorelli 1956; Van Vliet 1979; InsideWood 2004-onwards) instead of in the fibres as in R. multiseptatus. However, despite the similarity in size and shape of intervessel and vessel-ray parenchyma pits in these Combretaceae species and R. multiseptatus, in the Combretaceae species the vessel-ray parenchyma pits have distinct borders (Tortorelli 1956; Van Vliet 1979; InsideWood 2004-onwards), while in R. multiseptatus borders are slightly to obviously reduced. These Combreta- ceae species have, in addition to vasicentric parenchyma, confluent (Tortorelli 1956), marginal, and parenchyma in narrow bands (Conocarpus erectus, Van Vliet 1979). Pteleopsis myrtifolia (Van Vliet 1979) and R. multiseptatus both have unilateral and aliform parenchyma, but the distribution of crystals in axial parenchyma distinguishes Pteleopsis myrtifolia from the El Cien fossil. Ruprechtiioxylon multiseptatus resembles a number of Leguminosae genera. It is particularly similar to some species in the Mimosoideae, such as Albizia greveana, Havardia leiocalyx, Havardia pallens and Inga umbellifera. However, A. greveana differs in having chambered crystals in axial parenchyma, marginal parenchyma and shorter parenchyma strands (3–4 cells) (InsideWood 2004-onwards) than the El Cien fossil (5–6 cells); it also lacks unilateral parenchyma. Although Havardia pallens has crystals in fibres, the presence of vessels of two distinct size classes and shorter parenchyma strands (3–4 cells) and marginal parenchyma (Cassens & Miller 1981; InsideWood 2004-onwards) makes it different from the fossil from El Cien. Havardia leiocalyx has marginal axial parenchyma and crystals are present in axial parenchyma (Cassens & Miller 1981; InsideWood 2004-onwards); both features are not seen in

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Table 1. Comparison of Coccoloba, Ruprechtia, Triplaris, and Ruprechtioxylon multiseptatus. Mean and, in parentheses, standard deviation. * = data from Carlquist (2003); ** = the data presented are the range of values for several species (four species for Triplaris and seven for Coccoloba); *** = range in number of septa per fiber based on Coccoloba cruegeri, C. diversifolia, C. excelsa, C. latifolia, C. mollis, C. uvifera (material from the National Herbarium of the Netherlands), see text. µ m) Vessel diameter ( Vessel frequency (v/mm)Vessel Mean number of vessels per group Rays /mm ( µ m) Uniseriate ray height (# cells) Uniseriate ray height Number of cells in parenchyma strand Number of septa per fiber Ruprechtia ramniflora 121 (21) 7.8 (2.2) 1.78 10.1 (1.6) 152 (49) 10.4 (3.5) 5 3–5 Uw13332 Ruprechtia brachisepala 95.3 (15) 12.5 (3.1) 2.97 12.5 (2.2) 139.82 (47) 10 (3.1) 5 3–5 Uw1586 Ruprechtia laxiflora 81.8 (17) 19.1 (4.9) 1.87 10.5 (1.9) 133.2 (41) 9.8 (3) 5 3–5 Uw13701 Ruprechtia fusca 68 (16) 26 (6.8) 3.3 14 (1.6) 124.6 (65) 8.4 (4.5) 6 3–5 MEXU325 Ruprechtia sp.* 52 22 1.4 – 95 – 5 – Coccoloba 28–123 1.6– 41 1.7– 4.6 – – – 2–4 1–3*** spp.*, ** Triplaris 68–109 7–10 1.6–2.4 – – – 5 – spp.*, ** Triplaris surinamensis 120 4.2 1.5 8.4 – – 6 2–4 Uw50 Ruprechtioxylon 110.5 (10) 6.9 (2.3) 1.27 11.7 (1.5) 164 (74) 10.7 (5.1) 5–6 2–5 multiseptatus

