A Computational Analysis of Limb and Body Dimensions in Tyrannosaurus rex with Implications for Locomotion, Ontogeny, and Growth

The large theropod dinosaur Tyrannosaurus rex underwent remarkable changes during its growth from <10 kg hatchlings to>6000 kg adults in <20 years. These changes raise fascinating questions about the morphological transformations involvedı peak growth ratesı and scaling of limb muscle sizes as well as the body’s centre of mass that could have influenced ontogenetic changes of locomotion in T. rex. Here we address these questions using three-dimensionally scanned computer models of four largeı well-preserved fossil specimens as well as a putative juvenile individual.

Furthermore we quantify the variations of estimated body massı centre of mass and segment dimensionsı to characterize inaccuracies in our reconstructions. These inaccuracies include not only subjectivity but also incomplete preservation and inconsistent articulations of museum skeletons. Although those problems cause ambiguityı we conclude that adult T. rex had body masses around 6000–8000 kgı with the largest known specimen (‘‘Sue’’) perhaps ~9500 kg.

Our results show that during T. rex ontogenyı the torso became longer and heavier whereas the limbs became proportionately shorter and lighter. Our estimates of peak growth rates are about twice as rapid as previous ones but generally support previous methodsı despite biases caused by the usage of scale models and equations that underestimate body masses. We tentatively infer that the hindlimb extensor muscles massesı including the large tail muscle M. caudofemoralis longusı may have decreased in their relative size as the centre of mass shifted craniodorsally during T. rex ontogeny.

Such ontogenetic changes would have worsened any relative or absolute decline of maximal locomotor performance. Regardlessı T. rex probably had hip and thigh muscles relatively larger than any extant animal’s. Overallı the limb‘ ‘antigravity’’ muscles may have been as large as or even larger than those of ratite birdsı which themselves have the most muscular limbs of any living animal.