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

Finally, the possibility that the first angiosperms were aquatic has enjoyed renewed attention with the discovery of Archaefructus, a remarkably complete fossil with finely dissected leaves from Barremian-Aptian lake deposits of China. Archaefructus was interpreted as an aquatic herb (Sun et al. 1998, 2002), and a cladistic analysis by Sun et al. (2002) placed it below the common ancestor of all living angiosperms.

A New Perspective.-Discussions of the ancestral ecology and habitat of angiosperms have suffered from two main difficulties. First, previous hypotheses were not well constrained by robust phylogenies. However, independent molecular analyses have recently converged on similar rootings of the angiosperm tree. These studies placed Amborella, Nymphaeales (Cabombaceae and Nymphaeaceae, the water lilies), and Austrobaileyales (consisting of Austrobaileya, Trimenia, the star anise Illicium, Kadsura, and Schisandra) at the base of the angiosperm lineage (Fig. 1) (Soltis et al. 1997, 2000; Mathews and Donoghue 1999; Parkinson et al. 1999; Qiu et al. 1999; Barkman et al. 2000; Graham and Olmstead 2000; Zanis et al. 2002). Chloranthaceae, which occupied various near-basal positions in the molecular phylogenies just cited, branched immediately above this basal grade when molecular and morphological data were combined (Doyle and Endress 2000). In most studies the root was between Amborella and all other angiosperms, although trees in which Amborella plus Nymphaeales form a clade sister to the rest cannot be rejected (Barkman et al. 2000; Zanis et al. 2002). However, rooting the angiosperms near Magnoliales, Chloranthaceae, or Ceratophyllum can now be ruled out. Significantly, several morphological characteristics of Amborella, Nymphaeales, Austrobaileyales, and Chloranthaceae are known in Early Cretaceous fossil flowers, pollen, seeds, and leaves (Upchurch 1984b, 1995; Mohr and Friis 2000; Friis et al. 1997, 1.999, 2000, 2001; Doyle 2001), thus supporting the view that these taxa may represent appropriate ecological models for early angiosperms.

Here Amborella, Austrobaileyales, Chloranthaceae, and Nymphaeales are referred to as "basal" lineages, as contrasted with the much larger clade nested among them that includes all remaining angiosperms (called "core angiosperms"). The term "basal" is often criticized because it is sometimes misused as a synonym for "primitive." In fact, basal groups can be highly autapomorphic. Also, only nodes are actually basal. While recognizing these points, we believe "basal" is a useful shorthand for "a clade whose stem lineage is attached to a basal or near-basal node." More importantly, when several lineages branch off successively below a major clade, as is the case in angiosperms, any states that they share can be inferred to be ancestral, even if each of these lineages is autapomorphic in other characters.

A second difficulty is lack of detailed ecophysiological data for putative basal lineages with which to interpret the paleo-ecophysiology of Early Cretaceous angiosperms. Previous interpretations of early angiosperm ecology relied on structure-function relations taken from derived groups, in which the characters may have evolved convergently (Doyle and Hickey 1976; Doyle 1977,1978; Hickey and Doyle 1977; Taylor and Hickey 1996; Wing and Boucher 1998; Eriksson et al. 2000). Although this approach may be useful, direct physiological evidence demonstrates that inferences of physiological function based on distantly related proxies can be misleading (Feild et al. 1998).