Tyrannosaurus rex, Osborn, 1905

Blanco, R Ernesto, 2023, Tyrannosaurus rex runs again: a theoretical analysis of the hypothesis that full-grown large theropods had a locomotory advantage to hunt in a shallow-water environment, Zoological Journal of the Linnean Society 198 (1), pp. 202-219 : 215

TYRANNOSAURUS REX RUNS AGAIN

An interesting highlight is that, at least in water, Tyrannosaurus rex was probably able to adopt a running gait with a suspended phase. To walk in the water with increased buoyancy results in less contact time with the substrate. This effect has been likened to locomotion in a microgravity environment ( Coughlin & Fish, 2009, and references therein). In conditions of reduced gravity, humans switch from a walk to a run at slower speeds, but at approximately the same Froude number ( Kram et al., 1997). For example, if the Tyrannosaurus rex modelled here had been in the water at a depth of 3.2 m, the effective acceleration of gravity would have been reduced by approximately one-third. In that case, the change from walk to run would have been at a speed of ~ 2.4 m /s. According to my results, this is a speed easily attainable for Tyrannosaurus rex in such a situation. Also, the bone strength limitation for running suggested in previous studies would have lost relevance in this scenario. The ground reaction forces are reduced owing to buoyancy, and new gaits are available at lower speeds ( Martínez, 1996; Coughlin & Fish, 2009). This effect has been observed in crabs ( Martínez, 1996; Martínez et al., 1998), crocodiles ( Farlow et al., 2018), salamanders ( Ashley-Ross & Bechtel, 2004; Ashley-Ross et al., 2009) and hippopotami ( Coughlin & Fish, 2009). Observations on large vertebrates, such as the hippopotamus, suggest that with increased buoyancy to maintain stability, galloping can be performed at extremely slow speeds ( Coughlin & Fish, 2009). Studies on newts walking underwater and on land show that the submerged gait pattern is closer to a trot, including periods of suspension ( Ashley-Ross et al., 2009). These observations reopen the possibility that, at least in water, Tyrannosaurus rex could have used a running gait.

Biomechanical models generally suggest that Tyrannosaurus rex could not have run on land. However, several aspects of tyrannosaurid anatomy, such as the long legs and large pelvic limb muscles, which intuitively seem to indicate fast running capacity ( Bakker, 1986; Paul, 1998), are enhanced in Tyrannosauridae (and their immediate outgroups) relative to other theropods and within larger-bodied taxa ( Persons & Currie, 2016; Sively et al., 2019; Deccechi et al., 2020). Those anatomical features could also be a useful set for hunting swift prey by fast punting-running in water.

CONCLUSIONS

Large theropods probably had a locomotory advantage to pursue small prey in a shallow-water environment. Moreover, it is almost certain that the locomotory advantage of small prey on the land would be reduced drastically in aquatic environments. The shallow-water environment hunting scenario proposed here for large theropods could also apply to extant animals and other fossil vertebrate groups. The theoretical approach presented here could help researchers to understand hunting strategies and depict other palaeobiological scenarios involving extremely large predators. The hypothesis presented here raises several issues for further ichnological, functional and biomechanical studies.