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亦至关重要。迄今为止，化石中气囊存在的推断仅能依托保留内部气腔与气孔的骨骼作为骨学关联指征。本研究提出一类针对气囊的骨骼组织学关联特征，可作为识别这类呼吸系统结构的新型潜在鉴定工具。通过对多组鸟类与非鸟类恐龙样本的分析，研究团队在次级小梁骨与次级骨内膜骨中发现了细微纤维结构：其中鸟类的此类组织已知与气囊直接接触，而非鸟类恐龙的此类组织则被推测与气囊相连。这类“气骨性组织（pneumosteal tissue）”的骨骼组织学特征，与附着肌肉的区域（呈现典型穿通纤维（Sharpey’s fibers））存在显著差异。多个非恐龙类群的气骨化骨骼，并不具备这类“气骨膜（pneumosteum）”的特征。因此，这套基于组织学的全新研究方法，可为厘清恐龙类群中气囊的起源提供重要支撑，进而帮助我们深入理解呼吸系统演化史上这一非凡的创新事件。