T. Rex Had Weirdly Tiny Arms - Now We Have The Best Explanation Yet For Why That Was

Key Points:

• A new study by researchers at University College London and the University of Cambridge provides strong evidence that the evolution of smaller forelimbs in T. rex and other carnivorous theropods was primarily driven by the increasing strength and robustness of their skulls, which became the main weapon.
• The research analyzed measurements from 85 theropod species, finding that skull robustness was the strongest predictor of reduced arm size, more so than body size alone, challenging the idea that smaller arms were simply a byproduct of overall body scaling (allometry).
• Five theropod lineages, including Tyrannosauridae, showed significantly reduced forelimbs correlated with robust skulls, suggesting an evolutionary trade-off where powerful jaws reduced the need for large, grappling arms.
• The study counters alternative hypotheses such as arm reduction evolving first or being solely due to size constraints, noting that predators with small arms but weak skulls would be at a disadvantage, and pointing to ecological differences seen in large herbivorous theropods with large arms.
• While the study establishes a clear association between skull strength and arm size reduction, further research, including ongoing work on arm musculature, is needed to fully understand the evolutionary mechanisms behind these changes in theropods.