Scholarly article on topic 'Floristic changes in the Iberian Peninsula and Balearic Islands (south-west Europe) during the Cenozoic'

Floristic changes in the Iberian Peninsula and Balearic Islands (south-west Europe) during the Cenozoic Academic research paper on "History and archaeology"

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Academic research paper on topic "Floristic changes in the Iberian Peninsula and Balearic Islands (south-west Europe) during the Cenozoic"

Journal of Biogeography (J. Biogeogr.) (2009) 36, 2025-2043

ORIGINAL ARTICLE

Floristic changes in the Iberian Peninsula and Balearic Islands (south-west Europe) during the Cenozoic

Jose M. Postigo Mijarra1 , Eduardo Barron2, Fernando Gomez Manzaneque1 and Carlos Morla1

'Unidad de Botánica, Escuela Técnica Superior de Ingenieros dé Montes, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, Madrid, Spain and 2Instituto Geológico y Minero de España, Róos Rosas, Madrid, Spain

"Correspondence: José María Postigo Mijarra, Universidad Politécnica de Madrid, ETSI Montes, Unidad de Botanica, Ciudad Universitaria s/n, 28040 Madrid, Spain. E-mail: jm.postigo@upm.es

ABSTRACT

Aim The aim of this work was to identify the main changes in the flora and vegetation of the Iberian Peninsula over the Cenozoic Era, to record the disappearance of taxa associated with these changes and to determine the influence of climate and human activity on these events.

Location The Iberian Peninsula and Balearic Islands.

Methods A critical review was made of the palaeobotanical literature with the aims of detecting patterns of floristic change and extracting information on the disappearance of different taxa over the Cenozoic. These data are viewed alongside the most recent data for the climate of this period. A critical analysis is made of the role of Palaeotropical and Arctotertiary taxa in the forest communities of the Iberian Peninsula throughout the Cenozoic.

Results Although the Eocene-Oligocene transition was a time when many taxa disappeared, the most outstanding events occurred between the end of the Oligocene and throughout the Miocene. Substantial floristic changes took place over this period, including the disappearance of 177 Palaeotropical taxa. This was probably related to acute cooling and aridification; no evidence exists that the Messinian Salinity Crisis had any important effect in the Iberian Peninsula. The last great disappearance of Palaeotropical taxa (36 in total) ended in the Middle-Late Piacenzian; Arctotertiary taxa were most affected during the Pleistocene. The Lower-Middle Pleistocene transition, best represented by marine isotopic stages (MIS) 36-34 and 20-18 and characterized by a change in glacial cyclicity, was the time of the last notable disappearance of taxa.

Main conclusions This work provides the first chronogram of extinctions for the Iberian flora, and records the disappearance of 277 taxa during the Cenozoic. A clear relationship was detected between the main climatic events and the latest appearances of the different taxa.

Keywords

Balearic Islands, Cenozoic, floristic change, historical biogeography, Iberian Peninsula, palaeobotany, palaeoclimate, palaeoecology, plant geography, Quaternary.

INTRODUCTION

The palaeobotanical data provided by outcrops from around the Iberian Peninsula (Figs 1-3) suggest its Cenozoic flora to have been highly diverse and composed of taxa very different from those of today. The modern flora of the Iberian Peninsula is thus the product of the appearance and

disappearance of taxa during the Cenozoic. Climate, geology and ecology must all have had a direct impact on the survival of the different elements. Many studies on changes in the Cenozoic climate have been published in recent years, and the main trends have been established at a global level (Zachos et al., 2001; Lisiecki & Raymo, 2005; Mosbrugger et al., 2005). Determining the influence of these past

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doi:10.1111/j.1365-2699.2009.02142.x

episodes of change on the then existent flora, however, is a more complex matter.

Numerous studies on the palaeobiogeographical features of the Cenozoic flora of Europe - mainly of the Neogene - have been undertaken (Mai, 1989, 1991, 1995; Suc, 1989; Suc et al., 1992), although the characteristics of the palaeofloras of western Eurasia remain largely unknown (Utescher et al., 2007). Several different vegetation units have been described in Europe for these periods. Engler (1879-1882) was the first to use the terms 'Palaeotropical' and 'Arctotertiary' flora,

although it was Mai (1989, 1991) who proposed that these terms be used to describe ecological units (or geofloras). According to Mai, a 'Palaeotropical geoflora' was present in Europe from the Late Cretaceous to the Late Miocene, consisting of paratropical rain forests, subtropical rain and laurel forests, temperate laurel forests and laurel-conifer forests (the latter constrained by edaphic factors). At the same time, an 'Arctotertiary geoflora' existed, represented by warm temperate rain forests, lowland and swamp forests - the ancestors of mixed mesophytic forests. Later, for the

Figure 3 Quaternary sites of the Iberian Peninsula and Balearic Islands. Dark grey indicates areas with a high number of fossil localities; these are mainly in northern Iberia and the centre of the peninsula (Spanish Central Range). There is a notable lack of fossil records in the inner basins.

Palaeogene, Collinson & Hooker (2003) proposed: a polar, broad-leaved deciduous forest for the Palaeocene-Eocene; broad-leaved, paratropical to subtropical evergreen rain forests for the Late Palaeocene to the Eocene; and a broad-leaved, mixed deciduous and evergreen forest plus vegetation with sclerophyllous elements for the Late Eocene and Oligocene. Finally, Utescher & Mosbrugger (2007) and Utescher et al. (2007) describe, through the use of plant functional types, a large number of phytosociological classes for both the Palaeogene and Neogene.

Different floristic elements coexisted in the Iberian Peninsula for long periods of time, leading to complex mosaics of vegetation (Mai, 1989, 1991; Palamarev, 1989; Costa et al., 2005). Indeed, the Iberian Peninsula's fossil record is witness to this mixing of plant types. This is further corroborated by the modern-day existence of a significant group of species of Palaeotropical origin, the distribution of which is very localized and restricted to Iberia (Castroviejo et al., 1986; Costa et al., 2005).

The aims of the present work were to describe the main floral changes of the Iberian Peninsula registered in the Cenozoic, in the context of the available palaeoclimatic data, and to determine the origin of the peninsula's modern-day flora.

