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First record of fossil woods from the MariñoFormation (Miocene), Mendoza, Argentina and theirpalaeobiogeographical implicationsMaría Jimena Franco, Mariana Brea & Ana María ZavattieriPublished online: 29 Sep 2014.

To cite this article: María Jimena Franco, Mariana Brea & Ana María Zavattieri (2014): First record of fossil woods fromthe Mariño Formation (Miocene), Mendoza, Argentina and their palaeobiogeographical implications, Alcheringa: AnAustralasian Journal of Palaeontology, DOI: 10.1080/03115518.2014.951915

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First record of fossil woods from the Mariño Formation(Miocene), Mendoza, Argentina and theirpalaeobiogeographical implications

MARÍA JIMENA FRANCO, MARIANA BREA and ANA MARÍA ZAVATTIERI

FRANCO, M.J., BREA, M. & ZAVATTIERI, A.M., 2014. First record of fossil woods from the Mariño Formation (Miocene), Mendoza, Argentina andtheir palaeobiogeographical implications. Alcheringa 39, xxx–xxx. ISSN 0311-5518.

The first Miocene records of silicified fossil woods from the Mariño Formation, Potrerillos area, Andes Precordillera, Mendoza province, Argentinaare described. Rhaphithamnoxylon artabeae gen. et sp. nov. is described as the first fossil wood referable to Verbenaceae from Argentina. This newfossil species is related to extant Rhaphithamnus Miers, sharing the following anatomical features: diffuse porosity, distinct growth ring boundaries,numerous small to very small vessels, commonly in radial multiples, 1–3 seriate rays, and heterocellular and scarce paratracheal axial parenchyma.Rhaphithamnus contains only two extant species: R. spinosus (A.L. Juss.) Moldenke, which occurs in the Valdivian forests of Chile and Argentina,and R. venustus (Philippi) Robinson, which is endemic to the Juan Fernández Islands. Representatives of Verbenaceae are distributed predominantlyin the Americas from Patagonia (Argentina) to Canada, and they are inferred to have originated in South America. The fossil wood describedherein provides new age and geographical constraints on the raphithanoid lineage within Verbenaceae. Other fossil woods recorded from theMariño level are retained under open nomenclature, as they possess a combination of mostly solitary broad vessels, and smaller vessels in radialmultiples or in clusters, with numerous, vasicentric to confluent axial parenchyma, and heterocellular, high rays. Thus, they have features akin todicotyledonous lianas or vine-like or small shrub species.

María Jimena Franco [jimenafr@gmail.com] and Mariana Brea [cidmbrea@infoaire.com.ar], Laboratorio de Paleobotánica, Centro deInvestigaciones Científicas y Transferencia de Tecnología a la Producción, CICyTTP-CONICET, Dr. Matteri y España SN (E3105BWA) Diamante,Entre Ríos, Argentina; Ana María Zavattieri [amz@mendoza-conicet.gov.ar] Laboratorio de Paleopalinología, Instituto Argentino de Nivología,Glaciología y Ciencias Ambientales IANIGLA, CCT-CONICET Mendoza, Av. Adrián Ruiz Leal s/n., Parque General San Martín Mendoza (5500),Casilla de Correo 131, Argentina. Received 19.3.2014; revised 3.7.2014; accepted 29.7.2014.

Key words: Fossil woods, Mariño Formation, Miocene, Verbenaceae, Lianas or small shrubs, Argentina

THE NEOGENE MARIÑO FORMATION comprises ared-bed succession of mixed fluvial and eolian origin(Figs 1–3). Sedimentological evidence suggests that thisunit was deposited in a low-gradient setting where aridor semi-arid conditions prevailed. The occurrence ofstrongly oxidized floodplain deposits and sandy fluvialsequences indicates deposition under ephemeral flowregimes along with the development of a thick succes-sion of eolian deposits. These Neogene strata haveyielded mammalian assemblages (Simpson et al. 1962,Pascual & Odreman Rivas 1971, 1973, Cerdeño et al.2006, Cerdeño 2007) and have been dated radiometri-cally to ca 15.7–12.2 Ma age range (Marshall et al.1986, Yrigoyen 1993, 1994).

Here we describe the first Miocene plant fossilsfrom the middle member of the Mariño Formation atthe south-southwestern extent of the Precordillera inMendoza province. The fossils were recovered from thenorthern margin of the now-flooded Potrerillos Damreservoir. Remarkably, these are the first plant remains

recorded from the Neogene Mariño Formation inMendoza province.

One of the specimens collected from the MariñoFormation is assigned to Verbenaceae, which is mainlya New World family distributed in both temperate andtropical regions of America. Few genera are confined tothe southern part of South America (Atkins 2004). Thismaterial is the first Verbenaceae fossil wood found inSouth America and the first record of the family inArgentina.

