Putting the laws of time and plausiblity aside, picture a battle between the megalodon (a prehistoric shark with a six-foot jaw span) and a giant squid (reported to be the size of a school bus). Who would win?
I think greater surface area and the ability to strangle-hold the giant shark would beat out a few lost tentacles. (if the shark got a solid bite in first, maybe it'd go differently.)
Additionally, The Giant Squid still exists to this day, so from an evolutionary standpoint, VICTORY!