MATERIALS AND METHODS

Palaeobotanical data for the Cenozoic of other regions of the Northern Hemisphere are much more abundant than for the

Iberian Peninsula (Hably, 1987; Wolfe, 1992). Neither in Spain nor in Portugal have Cenozoic plant fossils enjoyed the traditional interest they have attracted in other European countries; the majority of Iberia's Tertiary floras are therefore only incompletely known (Barron et al., 1996). Nonetheless, those studies that have been published are of great palaeogeographical interest and provide very valuable data on the flora of south-western Europe and its evolution.

The present work involved an exhaustive review of the palaeobotanical studies of the Cenozoic in the Iberian Peninsula and the Balearic Islands (for convenience these will be referred to collectively here as the Iberian Peninsula or Iberia) (Figs 1-3), with the aim of describing the floristic changes that occurred during this time. A chronogram was constructed to reflect these data (Fig. 4). This included all the taxa recorded in the different Cenozoic outcrops so far examined. These taxa were mainly identified from their pollen, spores and leaf and wood remains; carpological studies of the Iberian Cenozoic are scarce (Alvarez Ramis, 1975; Menendez Amor, 1975; Gregor & Gunther, 1985; Cabrera et al., 1994; Barron, 1996; Barron & Dieguez, 2001; Martin-Closas et al., 2006), except for those performed on Holocene materials associated with archaeological remains (Ruiz et al., 1997). Similarly, very few studies containing data on flowers or inflorescences have been published (Barron, 1996; Barron & Dieguez, 2001; Postigo Mijarra et al., 2003).

Aegiceras/Brownlowia/Pelliceria AJangium Loropetalum Maasa Sesuvium Heteromorpha Normapollas Rourea Sfaphylea

Gossypium/Thespesia Phaleria- type Carpodlptera Stachytarpheta Calophyllum Koenigia Sinowilsonia Gleicheniaceae Podocarpus Bignoniacaae Ochnaceae Palaeogene aquatic tropical plants Tropical/subtropical Lythraceae Toumefortia Olacaceae

Tropical/subtropical Anacardiaceae Balanophora Commelinaceae Dennstaedtiaceae

Diphylleia/Nandina/Podophyllum

Helwingia Leptopteris Lithocaqjus/Pasania Mastixia Oligocene tropical group Photinia-t/pe Sladania-type Tetracentron Pandanusf Trigonobalanus Berchamia Chloranthaceae Corylopsis/Fothergilla Mahonia Catocedrus Disanthus/Exbucklandia Loranthus Tramophyllum Acanthaceae °Combretaceae Malpighiaceae

Grewia/Corchorus Zingiberaceae Icacinaceae Theaceae (non Pelliciera) Rest Ion aceae Avicennia

Tropical Apocynaceae Meliaceae Agavaceae Neurada Cycadaceae Nypa Myreinaceae °Simaroubaceae °Sterculiaceae

Palaeocene Dan Se Tha

Palaeogene

Eocene Y | Lu | Ba I Pri

Oligocene Ru Ch

Neogene

Miocene

Aq Bur Lan Ser Tor Mes

Pliocene Zan Pia Gel

Pleistocene L M | U I

Figure 4 Disappearance chronogram for the main plant taxa of the Iberian Peninsula and Balearic Islands during the Cenozoic. Symbols: *, taxa exclusively identified from megaremains; taxa identified from both pollen and megaremains (taxa without a symbol are identified only from palynomorphs); age at which a particular taxon was documented from megaremains; dotted line, time interval for which a given taxon has not yet been identified. Abbreviations: Dan, Danian; Se, Selandian; Tha, Thanetian; Y, Ypresian; Lu, Lutetian; Ba, Bartonian; Pri, Priabonian; Ru, Rupelian; Ch, Chattian; Aq, Aquitanian; Bur, Burdigalian; Lan, Langhian; Ser, Serravalian; Tor, Tortonian; Mes, Messinian; Zan, Zanclean; Pia, Piacenzian; Gel, Gelasian; L, Lower Pleistocene; M, Middle Pleistocene; U, Upper Pleistocene; H, Holocene.

Palaeogene Neogene

Palaeocene Eocene Oligocene Miocene Pliocene Pleistocene

Dan Se Tha Y Lu Ba Pri Ru Ch Aq Bur Lan Ser Тог Mes Zan Pia Gel L M и H

°Tropical/subtropical Rutaceae \ S,

Nyctaginaceae Colporate Buxus

Calpumia/Sophora Cercidjphyllum *Ficus (non F. caпса)

*Tormya Tropical Euphorbiaceae Tropical/subtropical Rubiaceae Melastomataceae

Ginkgo?

°Palmae (non Chamaerops and Nypa) \ \

Si \ Si V

°Magnoliaceae Platycarya \ \ \ \ V

°Caesalpiniaceae (non Catatonia and 4 N N N N N > y

Gledltsla) Monispormaceae Calastrus

°Pittosporvm N

°Saplndaceae Disty/ium Microtropis Notholaea Lauraceae (non Laurus) \ \ \ s, \

> \ \ > \

Parmtiopsis Rhoiptelea

*Rhizophora

S, s. 4 \ s,

°Zelkova *Cupressus Vigna °Engelhardia \ ч \ \

Sciadopitys

\ \ S \

Symplocos °Araliaceae (non Hedeia) Cathaya Vitaceae (non Vitis) °Liquidamber °Tsuga

\ S 4

°Ptemcarya \ N

s. s, S,

*Aesculus °Cerya N —

\ N N 4 N

Pinus type haploxykm

\ N N N

Syringe > N

Cedrus 9

Figure 4 Continued

All taxa identified from the earliest to the latest reports were considered (Heer, 1880, 1881; Bellot & Vieitez Cortizo, 1945). With the exception of a few cases mentioned in the

text, the ages and diagnoses attributed by the different authors consulted were respected. Notwithstanding, some interpretations are our own since botanical systematics and

Table 1 Genera that disappeared from the Iberian Peninsula and Balearic Islands during the Cenozoic grouped by their time of disappearance and listed within their respective families (as represented in Fig. 4). EOCENE Acanthaceae: Ruellia

Apocynaceae: Alstonia-type and Parsonsia Araliaceae: Tupidanthus-Schefleropsis Caesalpiniaceae: Daniellia and Julbernardia-type Euphorbiaceae: Amanoa, Austrobuxus-Dissilaria, Centroplacus,

Croton and Drypetes Icacinaceae: Gonocaryum-type Menispermaceae: Pachygone Ochnaceae: Schurmansia Palmae: Daemonorops and Sclerosperma Rubiaceae: Hedyotis, Oldendandia and Paedicalyx Rutaceae: Evodia-type OLIGOCENE

