December 15, 2011 6 Comments
This is a polished cross section of a fern trunk that I came across in the Natural History Museum while looking for other fossils. What great colors and anatomy! If you aren’t sure what you’re looking at, think of a typical tree first, like an oak tree. If you cut it down look down at the stump you would be looking at a cross-section of the trunk. You would see outer ring of bark surrounding wood with growth rings, and maybe a bit of pith tissue in the very center. If you cut down a tree fern like this one, this is what you’d see. There is no wood. The thick outer zone of small circles is a mantle of tough roots that provided support to the stem. The stem is the circular structure surrounded by the root mantle, and it has a lot of C-shaped leaf traces in the cortex. In the middle of the stem is another ring of little bundles. This is the stele, where the xylem and phloem is, and in the middle of the stele is that sort of pale-yellow area. That is the pith.
So who is this? This plant is a relative of modern Osmundaceae, a small but widely distributed family of ferns that includes the cinnamon fern, the interrupted fern, and the royal fern. This fossil would probably be assigned to the extinct subfamily Guaireoideae (woah, too many vowels, I know), but the taxonomy of Paleozoic and Mesozoic Osmunda-like ferns is not fully worked out, and so I don’t worry too much about subfamilies. This fossil comes from near the city of Rio Pardo in Rio Grande do Sul, Brazil. It was described by Henry Andrews in 1950 as Osmundites braziliensis. But today is called Guairea carneiri because trunks called Osmundites were transferred to Osmundacaulis, and then Osmundacaulis was divided up into three groups, Osmundacaulis, Millerocaulis, and Guairea; and then G. brasiliensis turned out to be indistinguishable from the earlier species G. carneiri.
In the late Paleozoic and during most of the Mesozoic the Osmudaceae was an impressive family in terms of growth form. Many of these plants were tall tree-ferns, and in some places quite abundant. Today we have shorter, though stately, ferns with mostly subterranean stems. Todea spp., though, can get somewhat “trunky” and may be the best plant to help give an image of the plant this colorful trunk came from. Here are two links to another site with pictures of an impressive Todea barbara. image 1 image 2
Here’s the reference for the original paper.
Andrews, H. 1950. A Fossil Osmundaceous Tree Fern from Brazil. Bulletin of the Torrey Botanical Club 77(1): 29-34