[Professor Mega-Mind (OP)]

How do the top marine monsters stack up to each other ?
I shall consider six cow gulpers as my contenders . 
All matches are "Cage Matches" ; held in a smallish bay .
First up is the " Man with a bat ! ", Aust Colossus  : 90 feet long , bite-tooth-less , flat-bodied , but with a club on his head for ram-beating his prey to death . 
Edged out by giant Mosasaur : 
60 ft. long , heavy lizard body w/ -eel-like tail for power-twisting . Monstrous triangle-shaped maw , a box-like skull for horrific twist-offs , superior agility and smarts.
Edged out by Basilosaurus :
70 ft. long , heavier , smarter, and w/ better endurance/metabolism
Beaten solidly by Megalodon :
60 ft. long , much heavier , much better armed , possessing extra-powerful , cookie-cutter bite .  Uses wound , and wait , strategy .
Edged out by P. Funkei (Ultra- Kronosaurus) :
..."Vengeance from the Deep" , jaw kept at University of Oxford . Killer of Monster of Arrumberri .
60+ ft. long , bite power : close to Megalodon , bite depth : 2 x Meg. , teeth : bone-shattering 14" railroad spikes ; effect of bite : Long , flexible head/neck gives  first-bite . The agile power-head , exposes opponent's guts on 1st good bite .
 Agility : Superior to Meg. , Endurance/Metabolism: superior to Meg., Intelligence : superior to Meg.
 Beaten by Livyatan MelvillI :
Almost 80 ft. of heavy muscle .
All of U.K. advantages writ large EXCEPT offensive agility .  This counter-balanced by much superior intelligence using equal defensive agility .  The whale's sonic shockwave weapon would confuse & disorient it's opponent , it's ship-sinking ram would rupture said opponent's organs . These two weapons giving it the opportunity to use it's skull-severing bite , almost at will .
Finally , the bronco-buck of this behemoth would amputate the Appalachian Mountains !
As Bill Duke said in Predator 1 ,
 " NO CHANCE  !! " .
 Alright , Ding-freaking-Ding !
...   P.M.

[Beesriwara]

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[Professor Mega-Mind (OP)]

    Bonus Fight : Semi-aquatics .
☆A fight that almost occurred :
  Spinosaurus A./Sarchosuchus
Spino.- Longer , heavier , taller , better reach , lethal claws , very heavy tail-strike , far better agility  on land & shallow water , better metabolism/endurance , much better intelligence .
Sarchosuchus- Armored, tougher against  injury ,  better swimmer , much better bite , leg-breaker tail slap , death-roll , much longer breath-hold .
 Result - On land , Spino out smarts & outmanouvers croc. , can-opens guts and/or throat once reptile fatigues .
 Shallow water - Spino. struggles to maintain advantage , as croc. moves much better while Spino. slows , trying to avoid powerful croc. weapons .
 Deep water - Spino. ambush and quickness allow some croc. injury , before Sarco. sinks a good bite in , drowning the now-hopeless kamikaze dinosaur in short order . 
P.M.
*Note - Spinosaurus was really conspecific with Machimosaurus Nowackianus .
This crocodylomorph replaced Sarcosuchus in Cretaceous Africa .