Data from: Bone histological correlates for air sacs and their implications for understanding the origin of the dinosaurian respiratory system

Air sacs are an important component of the avian respiratory system, and corresponding structures also were crucial for the evolution of sauropod dinosaur gigantism. Inferring the presence of air sacs in fossils so far is restricted to bones preserving internal pneumatic cavities and foramina as osteological correlates. We here present bone histological correlates for air-sacs as a new potential identification tool for these elements of the respiratory system. The analysis of several avian and non-avian dinosaur samples revealed delicate fibers in secondary trabecular and secondary endosteal bone that in the former case (birds) is known or in the latter (non-avian dinosaurs) assumed to have been in contact with air sacs, respectively. The bone histology of this “pneumosteal tissue” is markedly different from those regions where muscles attached presenting classical Sharpey’s fibers. The pneumatized bones of several non-dinosaurian taxa do not exhibit the characteristics of this “pneumosteum”. Our new histology-based approach thus can be instrumental in reconstructing the origin of air sacs among dinosaurs and hence for our understanding of this remarkable evolutionary novelty of the respiratory system.

气囊是鸟类呼吸系统的重要组成部分，其对应的结构对于蜥脚类恐龙的巨型化演化同样至关重要。迄今为止，推断化石中气囊的存在仅局限于以骨学关联特征（osteological correlates）保存有内部气腔和孔道的骨骼。我们在此提出以骨组织学关联特征作为识别这类呼吸系统结构的全新潜在工具。对多件鸟类与非鸟类恐龙样本的分析显示，次级小梁骨与次级骨内膜骨中存在细微纤维：前者（鸟类）的这类组织已知与气囊接触，后者（非鸟类恐龙）则被认为与气囊存在接触。这类"气骨性组织（pneumosteal tissue）"的骨组织学特征，与附着肌肉的区域呈现的经典夏贝氏纤维（Sharpey’s fibers）存在显著差异。多件非恐龙类群的充气骨骼并不具备这类"气骨性骨膜（pneumosteum）"的特征。因此，我们这套基于组织学的全新方法，可为重建恐龙类群中气囊的起源提供关键依据，进而助力我们理解呼吸系统这一非凡的演化新特征。