Many fossils of invertebrate (no backbone) animals have long shapes that have been confused with fossil bones. Cephalopods have an elongate shell. Most cephalopods are less than 12 inches in length, but cephalopods in excess of three feet have been found in Kentucky. Horn corals also can have an elongate shape that mimics the shape of a rib bone. Some horn corals at the Falls of the Ohio, near Louisville, are more than 4 feet long. Neither cephalopods or horn corals have the spongy interior typical of bone. Most horn corals and cephalopods have wrinkled or segmented exteriors, also not common in bone.
Bryozoans are another common invertebrate (lacking a backbone) fossil found in central Kentucky that are sometimes misidentified as bones. Some have small circular, “star-shaped,” or “asterisk-like” bumps that bone would not have. In section (look at the broken end of a fossil or edge of the rock containing the fossil), bryozoans will tend to be filled with calcite or concentric rings rather than exhibiting the spongy texture of bone.
Some bone-shaped rocks have also been misidentified as bone. The mineral siderite may fill the holes left by roots and invertebrate animal burrows forming tubular, long, and irregularly-shaped pieces that have been misinterpreted as fossil bones by amateur collectors.
If the fossil you have is elongate, does the outside, long part of the fossil have a smooth or polished appearance? Most elongate fossil bones are smooth with small cracks or breaks from compaction during the fossilization process. They may have a very porous (spongy), or feathery (fine, radiating lines, grooves, and chambers) texture, especially towards the thicker ends of the bone. Large fossil bones found in Kentucky are most often mammal bones. As such, they have shapes similar to modern mammal bones. Arm and leg bones will have ball-and-socket shapes toward the end just like human arm and leg bones. If there are cracks or breaks in the fossil, the spongy interior texture can generally be seen, if the fossil is really a fossilized bone.
Elongate fossil bones tend to be smooth, almost polished in appearance. Leg and arm bones will end in a rounded knob or protrusion, just like a human leg or arm bone.