4.7" Alethopteris Fern Plant Leaf Fossil Carboniferous Age Llewellyn FM ST Clair, PA
Location: Llewellyn Formation, St Clair, Pennsylvania
Weight: 5.9 Ounces
Dimensions: 4.7 Inches Long, 4.2 Inches Wide, 0.4 Inches Thick (Plate)
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Pennsylvanian Age, 300 million years old.
Alethopteris Sp Fern Fossil
The Llewellyn Formation, dating back approximately 300 million years to the Pennsylvanian Period, is one of the rare sites where exceptionally preserved plant fossils can be found. These fossils often feature white or pale yellow ferns set against the dramatic black shale, providing a striking contrast that highlights their intricate details.
During the Pennsylvanian, these plants fell into swampy environments where low temperatures, high pressures, and low oxygen conditions allowed their tissues to slowly mineralize. Initially, pyrite replaced the organic material, and later, as sediments accumulated and pressure and heat increased, pyrophyllite (aluminum silicate, whitish in color) replaced the pyrite, preserving the delicate structure of the ferns. The most commonly found genera include Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum, many from plant families that are now extinct, such as Macroneuropteris.
Fossil plants from this region are generally Carboniferous in age or younger. Ferns, horsetails, and club mosses dominated the landscape, though tree-like club mosses and horsetails declined by the end of the Carboniferous due to shrinking swamps and cooler climates. Some coal swamps persisted into the early Permian, forming coal seams intertwined with fossilized plant layers.
Excellent specimens are typically collected from thinly laminated shale layers, which split easily with a chisel or splitting hammer, making them accessible for both fossil collectors and researchers. These Llewellyn Formation fossils offer a rare glimpse into the diverse swamp ecosystems of the Pennsylvanian Period.
