Kronosaurus for both superior maneuverability and killing weapons.
The four-flippered, sea turtle/penguin-like method of locomotion utilized by plesiosaurs would allow for greater maneuverability than the axial swimming locomotion of a whale because the former would have a far easier time generating torque in a different direction.
Kronosaurus has the advantage in the jaws department hands down due to the following reasons:
1.) It appears to have a *much* larger skull.
Skulls Unlimited's orca skull replica is apparently only 87cm (hyperlink). On the other hand, a ~6t
Kronosaurus has a skull 180cm long.
Kronosaurus would also have a (probably much) heavier weapon, and I think this could increase damage because the weight of the head provides more force behind the teeth and jaws to sink in during a bite.
2.) It almost certainly has a far more powerful bite.
3.) Its teeth appear to be much larger. The tooth crowns of
Kronosaurus are 10cm (~4") long (reference shown below)
while this is how long a whole orca tooth can get, i.e. including the root (hyperlink). When you look at an orca tooth:
the root appears to be ~1.5 times longer than the crown. So with simple math (x+1.5x=4"->2.5x=4"->x=1.6" or 4.064cm), you can deduce that an orca's tooth crown is a little over 4cm in length.
4.) Its teeth also look more pointed IMO.
Kronosaurus teeth:
Killer whale teeth:
5.) Judging from pictures online and how orcas seem to have somewhat smaller heads for their size, the delphinid doesn't seem capable of a very wide gape. By contrast, by virtue of its enormous skull, the pliosaur should have been capable of a very large gape (although in terms of actual degrees in numbers, neither have been measured to my knowledge).
And while the orca can ram, I still feel as if the pliosaurid's jaws will kill more quickly and efficiently than the orca's rams. The killer whale could ram the pliosaur once but it certainly won't be enough to end its life; it will take repeated rams with time to kill. On the other hand, once the pliosaur apprehends the orca in its jaws, it will easily maul its victim to death with its powerful, enormous jaws. Its aforementioned agility advantage could help it avoid those attacks; IMO agility matters more underwater than on land. I suspect that ramming might not be exclusive to the delphinid here either; a few publications seem to imply pliosaurs could have rammed as well (I also got the figure for
Kronosaurus' tooth crown height from the former).
www.app.pan.pl/archive/published/app56/app20100117.pdfonlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1998.tb00520.x/abstractIn any case, I still think
Kronosaurus is left in the possession of the better weapon, whether you're talking about the orca's battering ram head or jaws.
So...yeah...
Kronosaurus wins.