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Just to let people know, the blurry end of the tail on my restoration of
Apatosaurus in the Tuseday NYTimes is meant to represent supersonic movement,
not some sort of fleshy end of the tail as some have thought. 

The probability that all sauropods used their tails for defense is high.
Sauropods were often hunted by predators as big as rhinos and elephants. They
were therefore in much greater danger of attack than are elephants. Female
african rhinos tend to have longer, more forward directed nose horns than
males. It is probable that they use these horns to defend their calves, which
 are nearly defenseless against lions and hyaenas (see R. Owen-Smith's 1988
classic Megaherbivores). The notion that big ungulates hardly ever use their
horns for anti-predator defense is something of a myth, have seen many videos
of them doing so. 

The situation faced by sauropods was akin to that faced by similar sized
whales, which come under attack from elephant sized orcas (a few million
years ago, it was even more enormous sharks), as well as pygmy killer whales.
Orcas have been filmed attacking solitary blue whales, just this year a pod
of sperm whales. In the latter case the whales tried to use their tails for
defense. Sperm whale pods have also been filmed under attack from pygmy
killer whales. The latter are much to small to kill the giants. They just
take big bites for their afternoon meal! In the Morrison, the 1-1.5 tonne
common Allosaurus may have been attacking adult sauropods not to kill them,
but to dine on their living flesh. Way cool if true. Young were subject to
death. The much bigger, rarer Saurophagus (cool name) were probably out to
kill adults. Large Yangchuanosaurus could probably kill Mamenchisaurus,
elephant sized Giganotosaurus could probably kill fin whale sized

A basic method of tail defense used by all sauropods was probably simply to
use the fleshy midsection of the tail as a swinging beam to knock the bipedal
predators off their feet. For example, the tails of Morrison diplodocids
weighed about a tonne and a half (according both to my models, and to
Myrvhold & Currie's calculations), about the same as the common allosaurs.
The tail of Argentinosaurus probably weighed as much as Giganotosaurus. The
effect of being walloped by a few tonnes of tail is obvious. Much more
serious than the impact form a whale fluck, because of the associated effects
of a fall in 1 G. 

Another mode of caudal defense,  the tail clubs found on some of the Chinese

The question is whether supersonic whip tails were used for defense.
Certainly they would be effective as a pain inducing deterrent. Force
increases by velocity squared, so the impact of a supersonic whip an inch or
more wide would leave a long, nasty, bleeding cut. Theropods could come to
associate the sound of tail cracking with pain, and might be deterred from
attacking by the sonic cracks produced by the members of a threatened
sauropod/herd. The one problem with this scenario is one that Nathan has
pointed to. It is not clear that sauropod whip tails could withstand such
high levels of stress, nor are the the expected signs of damage present.
Therefore, the whips are the least likely part of sauropod tails to be used
for defense. However, even if the whips were not used for physical defense,
tail cracking may have acted as a sonic clue to warn theropods that their
targets were armed with very long, massive tails that could sweep them off
their feet if the predators dared come too close.