Araliaceae: Acanthopanax, Brassaiopsis, Gilbertia, Heteropanax, Panax, Polyscias, Schefflera, Scheffleropsis and Tetrapanax Caesalpiniaceae: Crudia and Tamarindus Cupressaceae: Chamaecyparis? Ebenaceae: Euclea

Euphorbiaceae: Antidesma, Claoxylon, Homalanthus, Macaranga, Mallotus, Spondianthus and Phyllantus Icacinaceae: Desmostachys

Lauraceae: Lindera, Litsea, Nectandra and Phoebe Meliaceae: Aglaia-type

Menispermaceae: Arcangelisia-type, Cocculus,

Tiliacora-type and Tinomiscium-type Nyctaginaceae: Pisonia Ochnaceae: Ochna Olacaceae: Olax

Palmae: Areca, Arenga?, Borassus, Calamus, Caryota-type, Flagellaria, Lepidocaryum-type, Oncosperma and Trachycarpus Rubiaceae: Psychotria

Rutaceae: Clausena, Fagara, Orixa-type and Ptelea Sapindaceae: Litchi-type Sapotaceae: Chrysophyllum Schizaeaceae: Anemia/Mohria Sterculiaceae: Sterculia

Taxodiaceae: Metasequoia? and Sequoiadendron Theaceae: Gordonia-type Vitaceae: Ampelopsis and Cayratia MIOCENE Apocynaceae: Echitonium Araliaceae: Aralia

Caesalpiniaceae: Caesalpinia and Cassia Combretaceae: Terminalia Cycadaceae: Zamia

Euphorbiaceae: Alchornea, Jathropa, Mussaenda and Ricinus

Fabaceae: Dalbergia

Lauraceae: Neolitsea

Malpighiaceae: Hiraea

Myrsinaceae: Ardisia and Myrsine

Palmae: Sabal

Rubiaceae: Cephalanthus

Rutaceae: Toddalia and Zanthoxylon

Salviniaceae: Salvinia

Sapotaceae: Mimusops

Table 1 Continued

Schizaeaeceae: Lygodium Simaroubaceae: Ailanthus and Picrasma Sterculiaceae: Byttneriophyllum and Reevesia Taxodiaceae: Cryptomeria Vitaceae: Cissus and Leea

the study methods used in palaeobotany have substantially changed since the 19th century (Jones & Rowe, 1999). Hence, the true presence of certain taxa cited for the Iberian fossil record is questioned, and these dubious taxa have not been included in the figures or tables showing taxa that have disappeared. For example, some fossils taken to indicate a taxon show ambiguous morphology; such is the case for the remains of one Oligocene leaf from Mallorca attributed to the genus Cercis (Colom, 1983), which, based on its venation pattern, could be assigned to a member of the genus Daphnogene (Lauraceae). Further, the requirements of some cited taxa make their presence in Iberia in the associated geological period unthinkable; e.g. Larix, an Arctotertiary genus currently with a boreal or Alpine distribution, is cited by Cavagnetto (2002) for the Oligocene in the As Pontes Basin, but at that time the region had a tropical/subtropical climate. Finally, certain typical Gondwanan genera (Banksia, Protea, Dryandroides and Conospermum) represented by large numbers of fossils have long been attributed to the family Proteaceae (Heer, 1855-1859; de Saporta, 1863). However, morphological and anatomical studies would seem to relate these finds to the family Myricaceae (Ferguson, 1971) or to certain members of the Juglandaceae (Menendez Amor, 1955; Jungwirth, 2004). Further, no pollen grains that could be related to this family have yet been found in the Iberian Peninsula.

The taxa cited in palynological studies may sometimes appear controversial. Few reports actually contain photographs or other graphical representations of palynomorphs (for example Suc, 1980; Pais, 1981; Van Campo, 1989; Rivas Carballo et al., 1994; Barron, 1996; Barron & de Santisteban, 1999; Cavagnetto, 2002; Barron et al., 2006a). In addition, in most of these studies (i.e. except for those of Pais, 1981; Van Campo, 1989; Barron, 1996), identifications have been based on traditional light microscopy (LM) observations; this is a problem since some taxa are only distinguishable by scanning electron microscopy (SEM). This is true of some Hamamelidaceae species belonging to the genera Corylopsis, Distylium and Hamamelis, the pollen grains of which are of similar morphology in LM (see the pollen databases of the Florida Institute of Technology and the University of Arizona: http://research.fit.edu/paleolab/ pollen.php and http://www.geo.arizona.edu/palynology/ polonweb.html, respectively). These Hamamelidaceae identifications, plus those referring to Taxodiaceae, have, nevertheless, been included (Suc, 1980; Diniz, 1984; Bessedik,

1985; Cavagnetto & Anadon, 1996; Jimenez Moreno & Suc, 2007; Jimenez Moreno et al., 2007). All these identifications, however, require confirmation by SEM studies, since the modern species of these genera do show some exclusive pollen characteristics.

Table 1 shows the genera belonging to the families represented in Fig. 4. With the aim of summarizing as many data as possible in this figure, certain sets of taxa with clear ecological affinities were grouped together (e.g. as 'Palaeogene aquatic plants' or within the 'Oligocene tropical group'). The genera included in these groups are shown in Tables 1 and 2. On other occasions groups were formed following taxonomic criteria, e.g. Aegiceras/Brownlowia/Pelliceria, Gossypium/Thes-pesia, Disanthus/Exbucklandia, Lithocarpus/Pasania, Diphyl-leia/Nandinal Podophyllum and Corylopsis/Fothergilla. The studies in which these taxa are cited are recorded in Appendix S1 in Supporting Information. In general, the text refers to genera and families according to the traditional botanical taxonomic criteria of Cronquist (1981). For the construction of the chronogram, the scales proposed by Calvo et al. (1993) and Gradstein et al. (2004) were used. The beginning of the Quaternary was considered to be 2.58 Ma, the current boundary of the Gelasian Stage (Gradstein et al., 2004; Pillans, 2004; Clague, 2006). The change of the Quaternary limit from 1.8 to 2.58 Ma allows the inclusion of areas considered to be of Tiglian or Plio-Pleistocene age within the early Quaternary (Pillans, 2004; Clague, 2006). Phytochorological criteria were used to define the two large biogeographical units of the Iberian Peninsula, following, as much as possible, the proposed nomenclature of Ebach et al. (2008).

