March 28, 2013 Leave a comment
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.
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…
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.
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.
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.
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.
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!