Tyrannosaurus rex and the Seismic Monitoring Hypothesis: A Sensory Interpretation of Reduced Theropod Forelimbs

The repeated evolution of reduced yet structurally robust forelimbs among theropod dinosaurs represents a persistent problem in vertebrate functional morphology. Although forelimb reduction is most famously associated with Tyrannosaurus rex, similar patterns evolved independently in multiple theropod lineages, including tyrannosaurids, abelisaurids, and alvarezsaurids. Traditional interpretations have emphasized roles in prey handling, mating behavior, locomotor assistance, and stabilization; however, these explanations do not fully account for the retention of dense bone architecture, substantial muscle attachment sites, and reinforced shoulder girdles in forelimbs that underwent dramatic reductions in length. This paper proposes a new functional hypothesis in which reduced theropod forelimbs participated in substrate-borne vibration monitoring. Drawing upon evidence from mammals, reptiles, and birds that utilize seismic information, the hypothesis suggests that shortened, mechanically robust forelimbs may have functioned as substrate-coupled receiving structures capable of transmitting environmental vibrations into the skeleton. The proposed sensory function is envisioned to operate primarily during resting or prone postures, when the body was supported by contact with the ground and the forelimbs maintained mechanical coupling with the underlying substrate. In this context, forelimb-ground contact may have facilitated the acquisition of environmental information beyond the immediate limits of vision, hearing, or olfaction. The hypothesis further suggests that forelimb reduction may represent functional transformation rather than simple evolutionary degeneration. By reducing transmission distance, increasing structural rigidity, and minimizing locomotor noise, shortened forelimbs may have been advantageous for receiving substrate-borne mechanical signals and conducting those signals through the skeleton toward central sensory processing structures. The model generates specific biomechanical, histological, behavioral, and ichnological predictions that can be evaluated through future research. Although direct evidence remains unavailable, the widespread occurrence of seismic sensing among living vertebrates, combined with repeated convergent forelimb reduction in theropods, supports the value of investigating a potential sensory role for these structures.