The tail of a thresher shark (Alopias vulpinus). From Wikipedia.

Thanks to sensational documentaries and summer blockbusters, we are all familiar with the anatomy of a shark attack. The victim, unaware that they are in peril, is struck from below and behind with such speed and violence that, if they are not actually killed during the first strike, they soon find themselves a few pounds lighter in the middle of a billowing red cloud. The trouble is that this stereotyped scenario does not hold for all sharks, particularly one peculiar group of deepwater sharks which has long puzzled naturalists.

Thresher sharks, represented by three species within the genus Alopias which ran between 10 and 20 feet