Late Cretaceous Expansion Of Megathermal Forests

Angiosperms, which dominate megathermal rainforests today, first radiated during the Early Cretaceous from mid- to low latitudes (Crane et al., 1995; Hickey and Doyle, 1977) in response to climatic stress (Stebbins, 1974; Doyle and Donaghue, 1987). They are unlikely to have become initially established in a closed, rainforest setting as previously inferred by Takhtajan (1969) and Thorne (1976) on the assumption that "primitive" angiosperms such as members of Winteraceae, Trochodendron and Tetracentron (with vesseless wood which require a mesic climate) evolved in such areas. The vesseless habit in these angiosperms is now considered a derived character (Doyle and Endress, 1997). They came to dominate over other plant groups in the Albian and Cenomanian (Crane, 1987). The equatorial zone at this time was likely to have been hot (Barron and Washington, 1985; Pearson et al., 2001) and strongly monsoonal (Parrish et al., 1982; Morley, 2000a), but not necessarily "semi-arid" as suggested by Herngreen and Duenas-Jimenez (1990) and Herngreen et al. (1996). The equatorial zone was therefore an unlikely zone for the establishment of the first megathermal, mesic forests. The paucity of mesic low-latitude settings in the Turonian is emphasized by the particularly low diversity of fern spores from the equatorial regions at this time, but their diverse representation in mid-latitudes continued (Crane and Lidgard, 1990).

It was in mid-Cretaceous mid-latitudes, which were in part characterized by perhumid, frost-free climates, that mesic forests first became an important setting for angiosperms in both hemispheres, and by the Cenomanian most of the physiognomic leaf types characteristic of megathermal forests—including simple entire leaves with drip tips, compound and palmate leaves—were already in place (Upchurch and Wolfe, 1987). From the Turonian to the Maastrichtian, many groups that we consider as strictly "tropical" have their first records from these areas, with families such as Bombacaceae, Clusiaceae, Cunoniaceae, Icacinaceae, Menispermaceae, Rutaceae, Sabiaceae, Saurauiaceae, Theaceae, and Zingiberaceae (Mai, 1991; Morley, 2000a; Davis et al., 2005) first appearing within northern hemisphere mid-latitudes, whereas southern hemisphere mid-latitudes saw the appearance of Aquifoliaceae and Protea-ceae, and became a harbour for Winteraceae and Chloranthaceae (Dettmann, 1994).

Within the equatorial zone, mesic angiosperm-dominated forests did not appear until some time after their appearance in mid-latitudes (Morley, 2000a). The first evidence for the development of everwet equatorial climates is probably from Nigeria, where coal deposits are represented from the Campanian to Maastrichtian (Reyment, 1965; Salami, 1991; Mebradu et al., 1986), suggesting an everwet climate. Groups that show their initial radiation in the Cretaceous of the equatorial zone are Annonaceae, Arecaceae, Ctenolophonaceae, Gunneraceae, Fabaceae, Myrtaceae, Restionaceae, and Sapindaceae (Morley, 2000a).

Molecular studies sometimes help to determine which taxonomic groups originated as Northern Megathermal (or Boreotropical) elements, and which have always been equatorial lineages; thus, Davis et al. (2002) indicate that Malphigiaceae are likely to be Boreotropical. Doyle and Le Thomas (1997) show that Anonaceae are an equatorial group, as did Givnish et al. (2000) for Rapataceae. However, care needs to be exercised in assessing the often geographically biased and scattered fossil record of groups being assessed by molecular analyses since the macrofossil record is strongly biased to Europe and North America where most collecting has been done (Morley and Dick, 2003).

The biogeographical histories of the major groups of megathermal angiosperms for the remainder ofthe Cretaceous and Tertiary periods can be divided into two main phases. During the first phase, from the latest Cretaceous to Middle Eocene, the Earth was characterized by greenhouse climates, and predominantly by plate tectonic disassembly (Morley, 2000a, 2003). This was a period of widespread range expansion and diversification of megathermal plants. The post-Middle Eocene, on the other hand, was a period essentially of global cooling and the successive expansion of icehouse climates, coupled with plate tectonic collision, and was mainly a period of range retraction of megathermal taxa.

