Nauganee Integrative Research Center, Field Museum of Natural History, 1400 S Lake Shore Dr., Chicago, IL 60605, USA.
Paleontology, North Carolina Museum of Natural Sciences, 11 W. Jones St, Raleigh, NC 27601, USA.
Proc Biol Sci. 2020 Nov 25;287(1939):20202258. doi: 10.1098/rspb.2020.2258.
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.
恐龙巨型化的独立演化一直是一个长期以来备受关注的话题，但巨型兽脚亚目恐龙（化石记录中最大的两足动物）是否都以相同的方式或通过不同的策略达到巨大体型，目前仍不清楚。我们对来自八个兽脚亚目科的广泛进化枝数据集进行了多元素组织学分析，重点关注巨型暴龙类和鲨齿龙类，以重建这些谱系的生长策略，并检验特定骨骼是否始终保存最完整的生长记录。我们发现，在骨骼成熟的巨型兽脚亚目中，承重骨骼始终保存着广泛的生长记录，而非承重骨骼则被重塑，对于生长重建的用处较小，这与较小的兽脚亚目和其他一些恐龙类群的观察结果相反。我们发现暴龙类的生长模式符合加速模型的种系发生模式，鲨齿龙类的异速生长模式更好地符合超形态发生模型。这些不同的生长模式似乎在系统发育上受到限制，分别代表了较小的虚骨龙类和鲨齿龙类中存在的生长模式的极端版本。这首次提供了证据表明，在化石记录中最大的两足动物中，体型进化没有强大的机械或生理限制，也为有史以来记录到的寿命最长的恐龙个体之一提供了证据。
