Cycadeoidea
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
Nathan
Great post, the photo’s are fantastic , I can bring them up as very large and detailed.
The wavy lines could be early developing leaves in an accordion – like state . Which will inflate quickly when growing?
Rick
I think so, and they are reminiscent of young palm leaves developing plications along witch the lamina will eventually separate. I think that might jive with the mature morphology of some bennettitalean leaves with which I am familiar, but it will take some research to test that interpretation. See Gunawardena and Dengler 2006.
Hey, how’d you find me Rick? I sure miss Vancouver.
You Linked me in Linkedin which led me to your blog
Lovely detailed photos! I’ve just shared this blog with my Plant Structure students.
Just a quick question about cycads….
It’s sometimes said that dinosaurs probably did not eat cycads because all living cycads are highly toxic. What is the likelihood that fossil cycads were toxic as well? Are fossil cycads phylogenetically bracketed by living cycads?
First the short answers: good and some of them. Now the long answer:
It is unfortunate that there are many different kinds of plants in the fossil record that are called “cycads” because it means that there has to be a long explanation whenever discussing fossil cycads. I’ll break them down into three groups for now, and hopefully this will give you a few leads. Group 1: These are fossil cycads that actually are fossil cycads in that they are closely related to the genera in the extant order Cycadales (such as Cycas [Cycadaceae], Zamia [Zamiaceae], or Stangeria [Stangeriaceae]). These were probably unappetizing in the Cretaceous as they are today (though their large fleshy seeds and young leaves may have been good eating for dinos). Group 2: Next there are the cycads in the Nilsonia group (often spelled Nilssonia, and comprising the family Nilssoniaceae). Based on reproductive organs associated with fossil Nilsonia leaves, these were most closely related to the modern cycad families despite the fact that they grew quite differently- they were shrubs and vines. Although many of these had thick foliage that may not have been appetizing, in my own field experience I’ve observed extensive insect damage on the leaves, so somebody was eating them at least. Finally, Group 3: Now we arrive at the real challenge. The order Bennettitales (including Cycadeoidea in the post above) has not settled into a stable phylogenetic position, primarily because the jury is still out on how the 5 living groups of seed plants are related. Part of the problem is that there are more extinct types of gymnosperms than there are living ones! Some think the closest modern relative to Bennettitales are the Gnetales, but I am not convinced. Others have suspected for some time that the bennes are relatives of flowering plants, but recently increased doubt has been cast on this model. One thing I can tell you about these plants is that they were diverse! There are 2-3 recognized families, but there were probably more, and Bennettitales are known as columnar trees, much-branched shrubs, and small plants that may have had underground stems. Again, however, most of these seem to have had thick leaves that look like they were well-defended against herbivory. In addition, the petrified trunks of the columnar trees and small plants show large canals that probably moved toxic resin around the plant as protection against damage and herbivory.
Thanks for the great answer! But what about Cycadales proper? Do living taxa phylogenetically bracket fossil taxa? Or do crown Cycadales not encompass extinct taxa?
Ah, some Mesozoic fossil cycads are assigned to extant families, and are therefore bracketed within the order (i.e. Brunoa and Worsdellia – Zamiaceae); however many, if not most extinct taxa, have not been incorporated into cycad phylogeny with strong support. I would say that it is clear that the cycad families, and some genera, are Mesozoic. This means that it is likely that some of the unplaced extinct Mesozoic genera belong to crown group Cycadales. Nilsoniaceae, on the other hand, are probably stem Cycadales, so there is a mix. If you need a recent citation, Nagalingum et al 2011 shows that many of the splits are ancient (if you really get into it, be sure to compare their topology with those found by other workers).
Great stuff. Thanks again!