Here is the third part of my Paleobotany for Paleoartists series, dedicated to a less well-known group that was however very successful during the Early Cretaceous: the Gnetales.
Extant Gnetales are comprised of three highly distinc genera, namely Ephedra, Gnetum, and Welwtischia. This last species is probably the most well known, mostly because of its unique lifestile and extreme habitat. However, Ephedra is probably the most common, living in Mediterranean and more arid climates around the world (except Australia). Gnetum includes vines and one short shrub living in wet tropical forest, and is also rather unique for its broad leaves that are uncannily similar to angiosperm leaves.
These three genera are considered relictual, although Ephedra and Gnetum have experienced species diversification in the Cenozoic. During the Early Cretaceous, however, Gnetales were extremely abundant both as pollen (the typically plicate ‘Ephedroid’ pollen is quite recognizable) and as macrofossils, and where distributed worldwide although mostly limited to low-latitude, more ‘arid’ environments.
Although they seem at first sight to be radically different, all Gnetales present a series of common characters that are quite evident in their fossils as well. First, they all have opposite leaves with a decussate branching patterns both in vegetative and reproductive structures. This means that the leaves are brought in pairs at each node, and the leaves of each subsequent node are rotated at 90 degrees compared to the previous one. In Welwitschia, though the main stem only bears two opposite, permanently-growing leaves, this decussate arrangement can be clearly seen in the reproductive stems. Second, they all have multiple axillary buds. This means that, unlike most other land plants, multiple stems (vegetative or reproductive) can emerge from the axil of a single leaf.
Many of the Early Cretaceous Gnetales are preserved as large fragments or almost entire plants, which makes it much easier to imagine how they would have been as living plants. Many of these reconstructions are easily available in the Extinct Plant Paleoart database. However, many are only known from dispersed seeds or small fragments.
Early Cretaceous Gnetales would probably form tussocks or bushes, and probably occupy a role similar to extant grasses in many extant biomes (see the excellent post by Maija Karala). Although extant Ephedra have scale-like leaves, Early Cretaceous Gnetales have a much wider variety of leaf morphology, with broad-leaved forms such as Drewria from the Potomac formation of the US and the Codó Formation in Brazil and Cearania from the Crato Formation of Brazil. Plants similar to extant Welwitschia were probably already present, and leaves suggestive of this morphology as well as seedlings are retrieved from the Crato formation.
Another bonus of including Gnetales in paleoartistic reconstruction is the possibility of including both animal dispersal and insect pollination. While the former has been discussed excellently by Maija Karala, the latter is probably a less well-known fact. Insect pollination appears to have evolved at the crown group of the Gnetales, and has been lost within the genus Ephedra. Instead of nectar, the Gnetales seem to offer a secretion called a pollination drop, secreted by the ovule. This is also present in other gymnosperms.
Animal dispersal is however not the only peculiar dispersal mechanism in the Gnetales. Extant Welwitschia, as well as a few fossil taxa such as “Chaoyangia” from the Yixian formation in China, are dispersed by wind, and the dispersal units form expanded ‘wings’.
I hope this will encourage people to add more Gnetales to their low-latitude Early Cretaceous environments!
For the people who are interested in the story of the relationship between Gnetales and angiosperms, you can check here.
For more references on the group:
Bolinder, K., Humphreys, A.M., Ehrlén, J., Alexandersson, R., Ickert-Bond, S.M. and Rydin, C., 2016. From near extinction to diversification by means of a shift in pollination mechanism in the gymnosperm relict Ephedra (Ephedraceae, Gnetales). Botanical journal of the Linnean Society, 180(4), pp.461-477.
Ickert-Bond, S.M. and Renner, S.S., 2016. The Gnetales: recent insights on their morphology, reproductive biology, chromosome numbers, biogeography, and divergence times. Journal of Systematics and Evolution, 54(1), pp.1-16.
Kunzmann, L., Mohr, B.A. and Bernardes-de-Oliveira, M.E., 2009. Cearania heterophylla gen. nov. et sp. nov., a fossil gymnosperm with affinities to the Gnetales from the Early Cretaceous of northern Gondwana. Review of Palaeobotany and Palynology, 158(1-2), pp.193-212.
Lindoso, R.M., Dutra, T.L., Carvalho, I.D.S. and Medeiros, M.A., 2018. New plant fossils from the Lower Cretaceous of the Parnaíba Basin, Northeastern Brazil: Southern Laurasia links. Brazilian Journal of Geology, 48(1), pp.127-145.
Roberts, E.A., Loveridge, R.F., Weiß, J., Martill, D.M. and Seyfullah, L.J., 2020. Reinvestigating the fossil leaf Welwitschiophyllum brasiliense, from the Lower Cretaceous Crato Formation of Brazil. Cretaceous Research, p.104471.
Rydin, C. and Bolinder, K., 2015. Moonlight pollination in the gymnosperm Ephedra (Gnetales). Biology letters, 11(4), p.20140993.
Yang, Y., 2010. A review on gnetalean megafossils: problems and perspectives. Taiwania, 55(4), pp.346-354.
Yang, Y., 2014. A systematic classification of Ephedraceae: living and fossil. Phytotaxa, 158(3), pp.283-290.
Yang, Y. and Ferguson, D.K., 2015. Macrofossil evidence unveiling evolution and ecology of early Ephedraceae. Perspectives in Plant Ecology, Evolution and Systematics, 17(5), pp.331-346.
Leave a Reply