Cortical bone geometry and static gravitational loads in neosauropod dinosaurs: Implications for biomechanical constraints on terrestrial gigantism

Sauropod dinosaurs achieved unprecedented terrestrial gigantism, raising questions about the biomechanical limits of bone under extreme gravitational loads. We estimated static gravitational pressures (Pg) on appendicular cortical bone in neosauropods, including diplodocoids, brachiosaurids, and titanosaurs. We did this by measuring cortical thickness, estimating body masses, and calculating pressures on limb bones. We compared the results with the theoretical fracture thresholds of composite cortical bone (~150-200 MPa) and pure crystalline hydroxyapatite (~1000 MPa). In the largest specimens, Pg remained two orders of magnitude below the compressive strength of hydroxyapatite. These estimates are a conservative lower bound because they assume simultaneous four-limb support. They are also substantially lower (>15 MPa) than the functional failure threshold of composite cortical bone. Dynamic behaviors raise these values: during slow locomotion, a single hindlimb may briefly support the entire hindquarter load, and the weight distribution between fore- and hindlimbs varies markedly with center-of-mass position. Even so, rearing on the hindlimbs in the largest titanosaurs (>25 metric tons) would remain within safe limits defined by composite-bone failure thresholds. Neosauropod lineages show distinct biomechanical strategies. Diplodocoids kept a posteriorly positioned center of mass compatible with facultative bipedal rearing. Brachiosaurids shifted it cranially through neck and forelimb hypertrophy. Titanosaurs reached extreme sizes through enhanced secondary bone remodeling and extensive postcranial pneumatization. Cortical thickening is consistently associated with increasing gravitational loads across Neosauropoda. However, this static model does not test whether it was the primary evolutionary response, compared with other strategies such as trabecular densification and postcranial pneumaticity.