Interspersed repeats that emerged at different evolutionary times are
informative in mammalian phylogeny. Here we show that the ancient shor
t interspersed elements (SINEs) ARE1 and ARE2 are abundantly present i
n the genomes of artiodactyls and cetaceans but not in other mammalian
genomes. This supports the classification of the cetaceans with the a
rtiodactyls by a shared character that is unlikely to be the result of
convergence.