Geological settingThick Cenozoic non-marine red-beds are exposed at thesouth and southwestern extent of the Andes Precordilleraat ca 33°S. The stratigraphy, sedimentology and verte-brate palaeontology of the synorogenic Neogene strata innorthern Mendoza province have been investigated byseveral authors (Rolleri & Criado Roque 1970, Pascual &Odreman Rivas 1971, Rolleri & Fernández Garrasino1979, Pascual & Bondesio 1981, Kokogián & Mancilla1989, Legarreta et al. 1993, Yrigoyen 1993, 1994,Irigoyen et al. 1995, 2000, 2002, Irigoyen 1997).© 2014 Association of Australasian Palaeontologists

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The stratigraphic nomenclature used in this contributionderives from Yrigoyen (1993). Detailed sedimentologicaldescriptions, palaeoenvironmental interpretations,palaeoclimatic inferences, radiometric dating and mag-netostratigraphic studies of the 3500 m thick well-exposed Neogene succession in the Cacheuta-Tupungatoregion were carried out by Irigoyen (1997) and Irigoyenet al. (1995, 2000, 2002).

According to these authors, the Miocene successionis divisable into four formations characterized by flu-vial, eolian, alluvial and some volcanic deposits (seeFig. 1). At the southern end of the Precordillera,approximately 35 km southwest Mendoza city, mainlyTriassic and Cenozoic continental strata are exposed(Figs 2, 3). In the study area, the oldest Palaeozoicrocks form Cacheuta Peak; the Cenozoic continentaldeposits unconformably overlie Middle to Upper Trias-sic non-marine strata of the Uspallata Group, which inturn onlap andesitic rocks of the volcanic and plutonicPermian–Triassic Choiyoi Group (Figs 2, 3).

The upper Eocene to lower Miocene DivisaderoLargo Formation crops out in the Potrerillos Valley andsurrounding areas between the eastern side of CacheutaPeak and the Potrerillos locality (Figs 2, 3). Smallexposures of sandstones with gypsum and anhydritelayers and varicoloured mudstones are exposed in theSSW foothills of Los Baños peak, and the formation islocally ca 35 m thick in the Cuchilla del Toscal sectionin the SSW foothills of Cacheuta Peak (Irigoyen et al.2000, 2002). The middle Miocene Mariño Formation,described below in more detail, unconformably overliesthe previous unit and crops out in the north butalso more extensively in the southern areas of thePotrerillos Dam and on the western part of CacheutaPeak (Fig. 2, 3). The upper Miocene (ca 11.7–9.0 Ma)La Pilona Formation (cf. Irigoyen 1997, Irigoyen et al.2000, 2002) unconformably overlies the MariñoFormation (Yrigoyen 1993) and consists mainly of thickgrey sandstones and conglomerates exposed in the westof the study area (Figs 1, 2).

Fig. 1. Map of the Potrerillos Valley showing the fossil locality, Precordillera of Mendoza province, Argentina.

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The Mariño Formation (Rolleri & Criado Roque 1970)

In the Potrerillos Valley, the Mariño Formation formsthe highest topographic elevations, named ‘Tosca de

Mariño’ and ‘Cuchilla del Toscal’ (Fig. 1) where thetypical succession is briefly described below (Fig. 2).This formation was subdivided into three members by

Fig. 2. Schematic stratigraphic column of the Toscal de Mariño and Cuchilla del Toscal Section of the Mariño Formation, Mendoza, Argentina(Modified from Irigoyen 1997).

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Yrigoyen (1993). Based on magnetic polarity stratigra-phy and ash chronology, Irigoyen (1997) and Irigoyenet al. (2000, 2002) assigned the entire Mariño Forma-tion to the interval ca 15.7–12.2 Ma (middle Miocene;Fig. 3). The following sedimentological descriptionscorrespond to the Toscal de Mariño and Arroyo AguasBlancas sections (Irigoyen 1997).

Lower member (‘Conglomerados violáceos’: purpleconglomerates). This member comprises two distinctpackages occurring in isolated and discontinuous expo-sures in the study area. The lower part of the unit ischaracterized by purple and greenish fine conglomerateswith sandstone matrix and planar-stratified sandstonebeds that alternate with thinner intervals of massivegrey tuffaceous sandstones and red mudstones

interpreted to represent channel-fill and floodplaindeposits. The upper part consists of grey tuffaceoussandstone beds alternating with red mudstones. Thelithofacies recognized in this member indicate that thesedeposits are of fluvial origin. The upper part of thismember, characterized by the occurrence of mottledsandstone beds, abundant gypsum and carbonate-richmudstone layers with sparse mud cracks, suggests semi-arid conditions where episodes of sandstone accumula-tion alternated with periods of floodplain mud deposi-tion. This lower member attains a thickness of ca 50 min the Tosca de Mariño section (Figs 2, 3).

Middle member (‘Areniscas entrecruzadas’: cross-bedded sandstones). This member typically crops out atthe northern and southern margins of the Potrerillos

Fig. 3. Stratigraphy of the Neogene deposits in the Cacheuta-Tupungato area. Modified from Irigoyen (1997); Stratigraphic nomenclature fromYrigoyen (1993).