Ruprechtioxylon multiseptatus. Inga umbellifera, as R. multiseptatus, has crystals in fibres, butI. umbellifera, and all other Inga species included in a comprehensive wood anatomical study of the Mimosoideae (Evans et al. 2006), have axial parenchyma strands of up to 4 cells, which contrasts with the 5–6 cells observed in the fossil. In addition, the vessel-ray pits in these species, as other Mimosoideae, have distinct borders, which differs from the slightly reduced to reduced borders observed in R. multiseptatus. Lastly, Ruprechtioxylon multiseptatus bears similarity with some Polygonaceae, particularly those in the tribe Triplarideae. Except for Triplaris and Coccoloba, few

Downloaded from Brill.com10/04/2021 03:56:43AM via free access Cevallos-Ferriz et al. – Miocene Ruprechtioxylon (Polygonaceae) 437 species in the Polygonaceae are trees (Carlquist 2003). This diversity of life forms in Polygonaceae (vines, shrubs, perennials with woody roots, etc.) is paralleled with a large wood anatomical variability. Many species have vascular cambium variants, e. g., successive cambia in Antigonum leptopus, Rheum, Rumex (Carlquist 2003). We compared Ruprechtioxylon multiseptatus with extant material from six of the Polygonaceae genera with a single cambium. Two different anatomical groups can be distinguished among the reviewed extant genera. One includes Coccoloba, Triplaris and Ruprechtia and is characterized by the presence of exclusively uniseriate, homocellular rays, although some species in these three genera have mainly biseriate rays (e. g., Coccoloba latifolia Lam. [Uw 1037]) and uniseriate rays composed of square and upright cells (e. g., Coccoloba excelsa Benth. [Uw 44343] and C. diversifolia Jacq. [Uw 48432]). In Triplaris biseriate rays are frequent. In the other group, Rumex, Polygonum and Eriogonum, rays frequently exceed three cells wide, are considerably higher, and are heterocellular or paedomorphic sensu Carlquist. Carlquist (2003) mentions that some species of Rumex, Polygonum and Eriogonum and other genera (Antigonon, Bilderdykia, Caligonum, Symmeria and Muehlenbeckia) have wide rays, while in most Polygonaceae the average ray width is around two cells. Ruprechtioxylon multiseptatus is closer to the first group, and shares with it the presence of septate fibres (with more than one septum per fibre), axial parenchyma that is scanty paratracheal to vasicentric, and homocellular and mostly uniseriate rays. Triplaris, Coccoloba, Ruprechtia and Ruprechtioxylon multiseptatus have fibres with crystals. Our observations indicate that these crystalliferous fibres tend to be more enlarged in Coccoloba than in Triplaris, Ruprechtia and Ruprechtioxylon multiseptatus. In some species of Ruprechtia, these crystalliferous fibres tend to be associated with vessels as is the case in Ruprechtioxylon multiseptatus. Based on quantitative characters like ray height and frequency, and vessel diameter and frequency, R. multiseptatus is similar to species in the genera Ruprechtia, Cocco- loba and Triplaris (Table 1). However, in other characters, such as vessel grouping, the fossil material (which has a lower average number of vessels per group) differs from Ruprechtia, Triplaris and Coccoloba. Except for Coccoloba rugosa, all species of the genera studied by Carlquist (2003) exceed two vessels per group (Table 1). In general, this is also true for the species in Ruprechtia, although the only species of Ruprechtia studied by Carlquist (2003) (Ruprechtia sp.) has the closest degree of vessel clustering compared to Ruprechtioxylon multiseptatus (1.4). Triplaris has a higher proportion of solitary vessels and thus it is similar to R. multiseptatus in this regard. Ruprechtioxylon multiseptatus is more similar to Triplaris and Ruprechtia than to Coccoloba in the number of cells per parenchyma strand and the number of septa per fibre. Ruprechtioxylon multiseptatus has 5–6 cells per axial parenchyma strand while Triplaris, Ruprechtia and Coccoloba have 6, 5 and 2–4 cells respectively. In Coccoloba, septate fibres have 1–3 septa per fibre (C. cruegeri Lindau [Uw 4785], C. diversifolia [Uw 8432], C. excelsa [Uw 4343], C. uvifera [Uw 10169], C. latifolia [Uw 1037] and C. mollis [Uw 109]) while Ruprechtioxylon multiseptatus, Ruprechtia and Triplaris have 2–5, 3–5 and 2–4 septa per fibre. The absence of vestured pits in Coccoloba (Jansen et al. 1998; Carlquist 2003) makes Ruprechtioxylon multiseptatus more similar to Ruprechtia and Triplaris.