Table 2 Genera and families of taxa that disappeared from the Iberian Peninsula and Balearic Islands during the Cenozoic based on the taxon groups shown in Fig. 4.

Palaeogene aquatic tropical plants

Oligocene tropical group

Tropical/subtropical

Anacardiaceae Bignoniaceae

Tropical/subtropical Lythraceae

Aponogeton (Aponogetonaceae), Halogaris (Halogaraceae) and Hydrocleys (Limnocharitaceae) Acrostichum (Pteridaceae), Albizia (Mimosaceae), Canna (Cannaceae), Castanopsis (Fagaceae), Diervilla (Caprifoliaceae), Eugenia (Myrtaceae), Fothergilla (Hamamelidaceae), Fragaea (Potaliaceae), Glehnia (Umbelliferae), Gynandriris (Iridaceae), Hemsleya (Cucurbitaceae), Heritiera (Malvaceae), Hydrocotile (Umbelliferae), Rhyticarpus (Umbelliferae) and Siphonodon (Celastraceae) Lannea, Mangifera and Spondias

Adenocalymna, Spathodea, Stereospermum

and Tecomaria Decodon, Lawsonia-type, Pemphis, Rotala and Woodfordia

RESULTS AND DISCUSSION

The Palaeocene-Late Eocene interval and the Eocene-Oligocene transition

Few palaeobotanical studies are available for the Eocene and Oligocene periods, and nearly all are palynological in nature and mostly restricted to materials from the Ebro Basin and the Betic and Pyrenean mountain ranges (Fig. 1). During the Upper Cretaceous the territories that today form part of the Iberian Peninsula were populated by Palaeotropical forests of high floral diversity (de Porta et al., 1985; Mai, 1991; Vicente i Castells, 2002). Of particular importance in their composition were plants producing Normapolles pollen grains, possibly assignable to primitive Fagales (Schonenberger et al., 2001; Friis et al., 2006).

According to sedimentological and faunal evidence and the presence of ferralitic crusts and bauxitization processes, the climate that developed during this period was tropical, probably with seasonal rainfall (Lopez-Martinez, 1989). During this period progressive substitutions of different types of Normapolles are observed, which must be related to the general evolution of this group in Europe. The Normapolles group last appeared in the Upper Eocene at Sosis (Lerida) (de Sitter, 1961).

Although the Palaeobotanical record for the Iberian Eocene is rather sketchy, it appears to indicate the disappearance of Normapolles as well as 31 Palaeotropical genera (Fig. 4, Tables 1 and 2) that now inhabit tropical and subtropical regions, mostly in eastern Asia. These include typical mangrove taxa such as Aegiceras, Brownlowia and Pelliceria (for which the only record known in Iberia is restricted to the Bartonian), Julbernardia-type taxa and Daniellia (members of the family Caesalpinaceae today restricted to savanna environments) and genera such as Alangium that show a continuous record throughout the Palaeogene and Neogene in other parts of Europe (Nagy, 1985; Gastaldo et al., 1998; Knobloch & Konzalova, 1998; Kovar-Eder et al., 1998; Mai, 1998). Their disappearance from the fossil record may be related to the notable climatic changes that took place during the second half of the Eocene and which culminated in the Eocene-Oligocene transition (EOT).

The growing trend towards aridity in the Late Eocene, manifested mainly in coastal areas, was probably caused by oceanic regression that generated a continentalization of the Eurasian climate along with a global fall in temperature (Lopez-Martinez, 1989; Mai, 1989). From the Early Eocene climatic optimum (50-52 Ma) until the start of the Oligocene (34 Ma) there was a progressive fall in temperature that lasted 17 Myr before culminating in the EOT (about 34 Ma) and a long glaciation (lasting 400 kyr) - the Oi-1 glaciation - that coincided with the appearance of an ice cap in the Antarctic (Miller et al., 1991; Zachos et al., 2001; Mosbrugger et al., 2005).

The palynological record of the Ebro Basin allows the identification of the first signs of floristic change associated

with the EOT in the Bartonian-Priabonian. Mangroves disappeared during this period, and plants associated with dry climates and open country, such as Ephedra and Caesalpiniaceae, increased. Barringtonia, Boraginaceae, Chenopodiaceae, Combretum, Linum, Plumbaginaceae and Thymelaeaceae also appeared (Cavagnetto & Anadon, 1996). The Early Oligocene ecosystems of the Ebro Basin detected in Unit T3 also show an important reduction in mega-meso-thermal and megathermal taxa. The appearance of the genus Acacia and the abundance of Cupressaceae pollen suggest a new phase of the dry period started in the Priabonian (Cavagnetto & Guinet, 1994; Cavagnetto & Anadon, 1996).

Stepwise changes in the Late Oligocene-Late Miocene

From the Late Eocene onwards the terrestrial climate became colder and drier, leading to important changes in the flora of Central Europe and North America (Wolfe, 1978, 1992; Collinson, 1992; Knobloch et al., 1993). Both the marine and terrestrial isotope records of Central Europe reveal a marked fall in mean annual temperatures during the Oligocene compared with the Eocene (Fig. 5) (Zachos et al., 2001; Mosbrugger et al., 2005). This cooling was maintained until the end of the Oligocene when a new warming began (the Late Oligocene Warming), although this was followed by a brief but strong glacial maximum (lasting 200 kyr) at the Oligocene-Miocene boundary (23 Ma) known as Mi-1. This was followed

by a series of less intense, intermittent glaciations (Miller et al., 1991; Paul et al., 2000; Zachos et al., 2001; Billups et al., 2004). According to Mosbrugger et al. (2005), this cooling was especially noticeable during winter, with marked thermal seasonality apparent at certain times during the Oligocene and Miocene.

Palaeontological and sedimentological evidence indicates the Miocene climatic optimum (MCO) to have lasted between 13.5-14.0 and 18 Ma in Central Europe (Bohme, 2003). At the end of the Burdigalian there was a time of high rainfall, although later, during the Langhian, there were droughts lasting up to 6 months. The MCO finished abruptly in the Middle Miocene (Langhian-Serravalian) between 13.5 and 14 Ma, when there was a significant fall in the mean annual temperature of at least 70C. Analyses for Europe show that the fall in temperature and rainfall from the Middle to Late Miocene also provoked a seasonal and latitudinal climatic gradient across the continent (Bruch et al., 2007). At the end of the Miocene the process of aridification intensified, extending the area occupied by open ecosystems, encouraging the appearance or expansion of several families of plants adapted to such environments (Singh, 1988).

