Fossil of the month: Cirratriradites

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This month’s fossil can only be seen through a microscope. It is the fossil plant spore Cirratriradites saturni. When most people think of fossils, they usually think of objects they can pick up and see with their naked eyes, but extremely tiny fossils are also preserved in rocks. For example, plant spores and pollen are commonly preserved in coals and gray shales. The study of plant spores is called palynology and scientists who study pollen and spores are called palynologists. Palynologists study these spores and pollens to determine the types of plants that existed in the past, provide information on the environment and climates of the past, and correlate rock units from different areas containing the same spores. This month’s fossil has living relatives that are not much different from its 300-million-year-old ancestors!

Description. Cirratriradites is a common spore in many of Kentucky’s coal beds, although generally it accounts for less than 5 percent of the fossil spores found in a coal sample. It has a distinctive sub-triangular shape with three long spines sticking out from the ends of the triangle. The center part of the spore, which has a rough surface texture, is surrounded by a lighter-appearing fringe or annulus called a flange. Specimens are commonly 50 to 75 micrometers across their longest dimension. For comparison, 50 micrometers is less than the width of a single piece of paper and is the average size of a cell in the human body. Although 50 micrometers is tiny, that’s relatively large for spores.

Species. Several species of Cirratriradites are known from Pennsylvanian coal beds and gray shales in Kentucky and surrounding states, but the most common is Cirratriradites saturni. Other species include Cirratriradites annulatus, C. annuliformis, and C. maculatus. Cirratriradites was originally named by Wilson and Coe (1940), who discovered this distinctive spore while studying Middle Pennsylvanian (Desmoinesian Stage) coals in Iowa. The etymology (origin) of the genus name Cirratriradites is unclear, but the species names saturni, annulatus, and annuliformis all appear to refer to the ring-like fringe or flange that surrounds the central part of these spores.

Two species of Cirratriradites, C. saturni and C. annulatus.
Two species of Cirratriradites, C. saturni and C. annulatus. Both are from the Middle Pennsylvanian Springfield coal, Carbondale Formation, western Kentucky.

Range. Cirratriradites is a long-ranging spore genus that occurs from the Devonian to recent. The species Cirratriradites saturni has a shorter range, occurring only in Early and Middle Pennsylvanian strata.

Parent Plant and Paleoecology. Cirratriradites saturni is the tiny spore of a larger lycophyte plant. Lycophytes were the dominant plants in the Pennsylvanian and included giant trees, shrubs, and herbaceous ground-cover plants. Cirratriradites saturni is the dispersed spore of a small ground-cover and sometimes vine-like lycophyte called Selaginella. Most other lycophytes from the Pennsylvanian Period, including lycopod trees, are extinct, but Selaginella still exists today! Although the modern Selaginella is similar in appearance to its ancient ancestors, the species of modern Selaginella differ from their ancestors (Banks, 2009).

Modern Selaginella is sometimes called spike moss and looks somewhat like tiny fir or cedar tree branches or fancy leaf lettuce. More than 750 species are known today, according to Wikipedia. Modern Selaginella is common in tropical climates, similar to the environment in which they are interpreted to have thrived 300 to 320 million years ago, in the coal swamps that formed Kentucky’s major coal beds.

Selaginella reproduces from spores held in cones (strobili). Fossil Selaginella cones have been found with microspores (male gametophyte) of Cirratriradites and megaspores (female gametophyte) of Triangulatisporites, which is how scientists know which plant the dispersed spores came from.The microspore Cirratriradites annulatus (shown above) has been found in fossil cones of Selaginella suissei, an extinct species of spike moss that had tiny, closely spaced to overlapping lance-shaped (spiky) leaves. Cirratriradites saturni (shown above) has been found in cones of Selaginella gutbieri, an extinct species with small oval (obovate), oblong, and lance-shaped leaves from the Middle Pennsylvanian of Germany (Rössler and Buschmann, 1994; Thomas, 1997). These species of Selaginella were tens of centimeters tall, with tiny (0.2-0.6 cm) leaves (Thomas, 1997; Bek and others, 2001).

Specimen of Acrocyathus floriformus on display in the Department of Earth and Environmental Sciences, University of Kentucky. Scale in centimeters.
An ancient Selaginella spike moss, the source of Cirratriradites spores in Kentucky coal beds. Illustration by Stephen Greb based on fossils shown in Rössler and Buschmann (1994) and Thomas (1997).

Fossils of Selaginella are rare in Kentucky’s coal fields, probably because their tiny leaves were thin and rapidly decomposed after death. A few samples have been reported from coal balls (carbonate concretions in coals) in Kentucky (Winston, 1988). Fossil spores from the plant (Cirratriradites saturni) appear to have been more resistant, however, and occur in many coals and gray shales surrounding coal beds. It is the spores of the plant that provide evidence of Selaginella in the ancient coal-forming swamps that once covered Kentucky.

References Cited

  • Banks, J.A., 2009, Selaginella and 400 million years of separation: Annual Review of Plant Biology, v. 60, p. 223–238.
  • Bek, J., Opluštil, S., Drábková, J., 2001, Two species of Selaginella cones and their spores from the Bohemian Carboniferous continental basis of the Czech Republic: Reviews Of Palaeobotany and Palynology, v. 114, p. 57–81.
  • Rössler, R., and Buschmann, B., 1994, Some herbaceous lycopods from the Westphalian D of Germany: Reviews of Palaeobotany and Palynology, v. 80, p. 259–275.
  • Thomas, B.A., 1997, Upper Carboniferous herbaceous lycopsids: Review of Palaeobotany and Palynology, v. 95 (nos. 1–4), p.129–153.
  • Wilson, L.R., and Coe, E.A., 1940, Description of some unassigned plant microfossils from the Des Moines Series of Iowa: American Midland Naturalist, v. 23, p. 182–186.
  • Winston, R.B., 1988, Paleoecology of Middle Pennsylvanian-age peat-swamp plants in Herrin coal, Kentucky, USA: International Journal of Coal Geology, v. 10, no. 3, p. 203–238.

 

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Last Modified on 2021-05-08
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