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，其对应结构对于蜥脚类恐龙的巨型化演化同样至关重要。目前在化石中推断气囊的存在，仅局限于保留了内部充气腔隙和孔道作为骨学标志（osteological correlates）的骨骼。我们在此提出针对气囊的骨骼组织学相关特征，作为识别这类呼吸系统结构的全新潜在鉴定工具。对多件鸟类和非鸟类恐龙标本的分析显示，次级小梁骨与次级骨内膜骨中存在纤细纤维；其中鸟类的这类组织已被证实与气囊直接接触，非鸟类恐龙的该类组织则被推测曾与气囊相连。这种“气骨性组织”（pneumosteal tissue）的骨骼组织学特征，与存在典型夏贝氏纤维（Sharpey’s fibers）的肌肉附着区域显著不同。多个非恐龙类群的充气骨骼并不具备这种“气骨膜”（pneumosteum）的特征。因此，我们这种基于组织学的新方法，可为重建恐龙类群中气囊的起源提供关键依据，进而帮助我们理解呼吸系统这一非凡的演化创新。