The Oligocene

In general, the Iberian Oligocene fossil record reflects a broad-leaved, paratropical to subtropical evergreen rain forest

ПТГГ

I'll—I I f

10 15 20 25 -30 35 -40 45 : 50

Figure 5 Continental temperature curves for Central Europe over the last 45 Myr in comparison with the global marine oxygen isotope record of Zachos et al. (2001), adapted from Mosbrugger et al. (2005) (© 2005 National Academy of Sciences, USA). The arrows show the time of the main floristic changes of the Iberian Peninsula and Balearic Islands in the Cenozoic. The inset highlights the data for the Molasse Basin (Central Europe) as provided in the original figure.

(Collinson & Hooker, 2003) well adapted to periods of seasonal drought - appropriate for a hot climate with no large seasonal variations. Sclerophylls were well represented in these Oligocene communities (Hably & Fernandez Marron, 1998) and laurel forests were common, although many were confined to riparian environments - areas where the microclimate and soil conditions were adequate (Sanz de Siria, 1992; Barron & Peyrot, 2006). The number of taxa that disappeared from the Iberian fossil record over the Oligocene is very high. To date the loss of 117 taxa has been recorded (as revealed by palynological evidence), all of which were Palaeotropical except for several genera of the families Cupressaceae and Taxodiaceae (Cavagnetto & Anadon, 1996; Cavagnetto, 2002) (Fig. 4, Tables 1 and 2). With the exception of the latter conifers, the rest of the taxa that disappeared at this time (e.g. Acanthopanax, Areca, Brassaiopsis, Caryota, Clausena, Diphyl-leia, Eustigma, Fragaea, Glehnia, Hemsleya, Heteropanax, Helwingia, Lithocarpus, Mangifera, Mastixia, Nandina, Onco-sperma, Pasania, Podophyllum, Poliscyas, Sinowilsonia, Siphon-odon, Sladenia, Trachycarpus, Tetracentron and Tetrapanax) now largely inhabit the tropical or subtropical regions of Asia and the Indo-Pacific. Diphylleia, Glehnia and Lithocarpus are also represented in North America. Adenocalymna, Canna, Carpodiptera, Decodon, Diervilla, Hydrocleys and Fothergilla also disappeared from the Iberian Peninsula and are now found only in North America, as did Spathodea, Tecomaria and Tamarindus, which now live in Africa. The reduction in the number of genera suffered by the families Araliaceae, Euphor-biaceae, Menispermaceae and Palmae is also remarkable (Table 1) and indicative of significant changes occurring in Iberian plant landscapes.

During the Lower Oligocene, temperate deciduous vegetation and high-latitude mixed deciduous forest (Utescher & Mosbrugger, 2007) progressively extended across Europe. According to Knobloch (1992) and Knobloch et al. (1993), and unlike in Central Europe which had been home to a large number of Arctotertiary taxa since the Oligocene (Knobloch, 1986; Prothero, 1994), the Iberian Peninsula experienced a progressive increase in the number of Arctotertiary taxa starting in the Lower Miocene. Until this time paratropical and subtropical evergreen forests (Collinson & Hooker, 2003) were dominant. Thus, there was a clear diachrony in Europe with respect to the replacement of Palaeotropical taxa by Arctotertiary forms.

The Miocene

The Miocene is represented in nearly all the sedimentary basins of the Iberian Peninsula (Alonso-Zarza et al., 2002; Civis, 2004) (Fig. 2). During the Lower and Middle Miocene, Iberia saw the expansion of Arctotertiary taxa, which eventually dominated the Atlantic face of the peninsula as well as certain mountain ranges (Pais, 1981, 1986; Alcala et al., 1996; Roiron et al., 1999; Barron & Dieguez, 2001; Barron et al., 2006a). In contrast, the abrupt climate changes of this period led to the disappearance from the

peninsula's fossil record of 65 Palaeotropical taxa (Fig. 4, Table 1). The latter are nowadays found in tropical and subtropical areas around the world, except for Corylopsis, Disanthus, Exbucklandia, Nypa and Reevesia, which are found only in Asia. Paratropical and subtropical evergreen taxa characterized areas close to the sea where they developed xerophytic formations with conifers, Mimosacae, Caesalpin-iaceae and Fabaceae; members of the Lauraceae continued to form part of the riparian vegetation (Fernandez Marron, 1979; Bessedik, 1984, 1985; Sanz de Siria, 1985; Barron, 1999; Barron et al., 2006b). In fact, the eastern face of the Iberian Peninsula remained home to mangroves characterized by Avicennia during the Burdigalian-Langhian (Bessedik, 1981, 1985).

During the Iberian Upper Miocene a series of very arid phases occurred. Around 8.5 Ma there was an extremely dry phase during which rain was almost absent in the centre of the Iberian Peninsula. The first signs of these conditions appear in the Middle Miocene record, but they became generalized in the Duero Basin during the Vallesian (Lower Tortonian) (Valle & Salvador de Luna, 1985a,b; Rivas Carballo & Valle, 1987; Rivas Carballo, 1991a,b; Rivas Carballo et al., 1994; Valle et al., 2006). These dry phases were almost certainly the cause of the disappearance during the Tortonian of Palaeotropical families such as Cycadaceae, Meliaceae, Myrsinaceae, Restionaceae, Simaroubaceae, Sterculiaceae, Bombacaceae and Nyctagina-ceae from Iberian ecosystems, as well as of aquatic plants such as Nelumbo. Although some of these taxa were well adapted to seasonal drought they probably could not stand the acute falls in temperature associated with the 18 glacial/interglacial cycles detected for the end of the Tortonian (Hodell et al., 2001).

There are very few data available on the Messinian vegetation of the Iberian Peninsula, and no outstanding floristic changes can be determined with respect to the Upper Tortonian and the Zanclian (Sole de Porta & de Porta, 1977; Suc & Cravatte, 1982; Valle & Penalba, 1987; Bessais & Cravatte, 1988; Van Campo, 1989; Valle & Rivas Carballo, 1990; Barron, 1996; Rivas Carballo & Valle, 2005; Agusti et al., 2006; Fauquette et al., 2006). The only tropical genera to show their last records during the Messinian were members of the family Melastomataceae, Euphorbiaceae (Ricinus) and Rubia-ceae (Cephalanthus).

