Botanical Journal of the Linnean Society, 2010, 163, 44–54. With 14 figures Detarieae sensu lato (Fabaceae) from the Late Oligocene (27.23 Ma) Guang River flora of north-western Ethiopia boj_1044 44..54 AARON D. PAN1*, BONNIE F. JACOBS2 and PATRICK S. HERENDEEN3 1 Fort Worth Museum of Science and History, 1600 Gendy Street, Fort Worth, TX 76107-4062, USA Roy M. Huffington Department of Earth Sciences, Southern Methodist University, P.O. Box 750395, Dallas, TX 75275-0395, USA 3 Chicago Botanic Garden, 1000 Lake Cook Road, Glencoe, IL 60022, USA 2 Received 15 July 2009; revision 15 July 2009; accepted for publication 8 March 2010 New species of caesalpinioid legumes, Cynometra sensu lato and Afzelia, are described from the Late Oligocene (27.23 Ma) Guang River flora in north-western Ethiopia. Both taxa show leaf characteristics that are shared with extant species in the Guineo-Congolian, Sudanian and/or Zambezian regions of Africa today. The presence of these two species in Ethiopia during the Palaeogene provides further evidence of the importance of the legume tribe Detarieae in northern and north-eastern Africa throughout much of the Cenozoic, even although the clade is poorly represented in these regions today. The fossil record documents a significant palaeogeographical and evolutionary history of Detarieae in Africa, especially compared with that of Europe and Anatolia. Based on this evidence, it is unlikely that significant diversification of extant African Detarieae took place on the Eurasian landmass. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54. ADDITIONAL KEYWORDS: fossil angiosperms – leaf morphology – leaf surface – Leguminosae – palaeobotany – stomata. INTRODUCTION The caesalpinioid legume tribe Detarieae DC. sensu lato (hereafter referred to as Detarieae) is well represented in tropical forests and woodlands of Africa today, with 53 genera (48 of which are endemic) and approximately 330 species on the continent (with an additional endemic 15 species in Madagascar; Du Puy et al., 2002; Mackinder, 2005; Bruneau et al., 2008). In fact, Detarieae are one of the few angiosperm groups that is more diverse in Africa than in tropical regions of the New World or Asia. Within forests, members of this clade are common components of the canopy, can tower above this layer as emergents and can even form monodominant communities (e.g. Brachystegia laurentii (De Wild.) Louis ex J.Léonard *Corresponding author. E-mail: apan@fwmsh.org 44 forest, Cynometra alexandri C.H.Wright forest, Gilbertiodendron J. Léonard forest; Connell & Lowman, 1989; Gross, 2000; Sheil & Salim, 2004). Detarieae also dominate southern and East African woodland communities (e.g. miombo), which typically contain several species within the Berlinia clade: Brachystegia Benth., Isoberlinia Craib & Stapf and Julbernardia Pellegr. (Wieringa & Gervais, 2003; Smith & Allen, 2004; Bruneau et al., 2008). Although Detarieae are important in Africa today, the tribe is not currently found along or near the coast in northernmost Africa or in the Sahara. The tribe is also scantily represented in the Sudanian region, where four to seven species occur among five genera. In addition, Detarieae is poorly represented in the Horn of Africa, where only one species occurs in Ethiopia (Tamarindus indica L.; Polhill & Thulin, 1989) and two occur in Somalia (Afzelia quanzensis Welw. and T. indica; Thulin, 1993). The tribe is not © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 OLIGOCENE ETHIOPIAN LEGUMES well represented in the northern portions of the continent today, but the plant fossil record indicates that this region had a diverse Detarieae flora during much of the Palaeogene and Neogene, especially during the Oligocene and Miocene. Herein, we report two new records of Detarieae leaf macrofossils from the Late Oligocene (27.23 Ma) Guang River flora of northwestern Ethiopia by describing new species within the genera Cynometra L. and Afzelia Sm. MATERIAL AND METHODS The Guang River flora is an autochthonous or parautochthonous plant fossil assemblage representing a