Mario Coiro

Angiosperms in paleoart, especially in the Early Cretaceous, are often limited to familiar forms such as Nymphaeaceae (waterlilies) or magnolias. Although there is evidence of both groups in the Early Cretaceous fossil record, other groups of angiosperms were probably much more abundant across a range of environments during that period.

The main example are Chloranthaceae, one of the first clades of flowering plants to reach almost global distribution. This family is a member of the so-called mesangiosperms, i.e. the clade that includes all angiosperms except the three clades of the ANA grade (Amborellales, Nymphaeales, and Austrobaileyales). Chloranthaceae has four extant genera living in tropical to subtropical climates mostly in the southern hemisphere. Although many people might never have heard of Chloranthaceae, they truly were the stars of the Early Cretaceous.

Pollen of chloranthaceous affinities (usually classified in the genus Clavatipollenites) is found in Early Cretaceous deposits all around the world and mesofossils of flowers of Chloranthaceae are also relatively abundant (a recent, free to access review of thes mesofossils can be found here).

Chloranthaceae, unlike many other ‘famous’ primitive angiosperms, have reduced, diminutive flowers, brought often in lax inflorescences. Their vegetative morphology is quite constant: the leaves are opposite-decussate and they have toothed margins (a particular tooth type with a secretory tip called a chloranthoid tooth). Chloranthaceae today include vines, small understorey shrubs, and herbaceous plants that grow in shaded environments. However, they probably occupied a wider range of habitats during the Early Cretaceous, including more open environments.

Austrobaileyales are also seldom represented in paleoart. This order includes three families: Schisandraceae (which include the star anise genus Illicium), Trimeniaceae, and Austrobaileyaceae. Among the fossil record of the Early Cretaceous, the Austrobaileyales are known as seeds (from localities in Portugal) as well as (potentially) leaves from the Potomac group. However, unlike Chloranthaceae, Austrobaileyales have a much wider variation in both vegetative and reproductive traits. While Austrobaileya and Trimenia have both opposite leaves, the Schisandraceae have leaves arranged spirally on the stems. The order includes both lianas and trees (Illicium and some Trimenia species). The flowers can be unisexual (in Kadsura and Schisandra) or bisexual, though all flowers of the Austrobaileyales have multiple organs arranged in a spiral, no differentiation between sepals and petals, and very broad stamens.

A good portion of the Lower Cretaceous leaf record includes leaves with lobed margins, sometimes clearly associated with plants with herbaceous of weedy habit. Some of these leaves represent the early radiation of the Eudicots, such as Fairlightonia from the Aptian of the Potomac Group in the US and Iterophyllum from the Barremian Las Hoyas locality of Spain. On the other hand, some of these leaves have a more divergent morphology, and present cuticular characters which are more typical of the early-diverging Austrobaileyales or Chloranthales, such as Mesodescolea from the Aptian Anfiteatro de Ticó Formation in Argentina. In some localities, these angiosperms were in fact rather abundant, such as in the Albian of Primorye, Russia. Artistic representation of these (potentially) understory, pioneer, often herbaceous angiosperms are quite rare, though a few spectacular exceptions do exist.

From the Albian onwards, another kind of angiosperms became quite abundant in certain localities. The eudicot leaf Sapindopsis, a relatives of moden Platanus, is quite abundant in Aptian to Cenomanian localities from North America and Asia, and was present (though not abundant) in Europe as well. Sapindopsis leaves differ from Platanus leaves in being more highly dissected, with clearly definite elongated leaflets arranged in an imparipinnate manner (i.e., lateral leaflets on the rachis and a terminal leaflet). Flowers associated with Sapindopsis plants are not unlike ones of extant Platanus, and are brought in dense inflorescences forming globose heads, but often smaller and arranged alternately along an axis like an elaborate P. orientalis.    

Given the enormity of angiosperm diversity, we will keep other groups (such as the waterlilies and other water plants) for a future post.

I wish to thank Nathan Jud for comments on this piece, and Brian Engh for allowing us to show here his work on Fairlingtonia.

References:

Barral, A., Gomez, B., Feild, T.S., Coiffard, C. and Daviero-Gomez, V., 2013. Leaf architecture and ecophysiology of an early basal eudicot from the Early Cretaceous of Spain. Botanical Journal of the Linnean Society, 173(4), pp.594-605.

Crane, P.R., Pedersen, K.R., Friis, E.M. and Drinnan, A.N., 1993. Early Cretaceous (early to middle Albian) platanoid inflorescences associated with Sapindopsis leaves from the Potomac Group of eastern North America. Systematic Botany, pp.328-344.

Doyle, J.A., 2012. Molecular and fossil evidence on the origin of angiosperms. Annual review of earth and planetary sciences, 40, pp.301-326.

Doyle, J.A. and Endress, P.K., 2018. Phylogenetic analyses of Cretaceous fossils related to Chloranthaceae and their evolutionary implications. The Botanical Review, 84(2), pp.156-202.

Friis, E.M., Crane, P.R. and Pedersen, K.R., 2018. Fossil seeds with affinities to Austrobaileyales and Nymphaeales from the Early Cretaceous (early to middle Albian) of Virginia and Maryland, USA: new evidence for extensive extinction near the base of the angiosperm tree. In Transformative paleobotany (pp. 417-435). Academic Press.

Jud, N.A., 2015. Fossil evidence for a herbaceous diversification of early eudicot angiosperms during the Early Cretaceous. Proceedings of the Royal Society B: Biological Sciences, 282(1814), p.20151045.