The Pliocene: the Middle-Late Piacenzian extinction

Generally, the subtropical climate that developed in the Zanclian became drier with more intense summer droughts during the Piacenzian, accompanied by a slight drop in temperature owing to the installation of a Mediterranean climate rhythm (Suc & Cravatte, 1982; Sanz de Siria, 1987). This led to a transformation of the plant cover, including the spread of xerophytes such as Olea, Pistacia and Artemisia, and the progressive disappearance of taxa such as Ginkgo, Andromeda, Pittosporaceae, Menispermaceae, nearly all the Palmae and a fair number of tropical-type Hamamelidaceae (Suc, 1980; Suc & Cravatte, 1982; Valle, 1982, 1983; Sanz de Siria,

1987, 1994; Suc et al., 1995). The palaeobotanical data agree with those provided by the marine isotope record. The cooling that occurred over the Zanclian, approximately during MIS TG5-MIS GI1, was progressive, leading to periodic variations of temperature (Lisiecki & Raymo, 2005). Later, during the Mammoth subchron (MIS M2), a significant change in the isotope record can be seen, marking a brusque cooling about 3.3 Ma. This might be related to the climatic changes already mentioned for the Piacenzian. Finally, the interval between the isotope stages MIS M2 and MIS 104 - about 2.7-3.3 Ma -reflects an acute cooling over a relatively short period of time compared to the cooling periods of the Zanclian and early Piacenzian (Lisiecki & Raymo, 2005, 2007). A progressive reduction in summer rainfall and the development of a dry season coinciding with the warmest period of the year occurred around 3.1-3.2 Ma, thus initiating Mediterranean seasonality (Suc & Cravatte, 1982). This led to the expansion of the palaeomediterranean-type xerophytic plants installed in the area since the Oligocene (Palamarev, 1989). During the Piacenzian (Fig. 4), evergreen laurel forests disappeared almost completely from the Iberian Peninsula - a clear indication of long periods of summer drought accompanied by a fall in temperature. Seven genera of the family Lauraceae disappeared during this period: Apollonias, Cinnamomum, Daphnogene, Lindera, Ocotea, Persea and Sassafras. However, although it suffered considerably, the family Lauraceae was not completely lost, as indicated by the persistence until modern times of Laurus nobilis. Nonetheless, it must have suffered great difficulty during periods of extreme cold during the last third of the Quaternary (Rodriguez-Sanchez & Arroyo, 2008). Its persistence, along with that of other (non-Lauraceae) lauroid plants, was possible because it was able to settle in places of high humidity where it is easier to survive the cold (Postigo Mijarra et al., 2008).

The final appearance of arborescent ferns such as Cyathea-ceae, and of the genus Gingko (Valle & Civis, 1978; Suc, 1980), is also notable. Although the family Taxodiaceae survived until the start of the Middle Pleistocene in the Iberian Peninsula, it suffered a notable reduction in its diversity, last appearing as pollen grains (Suc, 1980; Diniz, 1984) and wood specimens attributable to the genus Sequoia (Alcalde Olivares et al., 2004). Mangroves with Rhizophora also made their last appearance in the western Mediterranean at this time (Sendra et al., 2000). Until the Piacenzian, genera such as Dodonaea, Pittosporum and Notolaea - currently of some importance in Australia - arrived in the Iberian Peninsula. Finally, Distylium, Diospyros, Embo-lanthera, Liriodendron, Magnolia, Microtropix, Parrotiopsis, Platycarya, Rhoiptelea and Sapindus all disappeared; many of these are currently restricted partially or totally to Southeast Asia. A significant group of 36 taxa disappeared in this period.

The species that seemed to disappear from the Iberian Peninsula during the Zanclian may have persisted during at least some of the Piacenzian. Given the available data it may be more appropriate to speak of intense Piacenzian floristic change, an event that reached its high point at the end of this period between 2.7 and 3.3 Ma. As a whole, the Middle-Final

Piacenzian plant disappearance episode is one of the best characterized events of the Iberian Cenozoic, and was probably the last great change in Iberian plant landscapes.

The Pliocene-Pleistocene transition

At the end of the Piacenzian and throughout the Gelasian, between 1.8 and 2.58 Ma, thermal contrasts became ever greater and the progressive cooling that was under way became more acute (Lisiecki & Raymo, 2005). Only three sites with fossils from the time are known in the Iberian Peninsula, Crespia (de Villalta & Vicente, 1972; Suc, 1980; Roiron, 1983, 1992), Tres Pins (Leroy, 1987, 1997) and Caranceja (Alonso et al., 1999; Alcalde Olivares et al., 2004). These outcrops provide the last references to the genera Cupressus, Zelkova and Bumelia (Sapotaceae) for the Iberian Peninsula. Leaf remains resembling those of Zelkova carpinifolia have been found at Crespia (Roiron, 1983); no younger macroremains have been found. However, this may not mean that this genus became extinct in Iberia; the palynological record may contain evidence of its existence but this could be masked by the similarity of its pollen to that of Ulmus.

The Middle Pleistocene transition

The large number of data available for the Quaternary (although the total for the Pleistocene is much smaller than for the Holocene; Fig. 3) allow for an in-depth picture of the evolution of this period's floras to be drawn. The last great floristic change in the Iberian Peninsula took place in the Lower Pleistocene, culminating in the Middle Pleistocene transition (MPT). The first significant change in the climate of this period came about 1.4 Ma during the so-called Early Pleistocene transition, when there was an abrupt reduction in the amplitude of the 41-kyr cycle associated with a much shorter duration of glacial periods (Lisiecki & Raymo, 2007). The MPT started about 1250 ka and ended about 700 ka, and was characterized by an increase in the severity of glaciations and the emergence of 100-kyr cycles (with the widest temperature fluctuations about 0.7 Ma) (Fig. 6) (Raymo et al., 2004; Clark et al., 2006). The increased continental aridity was surely another notable effect of the MPT on the flora of the Iberian Quaternary. Such an effect has been detected in Africa and Asia (Tiedemann et al., 1994; Clemens et al., 1996; Williams et al., 1997). A notable fall in temperature along with marked dryness would have been decisive in the total disappearance of many species. However, this alone does not explain all the changes in flora during the MPT. The emergence of a 100-kyr cycle and much shorter interglacial periods must have had a determining influence on the recovery of some populations after periods of retreat and restriction to certain areas. Unfortunately, it is difficult to quantify the relative importance of these factors with respect to the taxa that became extinct in Iberia during this period. Tolerance of the cold probably had a very important effect on survival (Svenning, 2003). However, resistance to recurrent seasonal

