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.

Figure 1.

Scanning electron micrographs of fossil inflorescences of Araceae from the Early Cretaceous (Late Aptian–Early Albian of the Vila Verde 2 locality (sample Vila Verde 2 439), Portugal. (a–f) Araceae fossil sp. A, and in situ pollen. (a) Inflorescence fragment with densely spaced and spirally arranged staminate flowers on the central axis; S165015. (b) Inflorescence fragment with many spirally arranged staminate flowers; S165005. (c) Single inflorescence section with 10 stamens; S165002. (d) Inflorescence fragment with several flowers, central axis missing; S165001. (e,f) Pollen grains from staminate flower in inflorescence fragment shown in figure 1c; S165005. (g–i) Araceae fossil sp. B, with densely spaced and spirally arranged flowers and periporate pollen; S165007. (g) Internal view of inflorescence showing axis and attached flowers. (h) External view of inflorescence. (i) Periporate pollen grain from anther; arrows show three of several pores. Scale bars: a = 2 mm; c = 250 µm; b, d, g, h = 1 mm; e, f = 10 µm; i = 20 µm. All specimens housed in the palaeobotanical collections at the Swedish Museum of Natural History (S).

Figure 2.

Scanning electron micrographs of in situ pollen from Early Cretaceous mesofossil floras from Portugal. (a,b) Pollen from Araceae fossil sp. A shown in figure 1b; S165005 (sample Vila Verde 2 439). (a) Pollen showing long open colpus and coarse, loose reticulum supported by scattered columellae. (b) Detail of wall showing muri without supratectal ornamentation, but with a granular structure on the internal surface and on the columellae. (c,d) Pennipollis-type pollen from the Late Aptian–Early Albian Vale de Agua locality (sample Vale de Agua 139); S105603. (c) Pollen showing long colpus and very coarse, loose reticulum with spiny supratectal ornamentation. (d) Detail of reticulum showing spiny supratectal ornamentation externally (above) and granular internal surface of muri (below). (e,f) ‘Retimoncolpites’-type pollen from Torres Vedras (sample Torres Vedras 44). (e) Pollen showing long open colpus and coarse, loose reticulum with transverse supratectal striations. (f) Detail of reticulum from two different grains. The upper reticulum is loosed from the main body of the pollen grain and shows the granular inner surface of the muri; the reticulum of the grain below shows the striate supratectal ornamentation of the outside of the muri. Scale bars: a = 10 µm; b = 1 µm; c, e = 12 µm; d = 3 µm; f = 1.2 µm. All specimens housed in the palaeobotanical collections at the Swedish Museum of Natural History (S).