Triassic cycadophytes from Arizona

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This Triassic Cycadophyte is part of a collection I am making this week with three others for the new Smithsonian deep time exhibit!

Triassic

Here is a complete blog post

http://nmnh.typepad.com/smithsonian_fossils/2014/04/triassic-plants.html

 

A paleobotanical puzzle

The plant fossil record is composed of fragments that represent different parts of the plant body and different stages of the plant life cycle. Part of the challenge in paleobotany is putting the pieces back together and getting a concept of the whole organism. Recently I was looking through the Smithsonian’s collections from the Patuxent Formation in Virginia because I was searching for fossils of early flowering plants. One of the most common plant fossils in the Patuxent Fm. are the leaves known as Dioonites buchianus. Dioonites leaves look like the leaves of a cycad, but there are several extinct groups of gymnosperms that had cycad-like foliage. In Cretaceous collections, cycad-type foliage typically comes from plants either in the order Cycadales, or in the extinct order Bennettitales.

Dioonites buchianus leaf

Dioonites buchianus leaf with fern

The standard way to tell whether compressed cycad-type leaf fossils are cycads or bennes is to examine the stomata. Stomata are the pores in the leaves that plants use to transpire water and take up carbon dioxide. Bennettitalean stomata have a unique development and morphology that makes them recognizable. They have thickened cuticle on the outer and dorsal walls of the guard cells, they are arranged more or less in rows, and the stomatal pores are oriented perpendicular to the veins (Taylor et al. 2009). Lets have a look using an epifluorescence microscope…

dioonites buchianus epidermis

Epidermis of Dioonites buchianus

dioonites buchianus stomata

Stomatal pore of Dioonites buchianus

BINGO! Dioonites buchianus is a benne. In both photos, the veins run from the lower right to the upper left, but they are not visible.

I also note that it seems to be a general rule that the leaflets or blade of bennettitalean leaves attaches along the upper surface of the petiole (rachis), whereas in cycads it generally inserts along the middle of the rachis. In D. buchianus, the leaflets attach along the top.

Dioonites buchianus

Dioonites buchianus, scale=1cm

I also noticed that in some of the collections there are a few fossils that belong to the species Williamsonia virginiensis. Williamsonia fossils are cones or parts of cones produced by some bennettitalean plants. This intrigued me and I decided to test the hypothesis that the same plants produced the leaves called Dioonites buchianus, and the cones Williamsonia virginiensis.

USNM 3404 cpt

Williamsonia virginiensis

Williamsonia virginiensis

Williamsonia virginiensis

Williamsonia virginiensis cones consist of bracts (modified leaves) arranged around a central scar. The central scar is where the ovulate receptacle attached. The ovulate receptacle is a distinctively bennettitalean structure that bears the megasporophylls and seeds. See an example here, on the right. Sometimes these structures are found isolated with exceptional preservation (Stockey and Rothwell 2003). Unfortunately, I haven’t seen any of these ovulate receptacles in the collections.

USNM 3404

Williamsonia virginiensis

In the lieu of finding the Williamsonia cones and Dioonites leaves actually connected in a single fossil via a stem, I had to employ alternative approaches for demonstrating affinity. One way is to analyze association data, and another is to demonstrate morphological and structural similarities.

First, I looked at a table of all the individual sites where plant fossils have been collected from the Patuxent Formation and what species were found. I noticed that only some of the collections included both D. buchianus and W. virginiensis, but that fossils of Williamsonia virginiensis were never found without abundant fossils of D. buchianus from the same site. If I had found that each was often found without the other, I’d be more likely to conclude that they came from different species.

Next, I decided to compare the epidermal structure of the bracts of the Williamsonia cones with the Dioonites leaves.  Above we saw that the cells on the surface of the leaves have wavy (or crenulate) margins and stomata that are sunken, arranged in rows with the pores oriented perpendicular to the veins, and surrounded by two thickened cells that fluoresce brightly under the scope. Only one Williamsonia had the original carbon of the bracts preserved and thus the potential to see the epidermis under the scope. 

LJH 71 117 williamsonia

Williamsonia virginiensis

Cuticle of W virginiensis

Cuticle of Williamsonia virginiensis

The margins of the epidermal cells are less crenulate, but the stomata have similar structure! Although I think the similarity supports the hypothesis that these two go together, what I saw still surprised me. I expected to see the epidermal cells with the crenulated margins, and no stomata. I suppose the various illustrations that I have seen over the years of bennettitalean flower-like cones with petal-like white or otherwise colored bracts is what was behind this expectation. But the stomata are there, and they are abundant! This means that in life the bracts were probably green, and based on the density of stomata, I’ll bet they were important in supplying photosynthate to the developing ovules/seeds!

Stockey and Rothwell 2003

Taylor et al. 2009

Cycadeoidea

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

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