EVIDENCE OF THE OLDEST HEALING LIMB BONE FRACTURE IN AMNIOTES

In eastern Greenwood County, near Toronto, Kansas, a fossil locality in the Snyderville (Shale) Member of the Oread (Limestone) Formation (late Pennsylvanian Period, Virgilian) contains an abundance of disarticulated and broken vertebrate fossilized bones weathering out of a paleosol. One of the most notable specimens is a tetrapod bone that was fractured while the animal was alive and then partially healed prior to its death. This is shown by the presence of both an offset of the bone fragments at the fracture and by the presence of a preserved callous (healing growth zone). Although many features of the bone that would allow definite identification of the animal were removed after burial, the preserved parts appear to be consistent with the bone being of an amniote. In transverse cross-section, the bone has a teardrop shape with a rounded anterior that tapers back to a flattened posterior that slightly curves. This shape compares favorably with other fossilized limb bones found at the locality that can be identified as early reptile. The classification to reptiles is also suggested by the microscopic structure of the bone when compared with that of known living reptiles (amniotes) and amphibians (non-amniotes). In transverse cross-section, the bone has a sponge-like appearance that is consistent with that of living reptiles, and unlike that of living amphibians. Given the geologic age of this fossil, it is the earliest reported healed limb bone fracture from an amniote.