tropical riparian forest community as indicated by numerous entire-margined notophyllous to mesophyllous leaf species, taxonomic composition (e.g. Cola Schott & Endl., Eremospatha (G.Mann & H.Wendl.) H.Wendl., Sorindeia Thouars), preservation in overbank sediments (mudstones) and relatively high species heterogeneity over short lateral distances (Pan et al., 2006; Pan, 2007). This assemblage is located approximately 60 km west of Gondar in north-western Ethiopia (Pan et al., 2006; Pan, 2007). The geology of the area is dominated by Oligocene and Miocene flood basalts with interbedded fluvially reworked volcaniclastic and clastic sediments, airfall tuffs and lignitic deposits (Kappelman et al., 2003; Jacobs et al., 2005; Pan et al., 2006). Fossil specimens of Fabaceae were compared with extant plant collections at the Missouri Botanical Garden Herbarium (MO) and the United States National Herbarium (US). Macrofossils and plant herbarium specimens were photographed using a high-definition digital single-lens reflex camera (Canon EOS20). Many of the fossil leaves are well-preserved compressions with cuticle and epidermal structures. Cuticle was prepared from fossil fragments (0.5– 1 cm2) by immersion of the material in 10% hydrochloric acid (HCl), followed by 48% hydrofluoric acid (HF) for 24–48 h to dissolve away the matrix. The cuticle was then placed in a 20% bleach (NaOCl) in aqueous solution briefly (20 s to 1 min) to dissolve excess organics. The cuticle was then thoroughly rinsed with distilled water and placed on microscope slides in glycerine. Light microscope slides of the fossil samples are housed in the Huffington Department of Earth Sciences at Southern Methodist University in Dallas, Texas, USA. Images of extant leaf cuticle specimens were provided by one of us (P.S.H.). All of the Guang River plant macrofossil specimens are permanently housed in the Chilga collections at the National Museum of Ethiopia, Addis Ababa. 45 DESCRIPTIONS CYNOMETRA CHAKA SP. NOV. A.D.PAN, B.F.JACOBS & P.S.HERENDEEN Description Compound leaves are paripinnate, with no more than three to four pairs of leaflets that are oppositely inserted. The microphyllous leaflets are sessile and attached to an adaxially grooved winged rachis by untwisted pulvinate petiolules. The basal-most leaflets arise immediately above the wrinkled pulvinate petiole (Fig. 1). Leaflets range in size from c. 22–50 mm in length and 10–20 mm in width and are entire-margined, asymmetric and oblong–elliptic to ovate–elliptic in shape. The lowermost leaflets are more ovate than subsequent ones (Fig. 2). Leaflet bases are unequal and obtuse (Figs 1, 3). Apices are acute to rounded, retuse and mucronate (although the mucro may not always be present). Midveins are straight to slightly curved and located closer to the proximal margin. Secondary venation is brochidodromous in the apical half of the leaflet and leaflets lack a marginal vein (Fig. 4). A fan of veins is present at the base of the leaflets (Fig. 3). Strong intersecondaries are also present, extending from the midvein to the intra-marginal secondary loops. The acutely angled one to three basal-most veins are eucamptodromous. Tertiary venation is random reticulate. CUTICLE: Abaxial cells are 4–6 sided, the majority being 5-sided, and have straight to rounded anticlinal cell walls (Fig. 5). Cell lengths and widths range from 6 to 13 mm (Fig. 5). Stomata are paracytic and only found on the abaxial surface. Inner stomatal ledges are heavily cutinized (Fig. 5). Hairs are present on the abaxial surface and are uniseriate and multicellular (cells generally numbering more than four; Fig. 6). Hair bases are enlarged but not buttressed (Fig. 6). The adaxial epidermal surface is similar to that of the abaxial, except that it is glabrous and lacks stomata. Material Holotype CH52-99 (Fig. 1); CH40-38 A & B (counterparts), CH40-P99 (Fig. 3), CH41-86, CH52-49, CH5298, CH52-127 (Fig. 4) and CH52-P55 (Fig. 2). Etymology ‘Chaka’ is the transliteration of ‘forest’ in Amharic; referring to the presence of this fossil species in a forest