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Dam reservoir. It comprises a distinctive ca 180 m thickmonotonous fine- to coarse-grained and well-sortedsuccession of pale brown, parallel-laminated, cross-laminated or massive tuffaceous sandstone beds. Thismember has many sedimentological features of eolianorigin, such as moderate- to high-angle cross-beddingforming wedge planar or tabular planar sets. Low-reliefripples occur on exposed forest surfaces. Large andmedium-scale cross-beds are interpreted to representdune facies. The occurrence of this lithofacies strati-graphically above fluvial sequences (deposited insemi-arid setting) suggests that fluvial input wasprogressively reduced favouring the development of anextensive dune field. The fossil twigs described belowwere probably reworked. A series of in situ undeter-mined vertebrate traces were also recorded in thismember at the southern margins of the Potrerillosreservoir (ex Río Mendoza margins; Figs 2, 3).

Upper member (‘Estratos de Mariño’: marine strata).This member, more than 500 m thick, is well exposedat the southern end of the Río Mendoza Valley, on thehighest hills of the area, the ‘Toscal de Mariño’ and‘Cuchilla del Toscal’ (Figs 1–3). It consists of reddishsandstones and grey conglomerates alternating with redmudstones. The upper member is notable for the abun-dance of sandstone bodies and horizontally stratifiedconglomerates that show variable degrees of channeli-zation; their sedimentary structures are indicative ofbraided stream deposits, under low flow conditions. Thepredominance of parallel-laminated sandstones, in con-junction with other sedimentological features, is sugges-tive of arid or semi-arid conditions in which sporadicand perhaps heavy rainfall favoured episodes of upper-flow-regime discharge (Irigoyen 1997).

Materials and methodsThe fossil twigs were collected from exposures of themiddle member of the Mariño Formation close to thenorthern margin of the Mendoza River, now flooded bythe Potrerillos reservoir (32°57ʹ36ʺS, 69°09ʹ21ʺW; Fig. 1).

The fossil woods were preserved by siliceous per-mineralization. Transverse, tangential and radial thin-section of the woods were prepared for study usingstandard lapidary techniques.

Anatomical terms used in the descriptions follow therecommendations of the IAWA List of Microscopic Fea-tures for Hardwood Identification (IAWA Committee1989). The identification of fossil woods was performedby consulting classical descriptions of extant and fossilplants, such as those of Metcalfe & Chalk (1950),Tortorelli (1956), Rancusi et al. (1987) and Roig &Vidal (2006). In order to compare the samples withextant and fossil species, the InsideWood website (In-sideWood 2004–onwards) was used. The systematicclassification follows the APG I (1998) and APG III

(2009), and the bibliography list is that of Gregory(1994) and Gregory et al. (2009). Authorities and refer-ences to extant plant taxa are available at the Interna-tional Plant Names Index website: http://www.ipni.org/.

The quantitative values provided in the anatomicaldescription are averages from 25 measurements. Mini-mum, maximum and average values are cited to thestandard format of: minimum (average) maximum.

The fossil wood was studied with a Nikon EclipseE200 light microscope, and the photomicrographs weretaken with a Nikon Coolpix S4 digital camera. Thestudied fossil wood and microscope slides are registeredin the ‘Museo de Ciencias Naturales y AntropológicasJuan Cornelio Moyano’ at Mendoza city, under theacronyms MCNAM-PB1393, MCNAM-PB1393 PR1–3, MCNAM-PB1395, MCNAM-PB1395 PR 1–3 andMCNAM-PB1397, MCNAM-PB1397 PR 1–3.

Systematic palaeobotanyOrder LAMIALES Bromhead, 1838Family VERBENACEAE Saint-Hilaire, 1805

Rhaphithamnoxylon gen. nov. (Fig. 4A–K)

Diagnosis. Only secondary wood is preserved. Diffuse-porous wood; vessels predominantly solitary, sporadi-cally in radial multiples of 2–4, in clusters, and in shorttangential rows, small to very small in diameter, withsimple perforation plates; intervessel pits alternate; axialparenchyma sparse and vasicentric; rays 1–3 seriate,weakly heterogeneous.

Etymology. The generic name denotes the woods resem-blance to extant Rhaphithamnus Miers, 1870.

Type species. Rhaphithamnoxylon artabeae sp. nov.

Rhaphithamnoxylon artabeae gen. et sp. nov.(Fig. 4A–K)

Diagnosis. Growth rings distinct; septate and non-septatefibers present; rays heterocellular, composed of procum-bent and upright or square cells, and uniseriate rayshomocellular.

Etymology. The specific epithet is dedicated to Dr AnalíaE.E. Artabe for her important studies on the fossil florasof Argentina.

Material examined. Holotype MCNAM-PB1393,MCNAM-PB1393 PR 1–3.

