Unidentified fossil

David MacMillan, who wrote an 8-part series on creationism for us, sent us these 4 photographs, along with the following request:

“I recently moved back to central Kentucky. One of the things I came across while visiting my family was this fossilized object I discovered near my home here when I was about 9 or 10 years old.

“Back in the late 90s, we were living in a new development and there was a lot of excavation going on near our house. I believe I found this half-buried in the bottom of a rain-fed creek just after a particularly heavy period of excavation followed by some heavy rainstorms.

“It appears to be a vertebra, due to the shape and orientation of the various spurs, and what seems to be a very large nerve opening going in the side. The exterior is dotted with what appear to be marine fossil concretions, including scallops and similar creatures.

“This region of Kentucky comprises primarily Ordovician limestone and shales, which is puzzling because this would have to be a pretty large marine vertebrate, and there were virtually no large bony vertebrates in the Ordovician. Perhaps this is actually not a vertebra at all and is rather some sort of oddly-shaped shell?

“The largest human lumbar vertebrae are around 13 mm thick, while this measures over 5 cm thick. If it is a vertebra, it would have to come from an animal with a spinal column at least five times the length of a human spine.

“Basically, I’m stumped. Any idea whether any of the readers of Panda’s Thumb might be able to identify it?”