Vastatosaurus rex represents what Tyrannosaurus rex would have become if the species was allowed to evolve unabaited for another 65 million years. Like their prehistoric relatives, Skull Island V. rexes have little competition for their position as top predators. Despite their size, they can turn on a surprising burst of speed for short periods, often acheiving speeds of 25 mph.
V. rexes developed many unique features over the 65 million years since the age of dinosaurs, but they still bear several recognizable similarities to their Cretaceous cousins. Vastatosaurs have large heads, filled with large teeth that are constantly being regrown to replace those lost in conflict. Some V. rexes even have overlapping teeth like one would find in modern day sharks. Their heads are shorter and more compact than that of their ancestors, intensely reinforced with thick bone. As the primary weapon of the animal, an individual Vastatosaur head is often distinctive, being covered with scars and calluses. Abnormal bone growths form from old battles with prey, other predators, rivals, or even mates were not uncommon. Narrow, short rib cages and a large gap between the ribs and hips allow V. rexes surprising flexibility for animals of their size, a necessary adaptation to survive amongst the towering trees and broken terrain of Skull Island.
Like early tyrannosaurs, their forelimbs are small in comparison with the rest of their body. One distinctive feature that distinguishes them from Tyrannosaurus is the additional digit on their forelimbs. Although Tyrannosaurs were set apart from the rest of the theropod dinosaurs (Allosaurus, Ceratosaurus, Eustreptospondylus etc) by having only two fingers, V. Rexes have three of them. This has led to debate whether V. Rex was indeed related to the Tyrannosaurs at all, but gene tests have confirmed it as T. Rex's cousin. The extra digit, though small, acted like a thumb and was useful when dragging a dead animal; the tiny arms helped to pin swinging meat against the carcass.
While juvenile V. rexes tend to cluster in the thick jungle interior, the large adults hunt mostly in the open areas of the lowlands, where they can move more freely. Large males seek the most open territories, while mature females usually stake claim to areas on the fringes of the forests, where they can find cosey nesting sites.
Fiercely territorial, adult V. rexes suffer no rival encroachments on their hunting grounds. Territorial boundaries are regularly marked with urine, and dawn roaring warns other animals in the vicinity that they will not be tolerated. Other predators can detect a lot about the physical condition of the owner from the smell of its urine, and the size of the animal can be determined by the sound of a roar. Displays and scenting minimize potentially dangerous confrontations between animals of different sizes. Occasionally, disputes between evenly matched carnivores can erupt. With the threat of serious injury, these confrontations are usually resolved with intense roaring matches in which each animal tilts towarn the ground and bellows in an attempt to intimidate the other. If this fails, violence is used. Older V. rexes bear the crisscrossing scars of many such fights.
Exceptions to their solitary existence are made in the breeding season. Males leave their hunting grounds to seek females in season. If the female is receptive she will accept the mating proposal and the pair may stay together, hunting in the female's territory for several days before she tires of him and sends him on his way. Young V. rexes seeking to hone their hunting skills sometimes follow adults at a safe distance during the mating season, watching and occasionally stealing meals from unguarded kills. Taking advantage of the season of nomadic adult males, bold adolescents may move in to claim currently undefended territory as their own.
V. rexes are capable of tackling very large prey species, but their massive size is as often employed to intimidate smaller carnivores and bullying them from a kill. While effective hunters, a meal is safer to obtain by simply appropriating someone else's meal. Vastatosaurs have intensely acidic stomachs, capable of processing even the most rancid rotting meat, a feature that serves the species well. Their massive jaws can exert astounding pressure, shattering bones to expose the rich marrow less robust predators are unable to reach.
Vastatosaurs tend to employ ambush tactics, using cover in and around waterholes or in forested areas to surprise prey. Ligocristus are the primary prey of adults, being the most abundant. Ferrucutus and Brontosaurs are more dangerous prospects, although if a young or sick animal can be separated from the protection of the herd, it can be an easy kill. Smaller species are likely game, though most are too small to provide a sufficient supply of nutrition. The greatest challenge for the hunter lies in concealment. Full grown Vastatosaurs are black in coloring, and despite their size, can be surprisingly stealthy, using scrubland and unstable ruins for cover.
Their black scales also help them warm up quickly at dawn, boosting their energy levels to gain an advantage over still sluggish herbivores.
The extent to which Skull Island V. rexes have developed since the Cretaceous era T. rex is most evident when comparing skulls. Developments traced through an inferred missing-link species show how the skulls became more robust and heavily armoured, creating a new king of dinosaurs.
Skull Island V. rexes have broad feet with sharp claws. These big feet are an adaptation that allow the heavy animals to stalk in more uneven terrain than their bulk would otherwise allow. Their huge jaws are the largest of any land predator to have lived and are capable of exerting astonishing pressure to tear flesh and shatter bone. Their only enemies were the giant apes that lived in the mountains, called Megaprimatus kong. Eventually the great apes fell leaving Kong as the last of his kind, but as Kong is soon captured by humans, leaving the role of top predator on Skull Island to the V. rex.