Hymenaea, Linnaeus, 1753

Peris, David, Kraemer, Mónica M. Solórzano, Peñalver, Enrique & Delclòs, Xavier, 2015, New ambrosia beetles (Coleoptera: Curculionidae: Platypodinae) from Miocene Mexican and Dominican ambers and their paleobiogeographical implications, Organisms Diversity & Evolution (New York, N. Y.) 15 (3), pp. 527-542 : 536-538

Hymenaea dispersal

Hymenaea currently includes 14 species. H. oerrucosa , from Madagascar and Eastern Africa, is considered the most ancient representative of the genus. The other 13 species have Neotropical distribution ( Lee and Langenheim 1975; Langenheim 2003; Fougère-Danezan et al. 2010). There are various hypotheses that attempt to explain the historic distribution of the genus, which are proposed and discussed as follows:

1. Poinar ( Poinar 1991) and Poinar and Brown (2002) argued that the recent distribution of Hymenaea spp. is the result of a vicariance process.

Based on the presence of fossil caesalpinioid pollen in Late Cretaceous of South America ( Muller 1981) , they suggested that Hymenaea existed in Gondwana during the Cretaceous period before the rifting of South America and Africa (at about 100 Ma). However, vicariance in Hymenaea is difficult to sustain as the ancient records found to date of the genus are from Early to Middle Miocene ambers.

2. In contrast, Langenheim and Lee (1974), Lee and Langenheim (1975), and Fougère-Danezan et al. (2010) defend an African origin of the tree.

According to these authors, Hymenaea may have reached Central and South America through two pathways: (1) via the boreotropical corridor proposed by Wolfe (1985) and support- ed by Lavin and Luckow (1991, 1993); or (2) via sea currents ( Langenheim and Lee 1974). In the first situation, Hymenaea could have firstly dispersed to North America and later to Central America and Antilles during the Paleocene–Eocene Thermal Maximum (PETM, an episode of intense global warming at about 55 Ma, lasting 100 thousand years, Magioncalda et al. 2004; Secord et al. 2010). Following this, it would have dispersed to South America via land bridges between Greater Antilles and South America (Iturralde-Vinent and MacPhee 1999, but see Ali 2012).

Some examples of Leguminosae are cited in Calvillo-Canadell et al. (2010), who suggest their distribution expand- ed through B boreotropical flora^ patterns and the migration of certain Afrotropical lowland plants to South America via Europe (Erkens et al. 2009). Nevertheless, the fossil record supports the cited examples. By contrast, there are no fossil records (neither macro-remains nor pollen record) of Hymenaea in Laurasia or South America until the Middle Miocene, and therefore this alternative pathway cannot be considered. Furthermore, certain authors ( Morley 2003; Graham 2011) doubt that megathermal plants from the African tropical rainforest crossed land areas of the boreotropical region and dispersed to North America during the PETM. It seems that the majority of megathermal and mesothermal taxa that would have made this crossing developed in the boreotropical region (30– 45° N) at the precise time.

The other possibility is that the genus originated in Africa and dispersed from West Africa to America via sea currents ( Langenheim and Lee 1974). The authors proposed that the African stock was transported from West African coasts to South America or Antilles via the oceanic South Equatorial Current (SEC) ( Fig. 6 ), considering that the evergreen forests were more widespread in Africa during the Paleogene (see also Feakins and Demenocal 2010; Jacobs et al. 2010). A similar colonization was proposed for other examples involving plants ( Thorne 1973; Chanderbali et al. 2001; Dick et al. 2003; Gottschling et al. 2004; Lavin et al. 2004; Renner 2004; Antonelli 2008) and insects (see below), and was also demonstrated with diverse groups of vertebrates (Fleagle 1999; Houle 1999).

Nonetheless, it is not necessary to explain the distribution of Hymenaea stock from Western Africa to the New World. Oceanic currents in the South Atlantic are similar nowadays since the Late Oligocene ( Fig. 6 ). Shortly after the Late Oligocene the Antarctic Circumpolar Current (ACC) was established (Kennett et al. 1975; Lawver and Gahagan 2003; Pfuhl and McCave 2004). The only area of the surface circulation that is assumed to be particularly different from today’ s circulation occurred in the Caribbean, prior to the closure of the Panama isthmus ( Lawver and Gahagan 2003; Renner 2004). Therefore, Hymenaea might have reached the New World from the Eastern Africa (East coast and Madagascar), where H. oerrucosa is currently distributed.

Madagascar is surrounded by several important oceanic currents ( Lutjeharms et al. 1981). When in contact with Madagascar, the SEC is divided between the Tanzanian and Kenyan coasts and the Agulhas Current, which is located along the Eastern South African coast (Ali and Huber 2010). The South Atlantic currents comprise a gyre: the Benguela current north and westward and the Brazil current south and eastward ( Stramma and England 1999). The Benguela current has two branches, one that crosses the Atlantic to the region of Bahia ( Brazil), and the other parallel to the West African coast towards to the tropics. Both branches link with the westward SEC and reach the regions of Paraiba and Rio Grande do Norte ( Brazil) ( Séranne and Nzé Abeigne 1999). The SEC then runs through the coasts of north Brazil, Guyana (where it mixes with the westward North Equatorial Current (NEC)), Venezuela, Lesser Antilles, and reaches the Caribbean Sea ( Fig. 6 ).

The Hymenaea tree can produce more than 7,000 pods/ year ( Vaca et al. 2002) ( Fig. 7 ). Pods are considered tropical drift seeds because they are very buoyant and impervious to salt water; seeds ( Fig. 7e ) of some species can remain in good condition inside the pod for several months (Gunn and Dennis 1976; Roth 1987). It is possible that pods of the Hymenaea African stock, or partially incomplete tree individuals as well, arrived by sea currents to the Caribbean islands and/or to South America during the Late Oligocene to Early Miocene ( Walker 1990; Gonzoli and Gordon 1996; Bryden et al. 2005; Beal et al. 2011; Simon et al. 2013). Indeed, pods of Hymenaea courbaril ( Fig. 7a–b ) are often washed ashore on the beaches of the Caribbean islands, the Gulf of Mexico, and Florida ( Perry and Dennis 2003).

As well as pods, tangled plant parts, also called B floating islands^, are constantly carried out into the tropical Atlantic from the major African and South American deltas, e.g., Congo, Niger, Senegal, and Amazon rivers. Nevertheless, although eastward currents also exist, the currents may not be ideal for transporting plant remains from South America to West Africa (Fratantoni et al. 2000; Renner 2004).

Figures

Fig. 6 Paleogeographic map of the Oligocene showing simplified oceanic circulation system proposed as responsible for current distribution of Hymenaea spp. in the Antilles and fossil records for the genus. The map has been modified after Langenheim and Lee (1974), Stramma and England (1999), Ali and Huber (2010), and Beal et al. (2011)

Fig. 7 Pods and seeds of Hymenaea spp. Pods of H. courbaril, Simojovel de Allende, Mexico; (a) complete pod; (b) pod cut in half, note the thickness of the wall. Pods and seeds of H. oerrucosa, Ambahy (Nosy Varika), Madagascar; (c) complete pods in a bifurcated branch; (d) complete pods in a single branch; (e) sample of two seeds contained inside a pod