palaeo-community. Comments The grooved winged rachis, paripinnate compound leaves, midveins near the proximal margin, lowermost leaflets arising immediately after the pulvinate petiole and the oblong–elliptic shape of the leaflets © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 46 A. D. PAN ET AL. Figures 1–8. Cynometra leaves, leaflets and cuticular features. Figures 1–4. Cynometra chaka Pan et al. sp. nov. Scale bars, 10 mm. Fig. 1. Holotype CH52-99, Arrow indicates the grooved, winged rachis. Fig. 2. CH52-P55, leaf fragment with three leaflet pairs (3-jugate). Fig. 3. CH40-P99, Leaflets. Fig. 4. CH52-127, Emarginate leaflet. Figures 5 and 6. Cynometra chaka abaxial leaf surface cuticle. Scale bar, 10 mm. Fig. 5. CH52-127, Paracytic stomata. Fig. 6. Uniseriate multicellular hairs, CH52-79. Figures 7 and 8. Cynometra lujae, 3766526 (MO). Scale bar, 10 mm. Fig. 7. Leaves. Fig. 8. Epidermal leaf surface. Scale bar, 10 mm. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 OLIGOCENE ETHIOPIAN LEGUMES are characters found both in extant Cynometra and Hymenostegia (Benth.) Harms, but the fossils are considered here to represent the genus Cynometra (Fig. 7) because of the presence of heavily cutinized stomatal inner ledges and multicellular uniseriate hairs (Figs 6, 8). Both characteristics occur in Cynometra but have not been noted in the cuticle of species of Hymenostegia by Herendeen, Bruneau & Lewis (2003) or in our observations of H. aubrevillei Pellegr. and H. floribunda (Benth.) Harms. Additionally, retuse leaflet tips can be found in Hymenostegia (Herendeen et al., 2003), but this character is rare within the genus, only occurring regularly within H. aubrevillei and H. floribunda (and both species can also have acute, non-emarginate leaflet apices) and rarely in saplings of H. gracilipes Hutch. & Dalziel (Hawthorne & Jongkind, 2006). Retuse-tipped leaflets in Cynometra are typically more deeply notched than those found in Hymenostegia. The fossils appear to be more similar to Cynometra in this respect as well. A species of the south-east Asian, Malesian and Papuasian genus Maniltoa Scheff., M. polyandra (Roxb.) Harms, also resembles the fossil. However, there have been doubts as to the distinctiveness of this genus from Cynometra (Breteler, 1996), especially considering characteristics discussed in Knaapvan Meeuwen (1970), who segregated the two. The number of stamens in Cynometra is usually 10, but can range from 8 to 15 (Knaap-van Meeuwen, 1970). Maniltoa has c. 15–80 stamens according to Knaapvan Meeuwen (1970), but this is an estimate and the exact range is not provided. Also, whereas the filaments are not connate in Cynometra, this characteristic varies within Maniltoa (Knaap-van Meeuwen, 1970). The texture of the fruit, which was also used to segregate the two genera, is not helpful as smooth pods, which occur in Maniltoa, can also be found in some species of Cynometra. Wood anatomical characters such as ray height also overlap at the extremes (30 mm; Knaap-van Meeuwen, 1970). Recent phylogenetic analyses of the caesalpinioid legumes by Bruneau et al. (2001, 2008) and Herendeen et al. (2003), show that, although Cynometra and Maniltoa are closely related and form a clade, neither is monophyletic. Because of these studies and the questionable segregation of Cynometra and Maniltoa based on morphological characteristics, in this paper the authors consider Maniltoa as subsumed within Cynometra sensu lato. Based on the gross morphology and cuticle anatomy, the fossil most closely resembles Cynometra lujae De Wild. (Fig. 7), a shrub and tree species found in valley forests in Gabon, the Republic of Congo and the Angolan exclave of Cabinda (Léonard, 1951, 1952) and, less closely C. schlechteri Harms, an arborescent species found in periodically inundated forests in 47 Gabon, the Republic of Congo and the Democratic Republic of the Congo (Aubréville, 1968). Both Cynometra chaka sp. nov. and Cynometra lujae possess: (1) leaflets that are sessile or subsessile; (2) winged grooved rachises; (3) mucronate, retuse (typically sharply