Psaronius sp. - fossil tree fern stump from the Pennsylvanian of Ohio, USA. (CMNH P-26081, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

Plants are multicellular, photosynthetic eucaryotes. The oldest known land plant body fossils are Silurian in age. Fossil root traces of land plants are known back in the Ordovician. The Devonian was the key time interval during which land plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and simple and probably remained close to bodies of water. By the Late Devonian, land plants had evolved large, tree-sized bodies and the first-ever forests appeared.

The three-dimensionally preserved fossil shown here is "petrified wood", which is a horrible term for what is technically called permineralized wood. Biogenic materials such as wood or bone have a fair amount of small-scale porosity. After burial, the porosity of wood or bone can get partially or completely filled up with minerals as groundwater or diagenetic fluids percolate through. The end result is a harder, denser material that retains the original three-dimensionality (or close to it). The wood or bone has become “petrified”. Well, no - it’s become permineralized. The most common permineralization mineral is quartz (SiO2). Sometimes, fossil wood or bone has been permineralized with radioactive minerals such as black uraninite (UO2) or yellowish carnotite (K2(UO2)2(VO4)2·3H2O). Recently, fossil bones permineralized with cinnabar have been identified - cinnabar is HgS - mercury sulfide.

This Psaronius fossil tree trunk is a marattialean, which is a group of tree ferns. Marattialeans first appeared during the Carboniferous and the group is still alive today. The maximum height of marattialean tree ferns is about 20 to 30 feet and their trunks have maximum diameters of about 2 feet. They have false stems and pinnate fronds.

A well-known fossil tree-like marattialean is Psaronius from the Pennsylvanian. The vascular system is in an inverted cone trunk, which points downward. The remainder of the trunk, especially the lower part, is adventitious root mantle. The crown of the tree had pinnate fronds and leaves were shed as the plant grew larger. The foliage of Psaronius is known by the leaf-genus name Pectopteris, but that form genus also occurs on other fossil plants. The outer trunk surface of Psaronius trees had spirally-arranged petiole scars (= form genus Caulopteris) or vertical rows of petiole scars, arranged opposite to each other (= form genus Megaphyton).

Classification: Plantae, Pteridophyta, Marattiopsida, Marattiales, Psaroniaceae

Stratigraphy: unrecorded/undisclosed Pennsylvanian-aged unit

Locality: unrecorded/undisclosed site at or near the town of Athens, Athens County, southeastern Ohio, USA

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