American Alligator and Sarcosuchus

Most adult American alligators weight around 790 lbs  (360 kg) and they can get up to  14 ft (4.4 m) long and some really big ones can weigh over 990 lb  (450 khg).  The largest ever , found, in the state of Louisiana, was 19.2 ft (5.8 m) long.   There is also a species of Alligator in China.  The Chinese Alligator is only about 7 ft (2.2 m) long.   

American Alligator

Alligators evolved in the early days following the extinction of non-avian dinosaurs.  .    .  about 66 million years ago.  They evolved from crocodiles.  Crocodiles lived with the dinosaurs.  There were even giant crocodiles that were over 40 feet long.   

Sarcosuchus was the  biggest freshwater crocodile ever, Sarcosuchus imperator, lived 110 million years ago, during the Early  Cretaceous Period.   It grew as long as 40 feet (12 meters), and weighed up to  17,500 pounds (8 metric tons).  Sarcosuchus has been found in Africa and possibly Brazil.  Sarcosuchus most certainly ate dinosaurs that were small enough to kill and eat.  It probably never touched the bigger Theropods for fear of getting hurt or eaten itself.  A Carcharodontosaurus was a predator that also lived in Africa.  It was over 40 feet (12 m) long and that is longer than a T. rex.  The name: "Carcharodontosaurus" means "shark tooth."  A full sized Carcharodontosaurus would be a bit longer than a Sarcosuchus, but i think a teen-aged Carcharodontosaurus, at 30+ feet would have had a chance.   

Sarcosucus vs a young Carcharodontosaurus

Interesting note, in the new BBC Series called Planet Earth II... they show a small crocodilian getting killed by the powerful jaws of a jaguar.  So, we can infer that a larger Theropod might have given Sarcosuchus a run for his money.