notched) leaflets; (4) acute apices; (5) a fan of veins at the base of the leaflets (Léonard, 1951); and (6) multicellular uniseriate hairs present on the abaxial surface. However, there are differences between the fossil and this species. Cynometra lujae has > 8 leaflet pairs and sinuous abaxial anticlinal cell walls, Cynometra chaka has straight to rounded anticlinal cell walls and 3- or 4-jugate leaves (Figs 1–3, 5, 7, 8). Although C. schlechteri is similar to the fossil species in some leaf characteristics, namely three pairs of mucronate and retuse leaflets that are sessile or subsessile, leaflets with a fan of veins at their base and straight to rounded anticlinal abaxial cell walls, they differ from the fossil in having prominent acuminate leaflet apices and non-winged rachises. Thus, Cynometra chaka is distinct from all extant species. AFZELIA AFRO-ARABICA SP. NOV. A.D.PAN, B.F.JACOBS, & P.S.HERENDEEN Description Asymmetrical, oblong–elliptic microphyllous leaflet with rounded (or possibly retuse) apex and a rounded base. The sole specimen is 47 mm long and 31 mm at maximum width. The pulvinate petiolule is twisted and 4.9 mm long (Fig. 9). Secondary venation is weakly brochidodromous and the spacing between vein pairs decreases towards the base. Secondary veins arise from the midvein alternately and at shallowly acute angles. A single basal marginal secondary vein is present. CUTICLE: Abaxial epidermal cells have sinuous anticlinal cell walls with undulations that range from ‘艚’ to ‘W’ in shape (Figs 10, 11). Cells range in size from c. 19–27 mm in length and 11–26 mm in width. Faint striations occur on the periclinal cell walls. Stomatal complexes are paracytic. Stomata are generally 19–23 mm long and 16–19 mm wide. A single hair base was observed (19 mm in width; Fig. 11). Characteristics of the adaxial epidermal surface and micromorphology are unknown. Material Holotype – CH54-28 A & B (counterparts; Fig. 9). Etymology The epithet refers to the Cretaceous-Palaeogene continent Afro-Arabia, which included Africa and the Arabian Peninsula prior to their separation and formation of the Red Sea. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 48 A. D. PAN ET AL. Figures 9–14. Afzelia leaf and cuticular features. Figures 9–11. Afzelia afro-arabica Pan et al. sp. nov. Fig. 9. Holotype CH54-28, Arrow indicates twisted petiolule. Scale bar, 10 mm. Fig. 10. CH54-28, Abaxial stomata. Scale bar, 10 mm. Fig. 11. CH54-28, Abaxial hair base. Scale bar, 10 mm. Figures 12–14. Extant Afzelia leaf and cuticular features. Fig. 12. Afzelia quanzensis, 2324620 (MO). Scale bar, 10 mm. Fig. 13. A. africana abaxial epidermal leaf cuticle. Scale bar, 10 mm. Fig. 14. A. quanzensis abaxial epidermal leaf cuticle. Scale bar, 10 mm. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 OLIGOCENE ETHIOPIAN LEGUMES Comments Based on the leaf shape, the rounded apex and base, the twisted petiolule and the basal marginal vein, the fossil compression is similar to the genera Afzelia and Intsia Thouars. The fossil is considered here to represent the genus Afzelia because it shares striated periclinal cell walls with at least three species of Afzelia (e.g. A. bella Harms, A. quanzensis Welw. and A. xylocarpa (Kurz) Craib.) and this character is absent in Intsia. In addition, the stomatal lengths in the fossil fall within the range of at least four extant African Afzelia species (A. africana Pers., A. bella, A. pachyloba Harms and A. quanzensis), but are larger than those of Intsia bijuga (Colebr.) Kuntze (Kadiri & Olowokudejo, 2008). The oblong–elliptic leaflet shape and rounded or emarginate tip of the fossil is most similar to the extant species A. quanzensis (Fig. 12). This species includes medium to large trees found in thickets, woodlands and lowland dry evergreen forest (often along rivers) in northern Namibia, Botswana, Zimbabwe, Mozambique, Zambia, Malawi, Angola, Kenya, Somalia, Tanzania and the Democratic Republic of the Congo (Brenan, 1967; Germishuizen, Crouch & Condy, 2005). However, the anticlinal cell walls in A. quanzensis are straight to