Osteohistological analyses reveal diverse strategies of theropod dinosaur body-size evolution

Abstract

The independent evolution of gigantism among dinosaurs has been a topic of long-standing interest, but it remains unclear if gigantic theropods, the largest bipeds in the fossil record, all achieved massive sizes in the same manner, or through different strategies. We perform multi-element histological analyses on a phylogenetically broad dataset sampled from eight theropod families, with a focus on gigantic tyrannosaurids and carcharodontosaurids, to reconstruct the growth strategies of these lineages and test if particular bones consistently preserve the most complete growth record. We find that in skeletally mature gigantic theropods, weight-bearing bones consistently preserve extensive growth records, whereas non-weight-bearing bones are remodelled and less useful for growth reconstruction, contrary to the pattern observed in smaller theropods and some other dinosaur clades. We find a heterochronic pattern of growth fitting an acceleration model in tyrannosaurids, with allosauroid carcharodontosaurids better fitting a model of hypermorphosis. These divergent growth patterns appear phylogenetically constrained, representing extreme versions of the growth patterns present in smaller coelurosaurs and allosauroids, respectively. This provides the first evidence of a lack of strong mechanistic or physiological constraints on size evolution in the largest bipeds in the fossil record and evidence of one of the longest-living individual dinosaurs ever documented.

Growth and miniaturization among alvarezsauroid dinosaurs

“…12,17 This suggests two different growth paths toward similar tiny body sizes, where Albinykus baatar grows faster but with a short lifespan and Xixianykus zhangi shows greater longevity but a very slow overall growth rate, even in its juvenile period. Interestingly, giant theropods with similar body sizes but different life-history parameters were also reported recently, 19 suggesting divergent growth strategies might have been widespread among similarly sized theropods. In sum, our osteohistological comparisons among alvarezsauroids suggest that the main factor producing size variance among alvarezsauroids is annual growth rate rather than longevity, and different miniaturized species also grow at different rates.…”

Dinosaur senescence: a hadrosauroid with age-related diseases brings a new perspective of “old” dinosaurs

“…The presence of EFS or OCL is commonly used in assessing the ontogenetic age of dinosaurs 9 – 12 , 42 . The utility of this feature is justified, because both the EFS and OCL constitute an evidence of slow tissue deposition and functional stoppage (or near stoppage) of growth.…”

Intraskeletal Osteohistovariability Reveals Complex Growth Strategies in a Late Cretaceous Enantiornithine

“…Double LAGs (and even triple and quadruple) have been reported in other dinosaurs. Evidence suggests they are prevalent among theropods (e.g., Lee and O'Connor, 2013;Cullen et al, 2014;Evans et al, 2015;Cullen et al, 2020), and also occur in ornithischians (Werning, 2012), albeit more rarely. This is the first report of such a structure in an enantiornithine.…”

“…It is becoming increasingly common that anatomical descriptions include osteohistological data, and although this provides comparative material, these studies typically only sample just one or two elements (Zhang et al, 2013;Hu et al, 2015;Hu and O'Connor, 2017;Wang et al, 2017a). Recent research revealing high intraskeletal variability in bone tissue highlights the importance of studies utilizing greater numbers of bones whenever possible to facilitate interspecific comparisons between homologous elements (e.g., Horner et al, 2000;De Ricqlès et al, 2003;Woodward et al, 2014;Prondvai et al, 2018;Chinsamy et al, 2020;Cullen et al, 2020). As such, we sampled as many elements as permission allowed.…”