Type locality. Northern margin of the now floodedPotrerillos reservoir (Río Mendoza Valley, between

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Fig. 4. Rhaphithamnoxylon artabeae gen. et sp. nov. (MCNAM-PB 1393). A, General view of transverse section. B, General view of longitudinaltangential section. C, Solitary vessels (arrows) in transverse section. D, Ray in radial longitudinal section. E, Biseriate (white arrow) and uniseriate(black arrow) rays in tangential longitudinal sections. F, G, Vessels (white arrows) and ray (black arrows) in transverse section. H, Triseriate ray(arrow) in tangential longitudinal section. I, Solitary vessel (white arrow) with sparse, vasicentric parenchyma (black arrow) in transverse section.J, Multiple radial vessels in transverse section. K, Vessel with simple perforation plate (arrow) in tangential longitudinal section. Scale bars: A, B= 300 μm, C–H = 100 μm, I = 50 μm, J, K = 30 μm.

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Cacheuta Peak and Potrerillos village), south-southwest-ern extent of the Precordillera, Mendoza Province,Argentina (32°57ʹ36ʺS, 69°09ʹ21ʺW).

Stratigraphic unit and age. Middle member (‘Areniscasentrecruzadas’: cross-bedded sandstones) of the MariñoFormation; middle Miocene.

Description. The species is based on a single fragmentof secondary wood. The pith, primary xylem, cortexand secondary phloem are absent. In transverse section,the wood reveals distinct growth rings and diffuseporosity (Fig. 4A).

In cross-section, vessels are predominantly solitary(65%), but also occur sporadically in radial multiples oftwo (24%), three (6%), four (4%), in clusters (2%) and inshort tangential rows (Fig. 4A, C, F, G, I, J). Solitary ves-sels are circular to oval in transverse section (Fig. 4I);they have tangential diameters of 17–(35)–60 μmand radial diameters of 22–(35)–53 μm; vessel density is2–(5)–11 per mm2. Vessels have straight end walls withsimple perforation plates (Fig. 4K); intervessel pits verysmall to small (≤ 4 μm) and alternate; vessel-ray paren-chyma pits are similar in size and shape to intervesselpits. Vessels 77–(133)–195 μm long.

Fibers are usually non-septate and rarely septate(Fig. 4E), arranged in radial rows (Fig. 4F), with diame-ters of 7–(14)–20 μm, and medium- to thick-walled:wall thicknesses of 2–(6)–8 μm. Axial parenchyma isscarce and vasicentric (Fig. 4I).

Rays are mostly biseriate (48%), or uniseriate (42%),rarely triseriate (10%; Fig. 4B, E, H); 6–(10)–12 per lin-ear mm. Rays 38–(163)–470 μm high and 7–(26)–55μm wide. Rays 1–(6)–17 cells high and 1–(2)–3 cellswide. Multiseriate rays are heterocellular, composed ofprocumbent, upright or square cells, and uniseriate raysare homocellular (Fig. 4D).

Comparisons with extant species. Verbenaceae is char-acterized by the following combination of anatomicalfeatures: distinct growth rings; very small vessels thatare predominantly solitary, but also sparsely in radialmultiples and in clusters; simple perforation plates;sparse and vasicentric axial parenchyma; and 1–3 seri-ate heterocellular to homocellular rays (Record & Hess1941, Metcalfe & Chalk 1950).

The Mariño fossil wood was compared with variousgenera of Verbenaceae (Table 1) of which Rhaphitham-nus shares the following features: diffuse-porous sec-ondary xylem; distinct growth ring boundaries; vesselssmall (75 μm) to very small (30 μm), fairly numerous,commonly in short radial multiples, evenly distributed;rays 1–3 seriate, heterocellular, body ray cells procum-bent with 1–7 rows of upright and/or square marginal

cells and uniseriate rays numerous and composed ofupright or square cells; parenchyma sparsely paratrac-heal and in a narrow irregular band demarcating growthrings (Record & Hess 1941, Rancusi et al. 1987).

Using the InsideWood database, the following com-bination of features are shared with the Mariño fossilwood: 1p 2a 3a 4a 5p 6a 7a 8a 9a 13p 14a 19a 22p24p 25p 30p 40p 41p 56a 65p 66p 75p 97p. With 0allowable mismatches (see coding of IAWA Committee1989), this yields only one species, Rhaphithamnusspinosus, with a compatible affinity to Rhaphithamnoxy-lon artabeae gen. et sp. nov.

Comparisons with fossil species. The fossil wooddescribed herein was compared with other fossil speciesassigned to Verbenaceae (Table 2; Gregory et al. 2009):Gmelina hubeiensis Yang in Qi et al. 1997 (Neogene,China); Gmelina tertiara Bande, 1986 (Paleogene,India); Gmelina wahanensis Yang in Yang et al.1993 (Neogene, China); Tectonoxylon prambachenseHofmann, 1944 (Oligocene, Austria); Vitecoxylonaethiopicum Lemoigne, 1978 (Miocene, Ethiopia);Vitexoxylon africanum Prakash, Awasthi & Legmoigne,1982 (Mio-Pliocene, Ethiopia); Vitexoxylon indicumIngle, 1972 (Cretaceous to Eocene, India); Vitexoxylonmiocenicum Prakash & Tripathi, 1974 (Miocene, India);Vitex sp. cf. V. simplicifolia Oliver, 1875 (Mio-Pliocene,Ethiopia; Jolly-Saad & Bonnefille 2012).

