430 IAWAIAWA Journal Journal 35 (4), 35 2014: (4), 2014 430–443 RUPRECHTIA IN THE MIOCENE EL CIEN FORMATION, BAJA CALIFORNIA SUR, MEXICO 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 un- derstanding 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 & © International Association of Wood Anatomists, 2014 DOI 10.1163/22941932-00000076 Published by Koninklijke Brill NV, Leiden Downloaded from Brill.com10/04/2021 03:56:43AM via free access Cevallos-Ferriz et al. – Miocene Ruprechtioxylon (Polygonaceae) 431 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 men- tioned 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). Downloaded from Brill.com10/04/2021 03:56:43AM via free access 432 IAWA Journal 35 (4), 2014 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. Downloaded from Brill.com10/04/2021 03:56:43AM via free access Cevallos-Ferriz et al. – Miocene Ruprechtioxylon (Polygonaceae) 433 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