0 200 400 600 800 1000 1200 1400 1600 1800

Figure 6 The LR04 benthic d18 stack from 0 to 1800 thousand years ago (ka). Adapted from Lisiecki & Raymo (2005). Arrows show the times of the last significant disappearances of taxa in the Iberian Peninsula and Balearic Islands over the Pleistocene. The numbers on the figure indicate the marine isotopic stages.

drought or fire (as seen in the modern Mediterranean) and the inherent mechanisms of each species' reproductive biology may also have been important. This interval of the Quaternary is represented by an important group of Waalian (end of the Lower Pleistocene or the Lower-Middle Pleistocene transition, 1.4-0.8 Ma) sites in the Iberian Peninsula (Elhai, 1966; Geurts, 1977, 1979; Julia & Suc, 1980; Antunes et al., 1986; Leroy, 1987, 1988, 2008; Postigo Mijarra et al., 2007).

A significant number of taxa seem to disappear for ever from the Iberian fossil record at the start of the MPT (c. 1.2 Ma; MIS 36), including Cathaya, Elaeagnus, Engelhardia, Eucommia, Keteleeria, Liquidambar, Nyssa, Parrotia, Partheno-cissus, Pterocarya, Sciadopitys, Symplocos and Tsuga. Others lasted through to the Lower-Middle Pleistocene (c. 800 ka; MIS 20-18). Such is the case for Aesculus and the family Taxodiaceae, probably making the Iberian fossil record for these taxa one of the last in Europe (Postigo Mijarra et al., 2007, 2008). It would appear that MIS 36-34 and 20-18 were key points of inflection for the floristic groups of the Iberian Peninsula (Fig. 6). On the one hand there was the change from 41-kyr to 100-kyr cyclicity, and on the other hand the cooling that took place about 900 ka which led to the first long glaciation of the Pleistocene (80 ka) (Lisiecki & Raymo, 2005). Things were not much different in other areas of southern Europe, although Arctotertiary taxa have been found at numerous sites in the Italian Peninsula representing the first stages of the Lower Pleistocene, e.g. Fornace Tini (Urban et al., 1983), Rio Ferraio, Piedrafitta, Stirone (Lona & Bertoldi, 1972) and the Leffe Basin (Ravazzi & Rossignol, 1995). These sites date from before MIS 36-34. With the 41- to 100-kyr cycle change, a significant number of taxa disappeared from Tenaghi Philippon in Greece, the most significant changes occurring in MIS 22-16 (Tzedakis et al., 2006). Compared with sequences for northern Europe, the much earlier disappearance of taxa such as Aesculus, Engelhardia, Liquidambar, Nyssa, Symplocos and Zelkova during the Pliocene-Pleistocene transition (Zag-win, 1960) is a notable difference.

After the MPT the records of Carya and Ostrya are quite imprecise, although they probably became extinct in Iberia in the Middle Pleistocene (Geurts, 1977, 1979). It seems unlikely

that they could have withstood the hard climatic conditions reigning between 0.43 Ma (MIS 12) and 0.87 Ma (MIS 22) - a time of extreme Pleistocene cold (Becquey & Gersonde, 2002). However, pollen grains belonging to Mimosaceae have been found for the Middle Pleistocene (Geurts, 1979) - which is quite hard to interpret; further studies will be needed before any conclusions can be drawn. It does not seem likely that the aforementioned taxa would have withstood the second notable climate change of the Pleistocene - the Middle Brunhes Event (MBE) - which took place c. 430 ka (MIS 12/11) (Jansen et al., 1986; EPICA Community Members, 2004). This event occurred on a global scale that marked the start of greater variability in the length of glacial and interglacial periods.

Extinctions during the Upper Pleistocene and Holocene: natural or due to human activities, or both?

One of the genera that disappeared at the end of the Quaternary was Cedrus. Its presence in the Iberian fossil record is almost constant from the Eocene to the Upper Pleistocene (Fig. 4), but its citation within the last glacial-interglacial cycle may be due to pollen grains coming from trees in North Africa (Magri & Parra, 2002). Further studies are needed to determine whether this is the case.

Picea is another taxon well represented during the Tertiary in the Iberian Peninsula but which disappeared at the end of the Upper Pleistocene. In Europe, the Quaternary history of this taxon is well documented (Collignon & Fabre, 2000; Ravazzi, 2002; Ravazzi et al., 2006). In the interior of the peninsula it was continuously present from the Oligocene until the Upper Pleistocene (Fig. 4); wood, cones and pollen remains have all been collected (Menendez Amor & Florschutz, 1959; Uzquiano, 1995; Alonso et al., 1999; Barron & Dieguez, 2001; Alcalde Olivares et al., 2004; Desprat et al., 2005; Gomez-Orellana et al., 2007). Pollen is also known from the Holocene (Pantaleon-Cano et al., 2003). The disappearance of Picea during the Wiirm glaciation as a consequence of low temperatures is difficult to understand. Its microthermal nature could have allowed it to become established at the foot of Iberian mountain ranges during the coldest parts of the

Quaternary without much difficulty. In fact, it persisted throughout most of the cold phases of the Middle Pleistocene. However, the lack of water during these cold times and the influence of the Mediterranean climate (with its dry summers) must have taken their toll. Changes in water availability have been proposed to have had a critical impact on the survival of Picea in other parts of Europe (Ravazzi et al., 2006), although in the Iberian Peninsula competition with conifers such as Abies (that produce shady forests) also needs to be taken into account. There is no evidence of any anthropogenic influence in its final demise. New studies will be needed to determine the role of this taxon in the Quaternary landscapes of the Iberian Peninsula. Finally, Pinus haploxylon is another conifer that is well represented during the Quaternary. Pollen records are found in Plio-Pleistocene and Lower Pleistocene sequences in the north-east of the peninsula (Julia & Suc, 1980; Leroy, 1987, 1997). Burjachs (2006) cites this taxon for the Abric Romani site, but also records its disappearance in MIS 3 and cites no Middle Pleistocene reference.

