A new study from University College London and the University of Cambridge points out that the reason why large theropod dinosaurs such as Tyrannosaurus rex evolved "short arms" that are extremely disproportionate to their huge bodies is probably because they increasingly relied on huge and strong skulls and powerful bite forces to hunt prey during the evolution process, and their forelimbs gradually degenerated into "redundant configurations." Relevant results have been published in the British "Proceedings of the Royal Society, Series B".
The research team analyzed 82 species of theropods, a group of carnivorous, bipedal dinosaurs that include the famous Tyrannosaurus rex family. The results show that the phenomenon of forelimb shortening occurs independently and repeatedly in at least five major groups of theropod dinosaurs, including tyrannosaurids, abelisaurids, carcharodontosaurids (including Tyrannotitan), megalosaurids and ceratosaurids, where Tyrannosaurus rex is located. The researchers found that the shortening of the forelimbs of these dinosaurs was not so much a byproduct of "growing too big", but more closely related to the skull becoming thicker and the bite force stronger.
Charlie Roger Scheele, first author of the paper and a doctoral student in the Department of Earth Sciences at University College London, pointed out that people are familiar with the "funny little short hands" of Tyrannosaurus rex, but other giant theropods such as Carnotaurus have forelimbs even smaller than Tyrannosaurus rex. Through quantitative analysis, the research team found that species with shorter forelimbs tend to have stronger and more compact skulls, which means that they rely more on their skulls and mandibles rather than their forelimbs and claws when attacking. Scheler described this as a kind of "use it or lose it" evolution: now that the head is sufficient to complete the tasks of biting, grasping and subduing prey, the importance of the forelimbs continues to decline, and is eventually "marginalized" in evolution.
The study also linked this trend to the rise of giant herbivores during the same period. In some ecosystems, huge sauropod dinosaurs with a body length of about 30 meters appeared. These "behemoths" with long necks and long tails forced predators to change their hunting strategies. For dealing with a giant sauropod nearly a hundred feet long, trying to pull it with its forelimbs is not a good choice. It is obviously more efficient to bite and clamp the prey with a strong skull and powerful bite.
In order to further examine the relationship between "skull and forelimbs", the team proposed a new set of "skull solidity" assessment methods, which comprehensively consider indicators such as bite force, skull shape, and tightness of bone connections. In this evaluation system, the shorter and more compact the skull, and the "stronger" the overall skull, is considered to be more suitable for withstanding high-intensity bites and severe impacts. Tyrannosaurus rex tops all samples in this metric, and Tyrannotitan, a giant predator from Early Cretaceous Argentina, is close behind, showing that its skull was also extremely strong.
Interestingly, shortened forelimbs do not exactly equate to "individual gigantism." Research points out that Majungasaurus, which lived in Madagascar about 70 million years ago, is an example. It was the local top predator, but it weighed about 1.6 tons, only about one-fifth of Tyrannosaurus rex. It had an unusually thick skull and extremely small forelimbs. This shows that the combination of "short hands + hard head" can appear in different body types. The key lies in ecological role and hunting method, rather than pure body shape.
The research also revealed that different dinosaur lineages took different paths on the road to shrinking their hands. Among abelisaurids, the portions of the hand and forearm below the elbow have been particularly degenerated over time, with the forelimbs of later representatives such as Madagascarosaurus having been reduced to an extreme degree. In contrast, in tyrannosaurids, all parts of the entire forelimb shrank evenly and synchronously, with no particular "shrinking" of any particular section seen. This suggests that different families may have ended up with the same "short forelimb" morphology through completely different developmental and evolutionary mechanisms.
In terms of timing, the team believes that it is more in line with evolutionary logic for the skull to "strengthen first and then shrink its hands". According to correlation analysis, the characteristics of thicker skulls and stronger bite forces tend to appear first, while the shortening of the forelimbs is gradually manifested later. Scherer pointed out that these predators are unlikely to rashly "give up" the forelimbs as a weapon without first evolving an attack method that is sufficient to replace the forelimbs. Therefore, the short forelimbs are more likely to be a passive response to the "upgrade of the head weapon system."
The research team also proposed the concept of "evolutionary arms race". As herbivores continue to grow larger and stronger, predators are forced to upgrade their hunting equipment, evolving to extremes in body size, skull structure, and bite force. In this process, some predators themselves grew into giant beasts, and gradually shifted from the dual-weapon mode of "claw + bite" to the "heavy bite" mode that relied heavily on the skull and mandibles, further weakening the role of the forelimbs.