The time from which mesic megathermal forests can be visualized as closed, multi-storeyed forests, and thus resemble modern rainforests in terms of physiognomy, is debatable. Upchurch and Wolfe (1987) suggested that leaf morphologies from the Cenomanian Dakota Formation reflect such a setting, but at this time angiosperm wood fossils are generally small-dimensioned, and seed sizes small (Wing and Tiffney, 1987), militating against the presence of modern aspect rainforests at this time. A re-examination of leaf assemblages from the same Dakota Formation locality by Johnson (2003, and pers. commun.) show that this locality was dominated by large, lobed angiosperm leaves, not reminiscent of rainforest physiognomy. However, Davis et al. (2005) have used molecular evidence to show that the clade Malpighiales, which constitute a large percentage of species in the shaded, shrub and small-tree layer in tropical rainforests worldwide, radiated rapidly in the Albian-Cenomanian, and suggest that this radiation was a response to adaptations to survive and reproduce under a closed forest canopy.

The first evidence for typical closed, multi-stratal forest synusiae based on fossils comes from the latest Cretaceous of Senegal and Nigeria in West Africa. Evidence includes the presence of casts of large seeds from the Campanian of Senegal (Monteil-let and Lappartient, 1981), a large supply of endosperm in an enlarged seed allowing successful germination under a forest canopy (Grime, 1979). A molecular link between life form and seed size has recently been established (Moles et al., 2005) with large seeds being linked with tropical trees. The presence of seeds or fruit attributable to climbers from Nigeria (Chesters, 1955) and the presence of large-girth angiosperm wood (Duperon-Ladouneix, 1991) also suggests the presence of tall canopy trees. The oldest locality for multi-storeyed forests is therefore likely to have been in the

40 0 Number of species

Figure 1.3. Numbers of stratigraphically useful angiosperm pollen types per epoch, for: (a) South America (data from Muller et al., 1987) and (b) West Africa (data from Salard-Chebaldaeff, 1990), providing a rough proxy for angiosperm diversity through time (from Morley, 2000a).

Pliocene-Oligocene

Eocene

Paleocene

Late Cretaceous

Early Cretaceous

40 0 Number of species

Figure 1.3. Numbers of stratigraphically useful angiosperm pollen types per epoch, for: (a) South America (data from Muller et al., 1987) and (b) West Africa (data from Salard-Chebaldaeff, 1990), providing a rough proxy for angiosperm diversity through time (from Morley, 2000a).

equatorial zone. Subsequently, large-dimensioned seeds are widespread from the Paleocene onward in North America (Wing and Tiffney, 1987) suggesting that following the demise of the dinosaurs, closed multi-storeyed forests became widespread, perhaps coinciding with the radiation of frugiverous mammals. The appearance of evidence for multi-storeyed forests in West Africa coincides with a distinct diversity increase of fossil angiosperm pollen (Figure 1.3, from Morley, 2000a), which was thought to reflect evolutionary adaptations associated with the development of the forest canopy by Niklas et al. (1980).

Kubitski (2005) considers the development of the rainforest canopy in the Late Cretaceous of Africa and South America to be one of the major stages in the development of all land plants. The presence of the rainforest canopy not only facilitated the diversification of most angiosperm families in a manner not seen previously, but also provided a setting for the renewed diversification of pteridophytes, under its shadow, as suggested both from molecular data (Schneider et al., 2004), and also from changes in pteridophyte spore assemblages in the low-latitude palynological record from the latest Cretaceous onward, with the increased representation and diversification of monolete, as opposed to trilete spores from this time.

The K-T meteorite impact probably had a major effect on rainforests globally (Figure 1.3) but did not substantially affect the main angiosperm lineages that characterized each area. Gymnosperms, however, fared particularly poorly in the low latitudes following the K-T event. In the earliest Tertiary gymnosperms were virtually absent from each of the equatorial rainforest blocks. Recovery of rainforest diversity after the K-T event is generally acknowledged to have taken some 10Myr (Fredriksen, 1994), but a recently discovered leaf fossil flora from the Paleocene of Colorado (Johnson and Ellis, 2002) suggests much more rapid recovery, perhaps within 1.4 Myr, suggesting that much more work needs to be done to determine just how long it takes for rainforests to re-establish their diversity after a cataclysmic event.

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  • Sophie Walker
    Is equatorial region are megathermal?
    3 years ago

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