Among the angiosperms, Platanus has a long palaeontolog-ical history, its first Iberian records dating from the Aquitanian (Fig. 4). Although the fossil record is patchy, it is known to have persisted until the Holocene (Garcia Anton et al., 1990). The cold periods of the Quaternary are thought to have left it isolated in a few areas of the Iberian Peninsula providing refugia for flora of Arctotertiary origin. The case of Syringa, however, is quite different. The representation of this taxon in south-western Europe is reduced to a single record for the Upper Pleistocene in Catalonia (40-70 ka) (Burjachs & Julia, 1994). Currently its distribution is restricted to the eastern Mediterranean. The aforementioned single Iberian reference shows that this genus did not form an important part of the Iberian plant cover, but confirms Catalonia to have offered refuge to Arctotertiary flora during the cold phases of the Quaternary.

A number of other taxa with more or less ample Cenozoic records, and with representation in the Iberian Pleistocene and Holocene, can still be found growing naturally in the Iberian Peninsula today. Such is the case of Juglans, Castanea, Vitis, Ceratonia, Carpinus and Fagus (Aizpuru & Catalan, 1984; Stevenson & Moore, 1988; Garcia Anton et al., 1990; Carrion & Sanchez-Gomez, 1992; Martinez Atienza & Morla, 1992; Hewitt, 1999; Grau Almero et al., 2004; Krebs et al., 2004; Costa et al., 2005; Magri et al., 2006; Postigo Mijarra et al., 2008). Their Iberian fossil records contrast strongly with the final disappearance of these taxa in Central and northern Europe (Zagwin, 1960; Van der Hammen et al., 1971; Postigo Mijarra et al., 2008).

CONCLUSIONS

The great climatic events that led to substantial changes in temperature, water availability and rainfall patterns over the Cenozoic were probably the main reason behind the main changes in the flora of the Iberian Peninsula recorded in its fossil record. In total, 277 taxa became extinct in Iberia during

the Cenozoic. Such changes can be seen in the Palaeocene as substitutions of the different genera of Normapolles. Over the Eocene-Piacenzian, Palaeotropical elements found today in the different tropical and subtropical regions of the world successively disappeared (Fig. 5). The patent floristic change of the Priabonian-Chatian interval was related to the aridifi-cation that began in the Late Eocene.

Palaeotropical taxa survived longer in the Iberian Peninsula than in other areas of Europe, probably as a consequence of its palaeolatitude and its varied orography (including large mountain systems and sedimentary basins), which allowed for altitudinal displacements and stable microclimate conditions. Moreover, the Tethys and Atlantic oceans may have played a key role by attenuating temperatures and providing constant humidity across several peninsular areas. From the Oligocene and during the Miocene, many Iberian regions must have had floras of a mixed Palaeotropical/Arctotertiary nature. However, little by little, Arctotertiary taxa gained ground over their Palaeotropical counterparts as seasonality, the trend towards lower temperatures and reductions in rainfall, became more pronounced. Palaeotropical plants became ever more scarce, victims of new climates (especially from the MCO onwards) and of direct competition with new species arriving from Europe.

During the Miocene, Palaeotropical elements continued to disappear - a gradual disappearance due to the fall in mean annual temperature and recurrent drought. At the same time Arctotertiary taxa arrived to substitute them. However, some of these Arctotertiary genera, such as Calocedrus, Torreya, Cercidiphyllum, Robinia and Gleditsia, also found themselves unable to resist the climate changes of the Iberian Miocene. In total, 177 taxa became extinct in Iberia during the Oligocene and Miocene.

During the Middle-Late Piacenzian (2.7-3.3 Ma), Palaeo-tropical elements made their last appearance; 36 taxa are known to have been lost due to the notable cold and the onset of Mediterranean seasonality. During the MPT, and probably related to the change in glacial cyclicity, a notable disappearance of 14 Arctotertiary elements occurred. These losses, along with those among the remaining Palaeotropical flora (notably Engelhardia and members of the family Araliaceae that today live in tropical areas) were related to the increased severity of the glaciations, a reduction in the length of the interglacial periods, a change in cyclicity from 41 to 100 kyr, and the increased dryness of the coldest times. MIS 36-34 and 20-18 stand out as the key moments in the Quaternary disappearance of taxa from the Iberian Peninsula.

It is difficult to think of human activity as the cause of the complete or near-complete disappearance of some taxa. Certain genera, such as Platanus and Syringa, seem to have disappeared due to natural causes, although the non-persistence of Picea might be related to climate change, competition with other taxa and the activity of humans.

The disappearance chronogram for Cenozoic flora proposed in this work differs from the model established for Central and

northern Europe: the Palaeotropical flora of the latter areas disappeared much sooner than in Iberia. The mixing and coexistence of Palaeotropical and Arctotertiary floras over much of the Iberian Miocene and the first part of the Pliocene, along with the appearance of Mediterranean-type taxa from the Middle Pliocene, represent further differences with respect to Central and northern Europe. In addition, the disappearance of many mainly Arctotertiary taxa of ample Cenozoic presence occurred later in Iberia (at the Early-Middle Pleistocene boundary) than in Central Europe, confirming that the Iberian Peninsula had offered refugia to these taxa since pre-Quaternary times.

ACKNOWLEDGEMENTS

This work was performed as part of the PALEODIVERSITAS I research project (Plan Nacional I+D, CGL 2006-02,956-B0s). Adrian Burton is thanked for his assistance with the English manuscript.

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SUPPORTING INFORMATION

Additional Supporting Information may be found in the

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Appendix S1 Taxa included in Figure 4 and documents in

which they are recorded.

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BIOSKETCHES

Jose M. Postigo Mijarra has research interests in Neogene vegetation dynamics and environmental change. He has a particular interest in the Quaternary dynamics of species distributions. His current work focuses on the flora and vegetation of Pleistocene landscapes in the Iberian Peninsula.

Eduardo Barron is a palaeobotanist and is interested in systematics, palaeoecology, palaeobiogeography and biostra-tigraphy.

Fernando Gomez Manzaneque is a lecturer in botany and geobotany whose research interests include Cenozoic palaeoe-cology, forest biogeography and Iberian flora.

Carlos Morla Juaristi is a lecturer in botany and geobotany whose research interests include the biogeography of the Iberian Peninsula, palaeobotany and the palaeoecology of the Late Quaternary of the western Mediterranean.

Editor: Malte Ebach