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（air sacs）是鸟类呼吸系统（avian respiratory system）的重要组成部分，而对应的结构对于蜥脚类恐龙（sauropod dinosaur）的巨型化演化同样至关重要。迄今为止，在化石中推断气囊的存在，仅能以保留内部气腔（pneumatic cavities）与骨孔（foramina）的骨骼作为骨学关联特征（osteological correlates）来实现。本研究提出了针对气囊的骨组织学关联特征（bone histological correlates），以此作为识别这类呼吸系统结构的新型潜在鉴定工具。通过对多件鸟类与非鸟类恐龙（non-avian dinosaur）样本的分析，团队在次级小梁骨（secondary trabecular bone）与次级骨内膜骨（secondary endosteal bone）中发现了纤细纤维；其中前者（鸟类）的此类组织已被证实与气囊接触，后者（非鸟类恐龙）则被推测存在此类接触。这类“气骨性组织（pneumosteal tissue）”的骨组织学特征，与肌肉附着区域所呈现的典型夏贝氏纤维（Sharpey’s fibers）存在显著差异。多件非恐龙类群（non-dinosaurian taxa）的充气骨骼（pneumatized bones），均未表现出该“气骨膜（pneumosteum）”的特征。因此，这项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助力，进而帮助我们理解呼吸系统这一非凡的演化创新。