Carl Zimmer in his excellent blog, The Loom:
Charles Darwin was interested not just in how new things evolve, but also in how old things disappear. Often, they don’t disappear completely without a trace. We don’t have a visible tail like our primate ancestors did, but we still have a series of little bones tucked away at the bottom of the spine. While it may not function like a full-blown tail, it still anchors muscles around the pelvis. Blind cavefish may not have eyes of the sort found on their cousins in the outside world, but they still start to develop eyes as larva, before the cells start to die away.
Sometimes, though, the only place to look for vestiges of a lost trait is in a genome.
In the journal PLOS Genetics, Mark Springer of the University of California and his colleagues have published an intriguing study of how teeth–and the genes for teeth–have faded away over the past 50 million years.