Dark and disturbed: a new image of early angiosperm ecology

Paleobiology, Winter 2004 by Feild, Taylor S, Arens, Nan Crystal, Doyle, James A, Dawson, Todd E, Donoghue, Michael J

Chloranthaceae.-Our analyses also suggest that some Chloranthaceae may have been among the first terrestrial lineages to "break out" of the dark and disturbed environment. Unlike Amborella and most Austrobaileyales, Ascarina and Hedyosmum independently evolved seedling recruitment in wet, disturbed, and higher-light habitats, such as cloud forest land slips, large forest light-gaps, and roadsides (Fig. 2) (Todzia 1988; Martin and Ogden 2002). The ecophysiological traits underlying expansion into these zones require further study. However, greater ecological flexibility in Chloranthaceae is not linked to herbaceousness. Herbs with limited secondary xylem appear to be derived (once or twice) within Chloranthus, and these herbaceous groups occur in shady, disturbed habitats (Fig. 2) (Luo and Li 1999). Hedyosmum orientale, which occurs in sunny, disturbed habitats and has been described as herbaceous (Verdcourt 1986; Todzia 1988), is a possible exception. Recent results, however, showed that H. orientale's growth habit and stem vasculature are different from those of Chloranthus herbs; plants reach 3.5 m in height and produce woody, canelike shoots with up to 3 cm of secondary xylem (Feild unpublished data 2002).

Consistent with greater ecological amplitude, the fossil record indicates that Chloranthaceae were the first conspicuous and widespread extant angiosperm lineage (although their abundance may be overestimated by high pollen production for wind pollination [Endress 1987]). Fossil pollen types comparable to Ascarina and Hedyosmum, named "Clavatipollenites" and Asteropollis respectively, were abundant in Early Cretaceous lowland sediments worldwide (Archangelsky and Gamerro 1967; Playford 1971; Volkheimer and Salas 1975; Brenner 1996; Walker and Walker 1984; Burger 1990, 1993; Archangelsky and Taylor 1993; Dettmann 1994; Hughes 1994; Eklund et al. 2004). "Clavatipollenites" is not necessarily related to Ascarina, as it represents the ancestral pollen type for the family, but Asteropollis has been associated with Hedyosmum-like fruits (Friis et al. 1997, 2000; Eklund et al. 2004). Many Early Cretaceous leaf fossils, also found worldwide, show chloranthaceous features, including chloranthoid teeth, cuticular striations, and stomata with variable subsidiary cell arrangement (Upchurch 1984b, 1995; Pons 1984; Romero and Archangelsky 1986; Upchurch and Dilcher 1990; Cantrill and Nichols 1996; Kong 2001). These foliar features, however, also occur in Amborella and Austrobaileyales and may be plesiomorphic in angiosperms (Upchurch 1984b; Doyle and Endress 2000; Doyle 2001; Feild et al. 2003b).

Fossil Evidence Bearing on the Hypothesis

Although reconstructing ancestral traits on the basis of extant phylogeny is a powerful tool, it is desirable to test hypotheses thus generated with evidence from the fossil record. In the following section, we critically examine existing fossil data bearing on each component of the dark and disturbed hypothesis, and suggest ways in which morphological and sedimentological observations might be used, in combination, to diagnose ephemeral, shady, and wet habitats (Wing and DiMichele 1992; Arens 1997; Davies-Vollum and Wing 1998; Behrensmeyer et al. 2000). Many modern basal angiosperms grow on erosional substrates that are unlikely to be preserved in the sedimentological record (Behrensmeyer et al. 2000). However, stream margins and channels-two disturbed habitats-are common terrestrial depositional settings (Wing and DiMichele 1992).