Microstructure of axial bones of lithostrotian titanosaurs (Neosauropoda: Sauropodomorpha) shows extended fast-growing phase

Titanosaurs reached extremes in size and diversity, but many paleobiological aspects of these sauropod dinosaurs are still unknown. Here we infer life history traits based on the microstructure of axial bones of three lithostrotian titanosaur species: Austroposeidon magnificus, Gondwanatitan faustoi and Maxakalisaurus topai - Campanian-Maastrichtian of Bauru Group, Brazil. The present study is the first wide characterization of the microstructure of axial bones comparing distinct lithostrotian lineages with different body sizes. We analyzed the cross sections of cervical ribs, dorsal ribs and haemal arches. The medullary cavity in samples of A. magnificus are pneumatic, with the primary bone rich in fibrolamellar bone, with scarce growth marks, which is different from the Lines of Arrested Growth (LAGs)-dominated pattern of G. faustoi and M. topai, with parallel fibered bone. In general, the inner cortex exhibits fibrolamellar bone with longitudinal canals. The outer cortex is rich in parallel fibered bone rows of longitudinal canals and growth marks. Based on dense remodeling, decrease of LAGs distances and presence of an External Fundamental System in some samples; we concluded that all individuals reached at least the sexual maturity. These titanosaurs shared with basal neosauropods two main growth phases: modular (inner cortex) and LAG-dominated (outer cortex). Distinct from basal neosauropods, titanosaurs presented extended fast modular period (rather than LAG-dominated). This pattern supported that titanosaurs adopted a distinct strategy of bone mass gain in dorsal ribs, alternating the timing in rib growth. (C) 2022 Elsevier Ltd. All rights reserved.