Rhaphithamnoxylon artabeae differs from Gmelinahubeiensis and Gmelina wahanensis in the type ofporosity and ray width. Abundant tyloses in vessels arecharacteristic of Tectonoxylon prambachense, Vitecoxy-lon aethiopicum and Vitexoxylon africanum but thesewere not observed in the Argentinean fossil wood(Table 2). Characters such as the size of vessels and thepresence of metatracheal parenchyma or banded apo-tracheal parenchyma are present in Vitexoxylon indicumbut they differ from those of the Mariño fossil specimen(Table 2). Vitexoxylon miocenicum has 4–10 seriate raysand larger vessels than those of Rhaphithamnoxylonartabeae. Vitex sp. cf. V. simplicifolia has wider raysthan the fossil studied here. Gmelina tertiara is themost similar fossil species, but differs from the Mariñowood because it has larger vessels and aliform andconfluent axial parenchyma (Table 2).

Order and family INCERTAE SEDIS

Mariño unknown dicot 1 (Fig. 5A–K)

Material examined. MCNAM-PB1395, MCNAM-PB1395 PR 1–3 and MCNAM-PB1397, MCNAM-PB1397 PR 1–3.

Locality. Northern margin of the now flooded Potreril-los reservoir (Río Mendoza Valley, between CacheutaPeak and Potrerillos village), south-southwestern extentof the Precordillera, Mendoza Province, Argentina.

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Stratigraphic unit and age. Middle Member (‘Areniscasentrecruzadas’: Cross-bedded sandstones) of theMariño Formation; middle Miocene.

Description. The fossil fragments consist of two piecesof permineralized twigs (ca 9 cm) with poorly pre-served pith and well-preserved secondary xylem(Fig. 5A). Twigs are laterally compressed or flattenedwith large vessels, readily visible to the naked eye andeccentric pith in cross-section (Fig. 5A). Transversesections reveal indistinct growth rings and diffuse-por-ous secondary xylem (Fig. 5A–C).

Vessels are predominantly solitary (70%), in radialmultiples of two (18%), three (16%), four (1%) and upto 10 vessels (2%), and in clusters (1%; Figs 5A–G).Solitary vessels are circular in transverse section withtangential diameters of 83–(170)–253 μm and radialdiameters of 31–(140)–220 μm; occurring at densitiesof 10–(15)–24 per mm2. Solitary vessels are larger indiameter than in radial multiples or in clusters. Vesselshave walls 9–(16)–27 μm thick. Vessel end walls arestraight or oblique with simple perforation plates(Fig. 5I). Intervessel pits are alternate, very small(≤ 4 μm), with diameters of 3–(5)–7 μm (Fig. 5J);vessel-ray parenchyma pits not observed. Helical thick-enings present only in some narrower vessels; vessellengths of 60–(180)–360 μm.

Fibers are thick-walled with diameters of 11–(17)–26 μm (Fig. 5K). Paratracheal vasicentric axial paren-chyma are complete and in some cases confluent(Fig. 5E, F).

Rays are very difficult to observe owing to the poorpreservation of the material. Only uni- and biseriatewith 11–14 cells high (Fig. 5H) are preserved. Hetero-cellular rays are composed of procumbent and uprightor square cells in radial section.

Comparisons with extant species. The key anatomicalfeatures of the studied material (large and wide vessels,many narrow vessel elements in addition to wide vesselelements, wide vessels with solitary distribution andnarrow vessels grouped in clusters, and helical thicken-ings in vessels) are common features in dicotyledonouslianas or small shrubs.

In particular, the secondary xylem in the lianas,vines and small shrubs includes large and wide vessels,large rays and abundant axial parenchyma, increasingclimbing strategies, water conductivity and storage per-formance (Carlquist 1985, Wheeler & LaPasha 1994,Angyalossy et al. 2012, Masrahi 2014). Vasicentrictracheids, fibriform vessel elements, helical thickeningsin vessels, starch-rich parenchyma adjacent to vesselsand other parenchyma distributions are also present inplants with these growth habits. Many vines and lianashave narrow vessels grouped in clusters in addition to

small,diam

eter:60–

125μm

.Mostly

numerous.Verysm

all

intervesselpits.

form

edby

procum

bent

cells.

Rha

phith

amnusMiers

South

America

Diffuse

ring

porous

insomecases

Distin

ctSolitary,radial

multip

lesandin

clusters.

Smallto

very

small,

diam

eter:75

–30μm

.Num

erou

s.

Uniseriaterays

composed

ofup

righ

tcells

and

multiseriate

rays

compo

sedof

procum

bent

anderectcells

1–3cells

Scarceparatracheal

and

term

inal,Apotracheal

diffuseandscarce

Sparselyseptate

Table1.

Com

parisonof

anatom

ical

characteristicsbetweenRha

phith

amno

xylonartabeae

gen.

nov.

andsp.no

v.andmostrelatedgenera

ofVerbenaceae.