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Based on characters such as the number of septa per fibre, number of cells per parenchyma strand, prevalence of uniseriate rays and the distribution of crystalliferous fibres, we related the new fossil wood with Ruprechtia. However, quantitative differences and age suggest the presence of a new species, Ruprechtioxylon multiseptatus Cevallos-Ferriz, Martínez Cabrera et Calvillo-Canadell.

Comparison with fossil taxa Several fossil woods similar to Ruprechtioxylon multiseptatus have been described; however, none have vestured pits. Determining the presence of vestured pits in fossil wood is dependent on exceptional preservation. Therefore, the apparent absence of vestured pits in many other fossil taxa is not a certainty, so we do not use it in comparisons of Ruprechtioxylon to other fossil woods. Based on searches in InsideWood (2004-onwards) and literature, we found that the fossil species resembling Ruprechti- oxylon multiseptatus are mainly in the Combretaceae and Sapindaceae. In Combre- taceae, Terminalioxylon mortandrense (Navale 1956) from the Tertiary of southern India and T. tunesense (Delteil-Desneux 1981) from the Tertiary of central Tunisia, share many of the anatomical characteristics with R. multiseptatus, but the El Cien wood lacks crystals in ray cells as described for these two fossil woods. Sapind- oxylon jansonii (Tertiary of South Sumatra; Kräusel 1922), S. klitzing (Pliocene of Java; Pfeiffer & Van Heuren 1928), Sapindoxylon sp. (Eocene of Lower Saxony; Gottwald 1992) and Euphorioxylon indicum (Maastrichtian of India; Kar et al. 2004), all in the Sapindaceae, have prismatic crystals in ray cells instead of in fibres as occurs in the El Cien fossil. We therefore suggest that R. multiseptatus differs from previously described fossil woods. Discussion

Together, Triplaris and Ruprechtia (tribe Triplarideae) have approximately 55 species distributed in the Americas (Sanchez & Kron 2011). Difficulties in the morphological characterization of these two genera have made the generic classification of Triplaris and Ruprechtia unstable (Pendry 2004; Sanchez & Kron 2011). Recent molecular work based on four chloroplast and two nuclear regions suggests that Ruprechtia is polyphyletic, with one species, R. obidensis, being sister to the main Ruprechtia clade and Triplaris, which is monophyletic (Sanchez & Kron 2011). The closeness of these two genera is also evident as the differences in the samples we studied are only in quantitative traits (septa per fibre, number of cells per parenchyma strand, prevalence of uniseriate rays) and in the distribution of crystalliferous fibres. Significantly, most of the 37 species of Ruprechtia are distributed in seasonally dry forest from Mexico to South America (with their diversity peaking in Brazil and Venezuela; Sanchez & Kron 2011). Some species of Ruprechtia occur in seasonally inundated or gallery forest (Pendry 2004; Sanchez & Kron 2011). This pattern agrees with both the taxonomic composition of the El Cien paleoflora and paleoclimate estimates that strongly suggest climatic and compositional similarity of El Cien flora with the dry deciduous/semi-deciduous forests of Jalisco (Martínez-Cabrera & Cevallos-Ferriz 2008). Triplaris, with only 19 species (Brandbyge 1986), has similar