slightly rounded, whereas the sinuous anticlinal cell walls of the fossil are quite pronounced (Fig. 14). Sinuous anticlinal cell walls are more typical of the Guineo-Congolian (Fig. 13; A. bella, A. bipindensis Harms and A. pachyloba) and Sudanian (A. africana) species (Kadiri & Olowokudejo, 2008). The cell walls are most comparable with those of A. africana (this species has elliptic to ovate–elliptic leaflets with acute or acuminate apices; Fig. 13; Brenan, 1967). The presence of a hair base in the fossil is unusual for the genus. Trichomes have only been reported in A. pachyloba, a species that also has rounded and/or emarginate leaflet apices (Kadiri & Olowokudejo, 2008). As the fossil species has characteristics similar to A. africana, A. bella, A. pachyloba and A. quanzensis, but in a unique combination, it is considered here to represent a new species, A. afro-arabica. FOSSIL DISCUSSION RECORD AND CYNOMETRA AND AFZELIA The presence of both Cynometra and Afzelia in Ethiopia during the Late Oligocene (27.23 Ma) is significant because neither genus occurs there today (Polhill & Thulin, 1989). However, both genera are relatively well represented in the fossil record. The earliest fossils with affinity or purported affinity to Cynometra are wood (Cynometroxylon Chowdhury & Ghosh) and leaf fossils (one or two species cf. Cynometra) from the 49 middle Eocene of Myanmar and Tanzania, respectively (Herendeen & Jacobs, 2000; Privé-Gill et al., 2004; Kaiser et al., 2006). Cynometra leaf impressions are also reported from the Late Eocene and Early Oligocene of Egypt (Qasr el Sagha and Jebel Qatrani formations; Bown et al., 1982), although there is some question as to the affinity of these specimens which also look similar to several other genera of Detarieae. Other Cynometroxylon records include occurrences in the Miocene of Egypt, Ethiopia and Tunisia, the Neogene of Algeria and Myanmar and Cenozoic sediments of India (Chowdhury & Ghosh, 1946; Navale, 1958; Prakash & Awasthi, 1969; Prakash & Bande, 1977, 1980; Prakash, 1978, 1979–1980; Bande & Prakash, 1980; Ghosh & Roy, 1982; Guleria, 1982b; Lakhanpal, Guleria & Awasthi, 1984; Vozenin-Serra & Privé-Gill, 1989; Awasthi, 1992; Dupéron-Laudoueneix & Dupéron, 1995; Kamal El-Din & El-Saadawi, 2004). Fossil leaflets from Puerto Rico reported by Hollick (1928) as having affinity to Cynometra cannot be confidently placed within the genus (Graham, 1992). Afzelia afro-arabica, from the Guang River flora, is the earliest definitive record of the genus, although probably not the oldest example. Fossil wood purported to have affinities with Afzelia is known from the Palaeocene of Niger (Afzelioxylon furoni Koeniguer; Koeniguer, 1971), Oligocene of Algeria (Afzelioxylon kiliani Louvet; Louvet, 1965) and the Palaeogene Red Sandstone Group in Tanzania (Wheeler & Baas, 1992; Dupéron-Laudoueneix & Dupéron, 1995; Damblon et al., 1998; Roberts et al., 2004). However, although this wood may represent Afzelia, or the closely related Intsia, characteristics of these woods may also be found in other legume taxa, including some members of Mimosoideae (InsideWood, 2009). Pahudioxylon menchikoffii (Boureau) Müller-Stoll & Mädel, which may also have affinities to Afzelia, Instia or 18 other genera of Detarieae (Wheeler & Baas, 1992; Herendeen et al., 2003; Mackinder, 2005), is reported from the Eocene of Algeria (Müller-Stoll & Mädel, 1967; Dupéron-Laudoueneix & Dupéron, 1995). A number of other wood fossils with affinity to Afzelia and Intsia (Afzelioxylon Louvet and Pahudioxylon Chowdhury, Ghosh, & Kazmi) are known from the Neogene of Africa and Asia (Chowdhury, Ghosh & Kazmi, 1960; Navale, 1962; Prakash, 1965a, b, 1979–1980; Prakash & Awasthi, 1969; Lemoigne & Beauchamp, 1972; Prakash & Tripathi, 1973; Lemoigne, Beauchamp & Samuel, 1974; Louvet, 1974; Lemoigne, 1978; Vozenin-Serra, 1981; Ghosh & Roy, 1982; Guleria, 1982a, b; Lakhanpal et al., 1984; Vozenin-Serra & Privé-Gill, 1989; Awasthi, 1992; Dupéron-Laudoueneix & Dupéron, 1995; Damblon et al., 1998; Kamal El-Din & El-Saadawi, 2004). A fossil fruit of