A urostyle (indicated by the arrow) is formed from the fusion of pelvic vertebrae and it makes a really cool-looking tri-branched shape with the frog's simplified pelvis.    From Wordnik.com.  [Update] Reference

Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

As the tadpole approaches the frog stage the vertebral column in the tail is rapidly absorbed, and its vestiges appear in the adult as the urostyle.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

T., tibia. t.p., transverse process. ul., ulna. ur., urostyle.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference