Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod.
"Text Book of Biology, Part 1: Vertebrata" by H. G. Wells
Caudal vertebrae fused into a urostyle or coccyx.
"Encyclopaedia Britannica, 11th Edition, Volume 3, Slice 4" by Various
How are the urostyle and the pelvis connected, if at all?
"A Guide for the Study of Animals" by Worrallo Whitney