Bennettites – The Latest

Bennettite Banksia sketch

The life appearance of bennettites has been extremely hard to reconstruct. Just a handful of fossils of bennettite stems are known to science. Even fewer have attached leaves or cones, and some of these are not well preserved.

To complicate matters further, it has become clear that, in the Mesozoic, true cycads, the Cycadales – such as Ischnophyton and Nilssonia had slender, branching, even vine – like, stems that could have been confused with those of bennettites. Stems of other plants, like pentoxylaleans, may have also caused confusion.

Recent work by Christian Pott of the Swedish Museum of Natural History has given all of us paleoartists our dream come true. Pott has sorted through all of the confusion, finding and re – describing the 19th century type specimens, and publishing new and exquisite fossils from China. Taking his extremely important work into account, here is what I can report about the state of our knowledge.

To clarify my earlier posts on these ubiquitous Mesozoic plants, there are two families of bennettites. They are the Cycadeoidaceae and Williamsoniaceae.

  1. The Cycadeoidaceae superficially resemble extant cycads, with thick, scaly trunks and a crown of fronds at the apex. These have cones between each leaf scar. Monanthesia, Cycadeoidea, (and the former Bennettites) are member genera.

Between the bases of every frond were small cones. I think of them as being analogous to figs, in that they were closed capsules that contained the pollen organs and seed follicles. They were pollinated, in some cases, by beetles that bored into the cone and left coprolites that were sometimes preserved there by fossilization.

  1. The Williamsoniaceae are named for fossils of the reproductive structure Williamsonia. Often called an inflorescence or even flower, this was actually a female or bisexual cone surrounded by bracts (fossils of detached bracts are often found on their own and called Cycadolepis) in an artichoke – like arrangement. In some bennettites, such as Williamsonia margotiana from the Lower Cretaceous Wealden Formation of England, the bracts opened up into a crown of showy ‘petals’ as large as a magnolia. Other female cone genera include Bennetticarpus.

Male cones (microsporagiate organs), which produced pollen and not seeds, are often called Weltrichia.

As far as we know all williamsoniaceans had slender, branched stems.

As we saw in my earlier posts, Sahni reconstructed a tall Williamsonia sewardiana with Bucklandia as its trunk, Ptilophyllum cutchense leaves, and Williamsonia (possibly W. scotica) cones[1]. The reconstruction resembles an elongated cycadeoid, with an armor of persistent leaf scars. This is an iconic fossil species for paleobotany though it was questioned by Delevoryas and Hope (1976) [2] and, as Pott points out, the stem may actually belong to an early cycadeoidacean, and not a williamsoniacean.

Bucklandia pustulosa is a williamsoniacean From the Yorkshire Jurassic flora of England. It was reconstructed (Harris, 1969) with branches of continually tapering diameter, and with bark that was largely smooth, but dotted with lenticels, giving it an appearance superficially similar to many angiosperm trees (e.g. Ailanthus). [3] B. pustulosa was reconstructed with Ptilophyllum pecten leaves and Williamsonia leckenbyi cones.

Watson and Sincock reconstructed Bucklandia – type trunks from the Lower Cretaceous Wealden Formation with Zamites and/or Ptilophyllum foliage.[4]

Pott advances a new analog for Williamsoniaceans. He presents evidence that they may have grown much like the modern Banksia, of Australia. These plants are shrubs or small trees that are adapted to survive wildfire and which have a characteristically dense growth habit. Pott reconstructs Wielandiella from Nathorst’s original fossils, dating back to 1880. It is a shrub, perhaps 2 meters tall, with slender stems that branch widely and develop cones in the angles.[5] Vardekloeftia may be the mature seed cone, in which most follicles are unfertilized and just a few have developed into enlarged seeds with armored, ridged, hulls. The leaves are Anomozamites.

He has also suggested that Nilssoniopteris (= Taeniopteris) vittata leaves are associated with Williamsoniella coronata, and written that he intends to publish on this plant in the future.

Pott demonstrates that bennettites display a branching pattern known as divarication. Divaricating plants have arisen independently in different plant families and habitats, and the literature and debate on them is fascinating. Explanations for a divaricating growth habit include adaptation to glaciers, efficiency of light – harvesting, and others. Divaricating plants are still most common in New Zealand, where the giant moa went extinct just 600 years ago, and Madagascar, where the elephant birds went extinct 300 or 400 years ago. These are very different climates, one is cool / temperate and postglacial while the other is tropical and seasonally dry, suggesting that climate and glaciers are not the common factor.

The leading hypothesis suggests that the common evolutionary pressure was heavy grazing by dinosaurian herbivores: giant ground birds. In environments where mammalian browsers predominate, divaricating plants are rare and woody plants are often thorny, such as Acacia. Thorns slow feeding rates in mammals, which may tear down large branches and eat them on the ground but must protect their fleshy lips, cheeks and tongues and chew carefully. Thorns do not, however, deter ratites, which use hard beaks to feed.[6] Divaricating plants, in contrast, form dense tangles of stems, with tough leaves, and wide branching angles. The inner foliage is well protected in a woody cage, and birds, which cannot shear off large shoots like ungulates do, spend too much time pecking at widely spaced leaves to obtain adequate nourishment. In feeding experiments goats were able to take 4 times more leaves from divaricating plants than emus did.[7] Pott raises the fascinating possibility that bennettites, evolving as they did in ecosystems with gigantic sauropod grazers taking every leaf they could reach, hit on the same solution but far earlier in earth history.

[1] Sahni, Birbal. (1932) Reconstruction of an Indian Cycad. Nature 130, 24-24. 02 July 1932. doi:10.1038/130024b0

[2] Delevoryas T, Hope RC (1976)More evidence for a slender growth habit

in Mesozoic cycadophytes. Rev Palaeobot Palynol 2:93–100

[3] Harris, Thomas Maxwell. (1969) The Yorkshire Flora III: Bennettitales. Publication No. 675, British Museum of Natural History. London.

[4] Watson, Joan; Sincock, Caroline A. (1992) Bennettitales of the English Wealden. Monograph of the Palaeontographical Society. London.

[5] Pott, Christian. (2014) Revision of Wielandiealla angustifolia, shrub – sized bennettite from the Rhaetian-Hettangian of Scania, Sweden, and Jameson Land, Greenland. Int. J. Plant Sci. 175 (4) University of Chicago. DOI: 10.1086/675577

[6] Bond, W. J., & Silander, J. A. (2007). Springs and wire plants: anachronistic defences against Madagascar’s extinct elephant birds. Proceedings of the Royal Society B: Biological Sciences, 274(1621), 1985–1992.

[7] Bond, W. J., Lee, W. G. and Craine, J. M. 2004. Plant structural defences against browsing birds: a legacy of New Zealand’s extinct moas. _/ Oikos 104: 500_/508.

One response to “Bennettites – The Latest

  1. Pingback: PaleoNews #18 (National Fossil Day 2015 Special Edition) | An Odyssey of Time·

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