Afzelia bipindensis has been reported from the latest Miocene or earliest © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 50 A. D. PAN ET AL. Pliocene (6.5–5 Ma) Nkondo Formation of Uganda (Dechamps, Senut & Pickford, 1992). PALAEOBIOGEOGRAPHY, PALAEOECOLOGY AND EVOLUTIONARY HISTORY Today the Detarieae have a pantropical distribution, although close to 90% of the genera in the clade are restricted to a single continent or smaller geographic area (Mackinder, 2005). Cynometra and Afzelia are unusual in having much wider distributions than the majority of Detarieae genera (Mackinder, 2005). Cynometra (including Maniltoa) is pantropical, with the largest number of species occurring in Africa and Asia, but with over 20 species also occurring in the New World (mainly in Amazonia; Mackinder, 2005). Afzelia has a somewhat more restricted distribution, being found exclusively in Africa and Asia. However, Afzelia is closely related to Intsia, which extends from the east coast of Africa to the tropical Pacific, and Brodriguesia Cowan, which is restricted to the Atlantic coastal forests of Brazil (Mackinder, 2005; Schrire, Lavin & Lewis, 2005a; Schrire, Lewis & Lavin 2005b; Bruneau et al., 2008). While not all of the fossil wood with purported affinity to Cynometra (Cynometroxylon) and Afzelia (Afzelioxylon and Pahudioxylon) can be taxonomically confirmed, some probably represent these taxa and their African, south and south-east Asian occurrences, and distributions provide additional support for their origin or early evolutionary history in the area surrounding the Tethys Seaway (Schrire et al., 2005a, b; Bruneau et al., 2008). Some extant species of Cynometra sensu lato are found in beach, mangrove and riparian forest communities (e.g. C. dauphinensis Du Puy & R.Rabev., C. iripa Kostel., C. mannii Oliv. and C. ramiflora L.) and aquatic propagule dispersal has been noted in several of these species (Tomlinson, 1995; Clarke, Kerrigan & Westphal, 2001). Additionally, some members of the Afzelia clade are well known as marine/aquatic propagule dispersers; Intsia bijuga, a mangrove, strand and riparian forest tree, is the most widespread species in the group, extending from East Africa to the western Pacific, and has fruits containing lightweight, durable, flat seeds with hard seed coats that float in and are resistant to seawater (Guppy, 1903; Thaman et al., 2006). While it is possible that marine/aquatic dispersed propagules could be a recent development within both groups (Cynometra sensu lato and the Afzelia clade), marine dispersal would be an advantageous adaptation for a clade originating along the margins of the Tethys Seaway, which may be a plausible explanation for their pantropical distribution. Afzelia fruits and seeds are not aquatically dispersed. Their fruits are woody, heavy and hard and contain heavy black or brown seeds with orange or red arils that are dispersed by mammals (and presumably birds; Gathua, 2000). Such adaptations are not useful for aquatic dispersal, but endozoochory would have allowed dispersal further into continental interiors and drier environments of Africa and Asia than where Intsia is found today. THE ROLE OF AFRICA IN THE BIOGEOGRAPHY OF DETARIEAE Schrire et al. (2005a, b) provided an intriguing hypothesis calling for the origination and diversification of Fabaceae along the margin of the Tethys Seaway during the Early Cenozoic. This hypothesis is based on the likelihood that legumes originated in dry environments (consistent with the high nitrogen metabolism, compound leaves and deciduousness found in the family), the early (Eocene) presence of legumes in seasonally dry palaeo-communities and the sizeable number of early diverging legume taxa in their phylogenetic analysis that are restricted to semi-arid environments [termed Succulent (S)-biome; Schrire et al., 2005a, b]. This evidence, and the inferred presence of semi-arid to arid environments around the Palaeogene Tethys Seaway (from Scotese, 2001), allowed them to form a cohesive explanation for the contemporary widespread distribution of Fabaceae