Diversity in obscurity: fossil flowers and the early history of angiosperms

Abstract

In the second half of the nineteenth century, pioneering discoveries of rich assemblages of fossil plants from the Cretaceous resulted in considerable interest in the first appearance of angiosperms in the geological record. Darwin's famous comment, which labelled the 'rapid development' of angiosperms an 'abominable mystery', dates from this time. Darwin and his contemporaries were puzzled by the relatively late, seemingly sudden and geographically widespread appearance of modern-looking angiosperms in Late Cretaceous floras. Today, the early diversification of angiosperms seems much less 'rapid'. Angiosperms were clearly present in the Early Cretaceous, 20-30 Myr before they attained the level of ecological dominance reflected in some mid-Cretaceous floras, and angiosperm leaves and pollen show a distinct pattern of steadily increasing diversity and complexity through this interval. Early angiosperm fossil flowers show a similar orderly diversification and also provide detailed insights into the changing reproductive biology and phylogenetic diversity of angiosperms from the Early Cretaceous. In addition, newly discovered fossil flowers indicate considerable, previously unrecognized, cryptic diversity among the earliest angiosperms known from the fossil record. Lineages that today have an herbaceous or shrubby habit were well represented. Monocotyledons, which have previously been difficult to recognize among assemblages of early fossil angiosperms, were also diverse and prominent in many Early Cretaceous ecosystems.

Recognising angiosperm clades in the Early Cretaceous fossil record

“…Although not yet analysed phylogenetically, inflorescences of sessile, ebracteate and dimerous flowers from the probable early Albian of Portugal (Friis et al 2010b; Figure 1) provide evidence that monocots had begun to diversify by this time, because the floral features listed are apomorphies of Araceae, in the Alismatales, an order that diverges one node above the base of the monocots (after Acorus) in molecular trees. Another probable early representative of Araceae is Spixianthus, based on aroidlike leaves and a stem bearing roots and leaves from the late Aptian Crato Formation of Brazil (Coiffard et al 2013b).…”

Exceptional preservation of tiny embryos documents seed dormancy in early angiosperms

“…Prevailing hypotheses, based on evidence from both living and fossil plants, emphasize that the earliest angiosperms were plants of small stature 7-12 with rapid life cycles 7,8,12,13 that exploited disturbed habitats 3,9,11,13,14 in open 3,9,11,13,14 , or perhaps understory conditions 15,16 . However, direct palaeontogical data relevant to understanding the seed biology and germination ecology of Early Cretaceous angiosperms are sparse.…”

Conservation and canalization of gene expression during angiosperm diversification accompany the origin and evolution of the flower

“…The presence of a diverse assemblage of floral forms shortly after the sudden appearance of angiosperm fossils in early Cretaceous deposits ca. 130 Mya suggests that a rapid radiation established most of the modern lineages within a few million years (1). Famously declared an "abominable mystery" over a century ago by Charles Darwin (in a letter to J.D.…”