ALCHERINGA MIOCENE FOSSIL WOODS FROM ARGENTINA 9

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Fossilspecies

Porosity

Growth

ring

sVessels

Ray

s

Axial

parenchym

aFibers

Typ

eWidth

Rha

phith

amnoxylon

artabeae

gen.

etsp.no

v.Diffuse

Distin

ctMostly

solitary,

also

inradial

multip

leof

2–4andin

clusters.

Tangentialdiam

eters

17–(35

)–60

μm.2–

(5)–11

permm

2.

Perforatio

nsimple.

Intervesselpits

alternate.

Heterocellularmultiseriate

rays

andhomocellular

uniseriate

rays

1–3cells

Vasicentric

andscarce

Arrangedin

radial

rows,thickwalls.

Gmelinahubeiensis

Yang,

1997

Sem

i-ring

toring

porous

Distin

ctMostly

solitary,

also

inradial

multip

les2–

4.Tangential

diam

eter:90

–140

μm.4–8vesselsper

mm

2.Sim

ple

perforationplates.

Alternateintervessel

pits.

Heterogeneous

Type

III

2–6cells

Vasicentric

andscarce

Thin-walled.

Septate

fibers

present.

Gmelinatertiara

Bande,19

86Diffuse

Distin

ctSolitary,in

multip

leradialsof

2–3

elem

entsandin

clusters.Tangential

diam

eter:50

–180

μm.Radialdiam

eter:

80–2

50μm

.Sim

ple

perforationplates.

Alternateintervessel

pits.

Weaklyheterogeneous

1–3cells

Paratracheal,vasicentric,

aliform

andconfl

uent

Septate

Gmelinawahanensis

Yang,

1993

Sem

i-ring

toring

porous

Distin

ctVesselsmostly

solitary.

Sim

ple

perforationplates.

Alternateintervessel

pits.Abu

ndant

tyloses.

Heterogeneous

Type

III

Multiseriate

Vasicentric,scarce,

paratracheal

andaliform

Thin-walled.

Septate

fibers

present.

Tecton

oxylon

pram

bachense

Hofmann,19

44Distin

ctSolitary,in

multip

leradialsof

2–3

elem

entsandin

clusters.Sim

ple

perforationplates.

Tylosespresent.

Hom

ocellular

1–3cells

Diffuse

and

paratracheal

Thick-w

alled

10 MARÍA JIMENA FRANCO et al. ALCHERINGA

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Vitecoxylonaethiopicum

Lem

oign

e,19

78Diffuse

Solitary,in

multip

leradialsof

2–5

elem

ents,sm

all.2–5

vesselspermm

2.

Sim

pleperforation

plates.Bordered

intervesselpits.

Abu

ndanttyloses.

Heterogeneous

1–3cells

Diffuse

andvasicentric

Non

-septate

Vitexoxylonafricanu

mPrakash,Awasthi&

Legmoign

e,19

81

Diffuse

Distin

ctSolitary

andmultip

leradial

elem

entsof

2–5elem

ents.

Tangentialdiam

eter:

120–36

0μm

.Radial

diam

eter:60

–400

μm.Vessels5–

7per

mm

2.Sim

ple

perforationplates.

Alternateintervessel

pits.Ty

losespresent.

Hom

ocellular

toheterocellu

lar

1–3seriate,

mostly

biseriate.

Paratrachealvasicentric

incomplete.

Apo

traqueal

inbandsof

2–3cells

atthe

beginn

ingof

thegrow

thring

Arrangedin

radial

rows.Septate.

Vitexoxylonindicum

Ingle,

1972

Diffuse

Distin

ctMostly

solitaryor

inradial

grou

psof

2–3

elem

ents.Tangential

diam

eter:90

–153

μm.Radialdiam

eter:

99–1

53μm

.8–20

vesselspermm

2.

Sim

pleperforation

plates.Verysm

all

bordered

intervessel

pits.

Hom

ocellular

1–3cells

Paratrachealandmetatracheal

Arrangedin

radial

rowsSeptate.

Vitexoxylonmiocenicum

Prakash

&Tripathi,19

72Diffuse

Distin

ctTangentialdiam

eter:

100–20

0μm

.Sim

ple

perforationplates.

Alternateintervessel

pits.

Hom

ocellularto

heterocellu

lar

4–10

seriate

Scantyparatracheal,vasicentric

Septate

Vitexsp.cf.simplicifo

liaOliv

er,18

75Diffuse

Indistinct

Mostly

solitary,

insomecasesin

radial

multip

lesof

2or

3.Tangentialdiam

eter:

31–1

94μm

.9–14

vesselspermm

2.

Sim

pleperforation

plates.Alternate

intervesselpits.

Heterocellular

multiseriate

rays

andhomocellular

uniseriate

rays

1–5cells

Paratracheal,vasicentric,

probably

confl

uent

Septate

Table2.

Com

parisonof

anatom

ical

characteristicsbetweenVerbenaceae

fossilgenera

andRha

phith

amno

xylonartabeae

gen.

nov.

andsp.no

v.

