January 29, 2012 10 Comments
Cycadeoidea is one of the classic genera of extinct Mesozoic plants. A 1971 reconstruction is widely reproduced online, and there was even a Fossil Cycad National Monument dedicated to these fascinating plants in South Dakota where many were preserved in place. Unfortunately, it was officially closed in 1975 because poachers had taken nearly all of the fossils.
Readers with something of a paleobotany background may already know that cycadeoids are not the same as modern cycads, and that early interest in these plants was driven by the hypothesis that they are closely related to flowering plants. Today it appears that cycadeoids were part of their own distinct lineage of seed plants, and the sister group of flowering plants continued to be debated.
Cycadeoids grew somewhat like palms, cycads, and some cacti today. These plants all have a primary thickening meristem. In other woody plants the growing shoot tip adds height to a plant whereas the vascular cambium adds thickness by producing wood and bark. In plants with a primary thickening meristem the growing tips add both height and girth. They either don’t produce wood at all (palms) or the vascular cambium produces relatively little wood (cycads and cycadeoids). Like many cycads, Cycadeoidea stems are covered in the hard, persistent bases of the old, shed leaves. Unlike in modern cycads where seed cones or pollen cones are produced terminally as a dichotomous branch, the outer armor of leaf bases in a Cycadeoidea is interspersed with the cones that produced both pollen and seeds (you may see them referred to as flowers).
The images here are taken from slides made in the early 1900’s for publications by Wieland (1916). The slides come from one silicified Cycadeoidea trunk. Images of similar petrified trunks are not hard to find online; however, it can be fairly difficult to see detailed pictures of the internal anatomy without subscriptions to a few different scholarly journals.
First I have a longitudinal section of a Cycadeoidea trunk, as though the stem was in half along the axis. Look at how small the seeds were! In some cycadeoids there were cones associated with every leaf. These plants had high fecundity. [UPDATE: I just wanted to point out that the little gray or brown bodies in the seeds below and in the second image are the cute little baby cycadeoids. In some of them you can make out a couple cotyledons.]
Here is a tangential section of the trunk through the outer armor of leaf bases and cones. Because the cones are borne laterally, you are looking at cross sections of the cones, as though the cone was cut in half perpendicular to the axis.
Last is a near-longitudinal section through a stem apex. Most of the stem tissue is opaque (black), but you can see the pith bounded by vascular tissue at the bottom of the photo, and the armor of leaf bases along the sides with a cone base in the lower left. The tip is where we would expect to see immature leaves developing, but I’m not sure precisely what the wavy, hair-like lines are, but I will come back to them in a future post, after I’ve done a little more research. Mature leaves have never been found attached to permineralized Cycadeoidea trunks, but we know they were thick pinnate leaves similar to cycads because the arrangement of the vascular bundles in the petiole or rachis of detached leaves can be matched with the pattern in the persistent leaf bases (Yamada et al. 2009)
Wieland, G.R. 1916 American Fossil Cycads. Vol. 1. Carnegie Institute of Washington, Washington D.C.
Yamanda, T. J. Legrand, and H. Nishida 2009. Structurally preserved Nilssoniopteris from the Arida Formation (Barremian, Lower Cretaceous) of southwest Japan. Review of Palaeobotany and Palynology 156: 410-417