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| Prairie Quillwort is 50 centimeters tall (© 2015 Robbin Moran); Scale Tree is 50 meters tall (source). Both are lycophytes—formerly "Fern Allies". |
This episode of
The Monthly Fern was going to feature
Isoetes melanopoda, the Prairie Quillwort, mentioned last month in the wildly popular
Prairie Spikemoss post (1). But in my search for information I fell down a rabbit hole and landed with a splash in an ancient Wonderland—a wet lush forest 350 million years ago near the start of the Carboniferous Period (2). After extricating myself from the muck I looked around. Tree ferns, horsetails and dragonflies looked familiar, though a bit large. But the trees were very strange.
It was during the Carboniferous that wetland forests with tall trees first appeared in the fossil record. These were hot humid riotous tangles of vegetation growing in shallow water and muddy peat that reeked of decay. Dense stands of curious trees rose high above the understory. The most common (or best preserved) was Lepidodendron, the Scale Tree. "Scale" refers to the distinctive bark—a network of diamond-shaped leaf scars (Halliday 2022).
Though they've been gone for 300 million years, we know a lot about these trees. Their fossilized remains are among the most extensive for any plant from any geological period, and for good reason. Not only was
Lepidodendron large and ecologically dominant, it lived in waterlogged conditions conducive to preservation. Paleontologists have been able to describe features ranging from spores to leaves to trees, and even stands of trees (Hetherington et al. 2016).
Lepidodendron differed in many ways from the trees we know. Stems of young trees were covered in long ascending needle-like leaves. These fell off as the tree grew taller and wider, leaving a spiraling network of diamond-shaped scars on the trunk. With age the trunk developed a thick tough outer layer—bark of sorts—but underneath was soft spongy tissue instead of wood. At maturity the stem branched dichotomously (repeatedly forked), forming a high crown as much as 50 meters above the ground. The final branchlets were tipped with strobili (cones) filled with spores, to be dispersed by wind (source). This so-called "tree" was an arborescent lycophyte, a fern relative.
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| Juvenile and mature Lepidodendron on left; trees to right are related lycophytes. Reconstruction from fossils, by Falconaumanni. |
Given
Lepidodendron's massive build, its lack of internal wood and the waterlogged habitat, it seems it would fall before reaching such heights. What kept it upright?
Some credit the thick tough bark. But others argue convincingly for the robust root system. From very long rhizomes grew a profusion of highly-branched rootlets covered in root hairs—on the order of 26,000 rootlets per meter of rhizome! They intertwined with those of adjacent trees, forming a strong anchoring network—"trees holding onto one another for stability" (Hetherington et al. 2016; Halliday 2022).
Lepidodendron truly was an arboreal superstar, dominating the wetland forests and producing immense amounts of biomass for tens of millions of years (3). But it was doomed. By 300 million years ago, the Carboniferous rainforests and arborescent lycophytes were gone, destroyed by widespread drought. Lepidodendron's only surviving relatives are little herbaceous plants—Isoetes, the quillworts.
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| Bolander's Quillworts in a lake in the Wasatch Mountains, Utah. Andrey Zharkikh photo. |
Now we return to the present—to a roadside waterhole along highway #44 in Mellette County, South Dakota, where W. H. Over (4) made his 15,878-th plant collection on July 10, 1924. Isoetes has not been collected in the state since.
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| Over correctly identified his collection to genus—Isoetes. Four months later, TC Palmer called it I. melanopoda, and Daniel F. Brunton agreed in 1995. The specimen resides at the Academy of Natural Sciences, where it has been digitized for all to enjoy (cropped here). |
Like all quillworts, the Prairie Quillwort has no stem. Instead a cluster of leaves develops from the rhizome. From a distance these look like clumps of grass, but up close the leaves are distinctive—long, slender, quill-like, and bright green. The leaf bases are broad and pale, forming a swollen rootstock. These usually become black with age, hence the alternative common name—Black-foot Quillwort (plants with leaf bases that remain pale have been called
I. melanopoda f.
pallida).
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| Isoetes melanopoda; MWI photo. |
Being a lycophyte, Prairie Quillwort produces no seeds. Nor does it have rows of spore-bearing sori on the undersides of its leaves as do most true ferns. Instead, spores are produced in a sac on the inner side of leaf bases.
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| Isoetes melanopoda; MWI photo. |
Surely some readers are wondering ... how is little
Isoetes like immense
Lepidodendron? Why do botanists think they're related? Answer: It's the roots, especially the way they branch.
"this architecture is conserved among [Lepidodendron's] only extant relatives, herbaceous plants in the Isoetes genus. Therefore, despite the difference in stature and the time that has elapsed, we conclude that both ... have the same rootlet system architecture." (Hetherington et al. 2016)
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| Lepidodendron and Isoetes rootlets branch dichotomously, narrowing in a stepwise manner. A is a diagram of a rootlet with 4 levels of branching. B and C show rootlets of Isoetes and Lepidodendron (scale bars are 5 mm). Hetherington et al. 2015, Fig. 1 in part. |
A final question: Is Prairie Quillwort gone from South Dakota? Has it suffered its own extinction? Maybe so. Skilled botanists have searched for it with no luck. But perhaps they were limiting themselves, looking only in "roadside waterholes" and such. Of great interest to me is its occurrence nearby in Minnesota where it's a state Endangered species. It's distribution there is quite limited—rainwater and seepage pools in quartzite rock outcrops,
in the southwest corner of the state.
This rock—
Sioux quartzite—also crops out in the
southeast corner of South Dakota. I visited several locations a few years ago, and am tempted to go back now that I have a such a vivid search image for Prairie Quillwort in my head!
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| Sign at Palisades State Park, SD. Pink marks exposures of Sioux Quartzite in southwest MN and southeast SD. |
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| See any habitat? |
Notes(1) Last week I noticed that views of my
Prairie Spikemoss post had skyrocketed. Now
Desert Mountain is getting the same level of attention. I'm suspicious. Are chatbots visiting, searching for information? Are AI models being built? Have you had this experience?
(2) In North America the Carboniferous often is treated as two periods— Mississippian followed by Pennsylvanian.
(3) Though long extinct,
Lepidodendron remains vitally important. Fossilized remains of those wetland forests with giant trees and abundant peat (aka
Coal Forests) drove rapid industrialization in the 18th and 19th centuries, and continue to sustain our dependency on fossil fuels (oil and gas are more common in Cretaceous rocks).
(4) W. H. Over must have been a bright and highly motivated autodidact. He quit school in Illinois after finishing the 8th grade, homesteaded in South Dakota, and by the time of his
Isoetes collection, was Museum Curator at the University of South Dakota. He would go on to become one of the state's great botanists. Among his many achievements is the "Flora of South Dakota"—the first comprehensive treatment of plants known for the state. For more about Dr. Over (he was awarded a Doctor of Science degree at age 70), start
here and
here. I'm still looking for a comprehensive biography.
Sources (in addition to links in post)
Halliday, T. 2022. Otherlands; a Journey through Earth's Extinct Worlds. Random House.
The Carboniferous Period is in Chapter 11, "Fuel". Halliday's descriptions of past worlds are surprisingly detailed, and supported with many citations.
Moran, Robbin. 2004. A Natural History of Ferns. Timber Press.
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