ALCHERINGA MIOCENE FOSSIL WOODS FROM ARGENTINA 11

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014

Fig. 5. Mariño unknown dicto 1 (MCNAM-PB 1395 and MCNAM-PB 1397). A, Transverse section showing indistinct growth rings and diffuseporous secondary xylem of MCNAM-PB 1395; the arrow indicates poorly preserved pith. B, Transverse section showing indistinct growth rings anddiffuse porous secondary xylem of MCNAM-PB 1397; C, Transverse section showing solitary and radial multiple vessels of four or more vessels(arrow) of MCNAM-PB 1397. D, Narrow and wide vessel elements of MCNAM-PB 1395. E, Radial multiple vessels of MCNAM-PB 1397. F, Trans-verse section showing solitary and radial multiple vessels of MCNAM-PB 1397. G, Vessels in clusters of MCNAM-PB 1395. H, Uniseriate ray (arrow)of MCNAM-PB 1397. I, Vessel with simple perforation plate (arrow) of MCNAM-PB 1397. J, Alternate invervessel pits of MCNAM-PB 1395. K,Detail of fibers (arrow) of MCNAM-PB 1395. Scale bars: A, B = 1000 μm, C = 300 μm, D, E, G, I = 100 μm, F = 200 μm, H = 50 μm, J, K = 30 μm.

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wide vessel elements as in the case of analysed wood(see Fig. 5D). These narrow vessels may take the formof fusiform cells, slightly wider than imperforate trache-ary elements, and they may function as mechanical sup-port, water storage cells and contribute to hydraulicstability (Gutiérrez et al. 2009). Lianas, vines and smallshrub plants have solitary broad vessels, whereas nar-row vessels are commonly distributed in clusters(Gutiérrez et al. 2009).

The presence of helical thickenings in vessels iscommon in woody temperate plants, chiefly those ofdry habitats and cold areas. Vines and lianas tend not tooccur in extremely cold or dry climates, although theymay occur where low temperatures and drought are epi-sodic and moderate (Carlquist 1985). The presence ofthickening of xylem walls provides strength and reducesthe risk of cell collapse and ensuing embolism in theadvent of high tension created by severe water deficitand high evapotranspiration (Masrahi 2014). Perforationplates in vessels of vines and lianas are predominantlysimple; this accords with the idea that simple platesoffer the least impediment to water flow, and therebypromote efficient conductivity. With respect to wall pit-ting in longitudinal sections of vessels, alternate pitsmay have greater selective value in self-supportinggrowth forms than in vines or lianas (Carlquist 1991).The presence of simple perforation plates in these fossilspecimens should enhance the conductivity of waterand reduce resistance to water flow, whereas the thick-ening of xylem walls provides strength against collapseunder increased tension (Carlquist 1991, Masrahi 2014).

Fabaceae, Sapindaceae and Bignoniaceae includemany vines with diffuse-porous wood or having indis-tinct growth rings, vessels in radial multiples of four ormore and commonly arranged in clusters, with simpleperforation plates, alternate intervessel pits and paratrac-heal parenchyma (InsideWood, 2004–onwards). Bignon-iaceae and Leguminosae are two of the most liana-richfamilies (Angyalossy et al. 2012).

Comparisons with fossil species. Just two fossil vinesor liana species have been recorded from Argentina.Lardizabaloxylon lardizabaloides Schönfeld, 1954 fromeither Upper Cretaceous or Palaeogene strata of Pata-gonia was closely related to Lardizabala Ruiz & Pav.of the Lardizabalaceae (Schönfeld 1954). This liana spe-cies differs from the fossil studied herein because it hasprimary and secondary xylem, and most importantlyincorporates widely spaced vascular wedges separatedby broad medullary rays (Schönfeld 1954, Tidwell et al.2010).

Lutz & Martínez (2007) described Ranunculodendronanzoteguiae, a fossil vine from the upper Miocene ofthe Palo Pintado Formation, northwestern Argentina.Ranunculodendron anzoteguiae has eccentric pith, withfour protoxylem vessels and secondary xylem dividedinto wedges by parenchymatous rays, all surrounded by

cambium, phloem and cortex (Lutz & Martínez 2007).The Mariño fossil differs from it in the great vesseldiameters and the presence of vessels in clusters andradial multiples of four or more.

Unfortunately, the preservation of this type of fossilwood is fairly poor, and it could not be assigned to anyknown taxon. It is difficult to distinguish between inter-vessel pits, vessel-ray parenchyma pits and ray cells.Fibers are poorly preserved, and so their lumens aredifficult to observe.

