A Devonian Callixylon Log of the Archaeopteris Tree Found in Marion County, Kentucky

Abstract

A rare fossil discovered in Kentucky gave geologists and paleontologists an opportunity to learn more about the natural history and origin of the rocks and fossils in the state. While excavating on his property in Marion County, a landowner, encountered what appeared to be a tree imbedded in the black shale. He contacted the Kentucky Geological Survey at the University of Kentucky and asked them to investigate the find.

The petrified log was horizontal in the Devonian shale. It measured 20 inches in diameter, and about 13 feet of it had been excavated from the bed however, more of it remained in place. Initial identification indicated that it was Archaeopteris (Callixylon), a Middle to Late Devonian progymnosperm with fern-like leaves and gymnosperous wood (Callixylon is the formal genus name given to the petrified wood of Archaeopteris). A paleobotanist at KGS further confirmed the initial identification. Parts of the tree were cut into slabs and polished for closer examination, which revealed internal cell structures of wood, growth rings, quartz crystals, and fragments of woody material. The cell structures are similar to those of modern conifers. The organic black color and minerals, mainly quartz, were absorbed into the cell structure from the sediments as the log underwent petrification. Analysis of the petrified wood and surrounding material confirmed that the three most prominent constituents, in descending order, were silicon dioxide, aluminum oxide, and iron oxide.

The Archaeopteris tree is Middle to Late Devonian, about 370 to 390 million years old. One possible explanation is that it was transported by ocean currents from a forest located to the northeast, coming to rest in shallow water on the Cincinnati Arch. The trees eventually became waterlogged, sank, and were embedded in accumulating organic-rich black sediments. Or it may have grown in close proximity to where it was found, but no root casts have been found to date.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016