Who was the biggest predator of them all? For as long as I can remember,
Paleontologists Francois Therrian and Don Henderson reviewed this problem in the paper “My Theropod is Bigger Than Yours… Or Not: Estimating Body Size From Skull Length in Theropods” which appeared in the Journal of Vertebrate Paleontology. Although there are many skeletons of Tyrannosaurus and scientists have a relatively complete view of what its skeleton was like, we are not so lucky with other theropods. In some cases, that of Spinosaurus in particular, much of the skeleton has yet to be found and body size must be estimated.
In developing a method to estimate body length and mass based upon skull size, Therrian and Henderson found that as theropods grew larger, their skulls became proportionally longer. This makes sense because the largest of the theropods were large-headed hunters that used their heads to attack, kill, and consume prey. (The giant Therizinosaurus was an exception to this, as its relatives had large arms, long necks, and small heads.) With this relationship established, the researchers could estimate the body length and mass of the large predators only known from skulls.
Although they stressed that they estimates were provisional and required more complete skeletal material to confirm, Carcharodontosaurus (43.5 feet; 33,345 pounds) and Giganotosaurus (42.6 feet; 30,438 pounds) appeared to be longer and heavier than Tyrannosaurus (39.3 feet; 20,085 pounds). Spinosaurus (41.2 feet; 26,428 pounds) was also slightly larger than Tyrannosaurus but not quite so big as had previously been estimated (47 feet;46,049 pounds).
Based upon these estimates, the little-known Carcharodontosaurus appears to be the “winner,” but time will tell whether this is true. Even if a complete skeleton is ever found, it is not likely to represent the largest member of this genus. Individual animals vary in size, and what we are really comparing are the largest members of different groups. There were probably some adult Tyrannosaurus that were longer and/or heavier than adult Giganotosaurus and vice versa; there was no single set length or weight that all individuals attained. What is clear, though, is that all of these dinosaurs were top predators in the places and times in which they lived and we would be little more than a snack to any of them.