What does evolution have against the tail?

Matthew:

I was lying awake the other night wondering where tails went. Almost all of the dinosaurs had tails -- tails aplenty, tails to write home about, even the fast and agile dinosaurs, for whom you'd think tails would be extra and unwanted weight. Most mammals, by comparison, have vestigial tails. Even those with generous tails (cat family, monkeys) have posterior appendages that seem to serve little practical use. And the mammals that are most like dinosaurs in size (elephants, rhinos) have the smallest and most ridiculous tails. What does evolution have against the tail? Or, conversely, what did the tail do 100 million years ago that we don't need now?

-- Tim, far away

I'm not sure what dinos' tails have to do with elephants' tails, except that they're both at the south end of large animals. Actually, we're not smart enough to know exactly what a dino's too-big tail was for. They didn't drag 'em, 'cause dino footprints don't usually have tail marks too. They may have been for balance. Or weapons. Today's reptiles use their tails defensively. One anthropologist surmises they cracked them like whips to attract mates. Elephants descend from things the size of pigs. Nature seems to have experimented a lot with body size and the front end of the animals -- trunks, tusks, ears -- so maybe the back end just never got the attention it deserved. Cats use their tails for balance, tho not all cats have tails. Monkeys use them for transportation. Nature has no universal tail attitude. No species has risen or fallen on its tail alone. And no more chorizo for you before bedtime.