Archaeorhynchus spathula is a basal member of the Ornithuromorpha, the lineage that includes neornithines. Although this is the fifth reported specimen, unlike the others it preserves significant soft tissue, revealing a tail morphology previously unknown in Mesozoic birds and an exceptional occurrence of fossilized lung tissue. This identification is based on topographical location and anatomical features revealed mainly by scanning electron microscopy. A structure similar to that in neornithines indicates that the 120-Ma-old Archaeorhynchus may have been capable of acquiring the large quantities of oxygen needed to support powered flight. Skeletal features related to respiration remain primitive, supporting inferences that many physiological adaptations preceded skeletal changes during the evolution of the anatomically modern bird.

Abstract

We describe a specimen of the basal ornithuromorph Archaeorhynchus spathula from the Lower Cretaceous Jiufotang Formation with extensive soft tissue preservation. Although it is the fifth specimen to be described, unlike the others it preserves significant traces of the plumage, revealing a pintail morphology previously unrecognized among Mesozoic birds, but common in extant neornithines. In addition, this specimen preserves the probable remnants of the paired lungs, an identification supported by topographical and macro- and microscopic anatomical observations. The preserved morphology reveals a lung very similar to that of living birds. It indicates that pulmonary specializations such as exceedingly subdivided parenchyma that allow birds to achieve the oxygen acquisition capacity necessary to support powered flight were present in ornithuromorph birds 120 Mya. Among extant air breathing vertebrates, birds have structurally the most complex and functionally the most efficient respiratory system, which facilitates their highly energetically demanding form of locomotion, even in extremely oxygen-poor environments. Archaeorhynchus is commonly resolved as the most basal known ornithuromorph bird, capturing a stage of avian evolution in which skeletal indicators of respiration remain primitive yet the lung microstructure appears modern. This adds to growing evidence that many physiological modifications of soft tissue systems (e.g., digestive system and respiratory system) that characterize living birds and are key to their current success may have preceded the evolution of obvious skeletal adaptations traditionally tracked through the fossil record.