Downloaded from Brill.com10/04/2021 03:56:43AM via free access Cevallos-Ferriz et al. – Miocene Ruprechtioxylon (Polygonaceae) 439 biogeographic and ecological patterns. However, most of its species occur as pioneers and in seasonally inundated or disturbed areas and only few species in seasonally dry forest. Previous comparative studies (e.g., Cevallos-Ferriz & Barajas-Morales 1994; Martínez-Cabrera & Cevallos-Ferriz 2004, 2006, 2008; Martínez-Cabrera et al. 2006) suggest that the landscape of the El Cien Formation was composed of a mixture of evergreen and deciduous plants, similar to those presently growing in the seasonally dry western coast of Mexico. Some other taxa of the El Cien Formation flora, such as Copaifera, Maclura, Tapirira and Tetragastris however, grow today under more hu- mid conditions, in different vegetation types (e.g., Cevallos-Ferriz & Barajas-Morales 1994; Martínez-Cabrera & Cevallos-Ferriz 2004, 2006, 2008; Martínez-Cabrera et al. 2006). The presence of Ruprechtioxylon multiseptatus in the El Cien Formation adds new evidence to the spatio-temporal trend exhibited by other taxa suggesting that some plants that are now part of the Neotropical flora were first present in high latitude North America. However, the extent to which lineages exhibiting this biogeographic-temporal pattern contributed to the extant configuration of the region relative to different patterns (described below) is still unknown. While exploring alternatives for the origin of the dry tropical flora, whereRuprechtia best grows today (Pendry 2004; Sanchez & Kron 2011), Martínez-Gordillo and& Morrone (2005) suggested that some lineages, including Euphorbiaceae, have a Gondwanan origin, and subsequently radiated north- wards into low latitude North America. In contrast, new fossil records, as well as molecular studies proposed that Burseraceae, another lineage important in seasonally tropical dry vegetation, made its first appearance in the fossil record in North America during the Late Paleocene/Early Eocene (Becerra 2003, 2005; Calvillo-Canadell et al. 2010; Becerra et al. 2012et al.), and that by the beginning of the Late Oligocene (23 Ma) dispersed to eastern Laurasia (Weeks et al. 2005; Dick & Wright 2005). Similar scenarios have been proposed for other groups such as Inga, Pithecellobium, and Prioria (Leguminosae), or Haplorhus and Loxopterygium (Anacardiaceae). If Ruprechtia (Polygonaceae) is indeed a near relative of Ruprechtioxylon, this record can be added to this growing list of neotropical plants with first occurrence in the fossil record in North America during Eocene to Miocene, before the Plio-Pleistocene great biota exchange between the Americas. There are, however, other hypotheses suggesting contrasting scenarios, like the one proposing that diversification and expansion of distribution areas to North America of taxa with Mid-Miocene to Pliocene presence in South America took place both at that time and during the Pleistocene, thus suggesting that the Neotropical plants expanded their distribution from the south (Pennington et al. 2004; Lavin 2006). Evidence from different taxa exhibiting these patterns suggests that the processes that gave rise to the extant vegetation of Mexico and the biogeographic distribution of taxa through time are more complicated than expansion and reduction of areas of distribution through time. It is becoming clear that the exchange of taxa between North and South America operated in both directions, and that the process may have started as early as the Eocene (c. 50 Ma) through long distance dispersal, and perhaps was invigorated as the connection of the Americas via the Panama Isthmus facilitated these movements, c. 3.5 Ma ago. Further work will help to address the present hypotheses, but it is clear

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Acknowledgements

We thank Josefina Barajas-Morales (MEXU), Paul Maas and Imogen Poole (NHN, Utrecht Branch) for allowing us access to the slide collections (spring 2003), and Elisabeth Wheeler and two anony- mous reviewers for their constructive comments. SRSCF, HIMC and LCC acknowledge support from CONACYT (82433, 104515) PAPIIT-UNAM (219810).

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Accepted: 10 March 2014

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