Taxonomic classification of amphisbaenians was traditionally based on morphological characters, such as the number of preanal pores, body annuli, and tail annuli. Such characters are vulnerable to convergent evolution; in particular, the loss of the forelimbs and the evolution of specialized shovel-headed and keel-headed morphs appear to have occurred multiple times in the history of the group. Classifications based on mitochondrial DNA sequences and nuclear DNA sequences better reflect their true evolutionary history, and are now being used to distinguish genera of amphisbaenians.