and the nearly instantaneous pervasiveness over a large global area shortly after their earliest occurrence in the fossil record (Schrire et al., 2005a, b). Although much of this hypothesis seems reasonable, the suggestion by Schrire et al. (2005b: 41) that, during the Eocene (and thereafter), legume dispersal from Eurasia to Africa probably increased because of the appearance of numerous islands around Turkey/ Anatolia and western Asia, and that dispersal direction was southward from the northern Tethyan (Eurasian) coast to Africa may be ignoring the more likely possibility that dispersal for some groups, such as the Detarieae, would have been from south to north. Dispersal of Detarieae into Africa from Europe is problematic because the fossil record for this group is much older in Africa than Eurasia, extending back to the late Maastrichtian or Palaeocene. These fossils include the pollen taxon Striatopollis (Striatricolpites) catatumbus from the Kerri Kerri Formation in Nigeria (Adegoke et al., 1978), which may be related to Anthonotha P.Beauv., Berlinia, Crudia Schreb., Didelotia Baill. or Isoberlinia (Muller, 1981; Rull, 1999; Morley, 2000; Vincens et al., 2007), Palaeocene legume wood from Mali, which possesses several characteristics typical of Detarieae (Crawley, 1988), and Afzelioxylon furoni wood from Niger mentioned above, which may also be of Detarieae affinity (Koeniguer, 1971). In addition, a number of early and middle © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 44–54 OLIGOCENE ETHIOPIAN LEGUMES Eocene Detarieae records are known from northern and eastern Africa (Herendeen & Jacobs, 2000; Eisawi, 2007). Except for the fossil material with affinity to Daniellia Benn. from the Early Eocene Paris Basin (De Franceschi & De Ploëg, 2003; Bruneau et al., 2008), few (if any) fossils with purported affinities to Detarieae are present in the macro- and palynofloras from the Palaeocene, Eocene and early Oligocene of Spain, central Europe and Anatolia (Hably & Fernández Marrón, 1998; Akgün, Akay & Erdoğan, 2002; Akkiraz & Akgün, 2005; Akkiraz et al., 2006). The Eurasian fossil taxon Podocarpium A.Braun ex Stizenberger may represent the Detarieae clade, but the earliest record of this genus is late Oligocene (Herendeen, 1992; Wang, Dilcher & Lott, 2007). The suggestion that Fabaceae may have a Tethyan origin is possible, perhaps likely, but that does not necessarily imply a Laurasian (northern) origin. The existence of legume fossils in Africa, South America, North America and Britain in the Palaeocene indicates that the family was already widely distributed at that time and suggests either a Late Cretaceous origin or a rapid initial diversification and dispersal in the Palaeocene. The Tethys Seaway may have been an ideal corridor for such dispersal(s), but there do not appear to be enough definitive data to pinpoint the landmass which was the place of legume origins. Whether the legumes originated in Eurasia, Africa or the New World, Africa has played a major role in the evolutionary history of Detarieae throughout the Cenozoic. This group disappeared from much of northern and north-eastern Africa only after the Miocene. The Guang River flora legume fossils provide additional evidence of the former prevalence of Detarieae in regions north of the contemporary diverse African Detarieae communities. ACKNOWLEDGEMENTS We would like to thank the Authority for Research and Conservation of Cultural Heritage, the Ministry of Culture and Tourism, Ethiopia and its Director Ato Jara for permission to conduct continuing research in north-western Ethiopia. We thank Mamitu Yilga, Director of the National Museum and her staff for their support in Addis Ababa, and the Gondar ARCCH and Chilga Ministry of Culture and Sports Affairs for permission and logistical support in the field. The Missouri Botanical Garden and the United States National Herbarium provided assistance and access to their collections and we thank the collectors of the herbarium specimens examined for this study. 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