DiscussionPalaeoenvironmental setting

The Miocene vegetation history of Patagonia and themid-west of Argentina is poorly known. Fossil woodsand phytolith assemblages from the Miocene are scarce(Schönfeld 1954, Hünicken 1995, Schöning & Bandel2004, Zucol et al. 2007, Brea et al. 2012) but palyno-logical evidence from the middle–late Miocene ofnortheastern Patagonia (Guler et al. 2001, 2002, Guler2003, Palazzesi & Barreda 2004, Barreda & Palazzesi2007) documents a flora characterized by an increasingdiversity and abundance of xerophytic taxa, includingAsteraceae, Amaranthaceae and Convolvulaceae.Expansion of these xerophytic taxa, coupled withextinctions of megathermal/non-seasonal vegetation,would have been associated with both tectonic and cli-matic forcing factors that promoted the development ofaridity and extreme seasonality. These arid-adapted lateMiocene floras are very similar to the modern Patagoniavegetation characterized by widespread steppe in extra-Andean Patagonia and with forest restricted to the wes-tern humid upland regions of the Patagonian cordillera(Barreda & Palazzesi 2007).

The Mariño Formation sedimentary succession wasdeposited in a complex low-gradient fluvial systemwhere arid or semi-arid conditions prevailed. Evidencefor this includes the occurrence of oxidized floodplaindeposits, incipient calcrete bands and fluvial sandstonelithofacies that suggest deposition under ephemeralflow regimes, and a thick succession of eoliandeposits.

Verbenaceae biogeography

Verbenaceae includes about 36 genera and ca 1035species of trees, shrubs, and some lianas and herbs(Schweingruber et al. 2013). They are particularlyimportant components of arid to semi-arid communitiesin North and South America where they are locallydominant in some communities, but they are also pres-ent in wet and dry tropical forests, high Andean grass-lands and cloud forests (Marx et al. 2010, Olmstead2013). A secondary centre of distribution is found inAfrica (Olmstead 2013).

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Distributions within the New World extend fromPatagonia in southern South America to Canada,although very few species distributions extend north ofthe arid southwestern United States. Generic diversity isat its greatest in southern South America, presumablybecause the Andean uplift contributed to arid environ-ments in what is now Argentina, where several cladeshave diversified, and to more mesic ecosystems nearbyin northern Argentina that host representatives of manyof the more wet-tropical groups (Marx et al. 2010). Thedistribution pattern described for Verbenaceae is similarto that of Bignoniaceae and Solanaceae, which origi-nated and diversified initially in the New World. Big-noniaceae and Verbenaceae have unequivocally SouthAmerican ancestral areas and diversified throughout theNew World, including North America, but also estab-lished toeholds on other continents, primarily in theSouthern Hemisphere (Olmstead et al. 2009, Marxet al. 2010, Olmstead 2013).

Divergence time estimates for the crown of the fam-ily are inferred to date to the Late Cretaceous or Paleo-gene, after the separation of South America from Africa(Olmstead 2013).

Rhaphithamnus species are spinescent shrubs. Thisgenus consists of two species restricted to southernSouth America, in a longitudinal range of 45–30°S.Rhaphithamnus spinosus (Juss.) Moldenke occurs inmainland Chile and adjacent Argentina (Moldenke1937); R. venustus (Philippi) Robinson, 1916 is ende-mic to the two islands (Masatierra and Masafuera) ofthe Juan Fernández Archipiélago (Sun et al. 1996, Marxet al. 2010, Olmstead, 2013).

The close distributional and morphological affinitiesof the Rhaphithamnus spinosus and R. venustus suggestthat at one time, they might have formed a single gen-eric complex throughout southern South America. Pro-gressive drying probably developed as a result of thesouthern Andean orogeny and subsequent Pleistoceneglaciation. These events gave rise to the vast, arid Ata-cama Desert in northern Chile, which may have isolatedthe populations and led to generic differentiation (Sunet al. 1996).

ConclusionsRhaphithamnoxylon artabeae gen. et sp. nov. (Verbena-ceae) has been erected based on a single sample ofwell-preserved fossil from Miocene strata of Argentina.It is very similar to extant Rhaphithamnus anatomically.The other fossil twigs found at the same stratigraphiclevel and described under open nomenclature are similarto the woods of dicotyledonous vines, lianas and smallshrubs. These are the first palaeobotanical records fromthe Miocene Mariño Formation.

The occurrence of Verbenaceae wood with affinityto Rhaphithamnus in the Mariño Formation is not sur-prising given the modern distribution of the latter is

restricted to 45–30°S. This fossil wood is affiliated withVerbenaceae and adds to the sparse fossil record of thisfamily.

AcknowledgementsThe authors are grateful to the authorities of the Museode Ciencias Naturales y Antropológicas ‘Juan CornelioMoyano’ of Mendoza city, Argentina, for grantingaccess to the collections and for having generouslyarranged the loan of the fossil material. The authorsexpress their thanks to Leandro C.A. Martínez (CONI-CET-Museo Argentino de Ciencias Naturales, BuenosAires, Argentina) and Adriana C. Kloster (Centro deEcología Aplicada del Litoral, CECOAL-CONICET,Corrientes, Argentina) for providing literature. Theauthors would like to express their thanks to the twoanonymous reviewers for their valuable help in provid-ing critical and constructive comments. We are alsograteful to the Editor, Steve McLoughlin, for his obser-vations, comments, corrections of the text and his con-structive criticism of the previous version of thismanuscript.

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