Sahnioxylon, with Ptilophyllum leaves. Here it is reconstructed with a conservative height of 15 meters.

Bennettitales (Cycadeoidales)

I write to the paleo community today not because we have failed, but because we are succeeding like never before. We are ready to take on a challenge.

There is so much good paleoart out there now that I no longer see any excuse for us to persist in our glaring oversight. Today I call on all the paleoartists and researchers of the earth.

Draw bennettites. Think about, read and write about bennettites. More properly the Bennettitales (Cycadeoidales). They should be readily apparent in most of the Mesozoic habitats in which we would place dinosaurs.

We don’t draw them. Wikipedia has next to nothing on them. There are only a handful of canonical depictions: the drawing of Williamsonia sewardiana from Sahni (1932) and the drawing of the same taxon by Douglas Henderson in 1989. There is Cycadeoidea from Delevoryas (1971), Monanthesia from Delevoryas (1959) and the model of Williamsoniella at the Field Museum, from Nathorst (1902). That’s about it for the canon. More recently, but largely unrecognized, are the excellent and authoritative reconstructions of the Yixian Tyrmia and Rehezamites by Sean Murtha and Kirk Johnson in the AMNH exhibit Dinosaurs: Ancient Fossils New Discoveries.

Bennettites (Cycadeoids) are not cycads. They are, however, placed with cycads within the division of gymnosperm plants called cycadophytes (Cycadophyta). The evolutionary relationship between cycads and cyadeoids is not clear, and it is possible that the later flowering plants (angiosperms) evolved from bennettites. Fossil cycad and cycadeoid leaves are, overall, quite similar, except in venation anatomy. The only definitive difference is in the structure of the stomata, and this can only be seen under a microscope in fossils with good cuticle preservation. There are greater differences in the reproductive structures of cycads and cycadeoids, which are  unisexual or bisexual cones. Perhaps the most important difference is that cycads survived to now, and all bennettites are extinct.

Yet bennettites dominated Mesozoic floras from the Triassic up until the K/T, with some evidence that they may have clung to life in Australia as late as the Oligocene.[1] Trying to reconstruct the environments of dinosaurs without bennettites is like depicting the great wilds of the earth today without broadleaf trees and shrubs. Take the Tiaojishan Formation, for example, where we find the earliest paravians. Bennettites account for 35% of the plants there, more than any other group, but I have never seen an illustration of any of them. [2]They dominated in classic dinosaur habitats like the Morrison Formation (Cycadolepis 4 sp.?, Nilssonia 5 sp., Otozamites, Pterophyllum, Ptilophyllum, Weltrichia, Zamites)[3] and they were abundant in the Yixian Formation (Baikalophyllum (formerly Yixianophyllum)[4], Rehezamites, Tyrmia, Williamsonia).[5]

We are not entirely at fault for avoiding them. Definitive whole plant reconstructions are few. There are tons of good bennettite leaf fossils, but just a few have ever been found attached to stems. Those include Ptilophyllum attached to Bucklandia, Nilssoniopteris to Wiliamsonia. Stem fossils are very rare. The stem fossils we do have range in form. Some are 1 m diameter spherical trunks as in Cycadeoidea. Some are 2 meter tall, 14 cm diameter,  palm – like columns like in Williamsonia and Bucklandia. Then there are others, like Ischnophyton, that are just 1 cm in diameter, without persistent leaf bases, and highly branched.[6] Whole terminal leaf clusters, with stem attached, are known for Williamsonia (Wu 1999) and Baikalophyllum  (Pott et al. 2012). Both show cycles of persistent leaf scars forming a collar at the terminal 4 cm or so of the stem, with leaves attached at the axis. Other bennettite stems, like Bucklandia, retain leaf scars for the entire length.

Bennettites were sometimes low and shrubby, but their maximum size is more hypothetical. Paleobotanists have long suspected that some bennettites were arborescent. Fossil wood has been tentatively assigned to bennettites, and sometimes this wood indicates an arborescent growth habit, up to medium – sized trees. Sahnioxylon is probably the trunk of a bennettite, possibly with Ptilophyllum leaves. It is known from India, China, and Antarctica, and ranges from the Triassic to the Cretaceous. It has been reported from the Tiaojishan Formation[7]. It is also known from Early Cretaceous fossilized forests in the Antarctic islands, where it grew fast and reached diameters of 25 cm and estimated heights around 25 meters.[8] Another bennettite trunk is Phoroxylon, which has been known since 1954 from the Yixian Formation. Phoroxylon reached 15 cm in diameter and may have been a small tree. Thickets of bennettites, cycads, and ferns were common around the world in the Jurassic.

Now let’s go out there and depict the Mesozoic world as it was: full of bennettites.