Bone Tissue Histology Of The Early Cretaceous Bird Yanornis: Evidence For A Diphyletic Origin Of Modern Avian Growth Strategies Within Ornithuromorpha

We present the first description of the bone microstructure of the ornithuromorph bird Yanornis from the Early Cretaceous Jehol ornithofauna. Yanornis is a derived member of independent ornithuromorphan clade, itself the sister group to the clade including Iteravis and ornithurans (including modern birds). We demonstrate that Yanornis grew continuously and probably was fully grown within the first or second year of life. No growth marks indicating cessations in bone deposition are present within the samples. However, significant transitions in bone and vascular texture are evident, features which are likely to signal the final stage of growth before reaching adulthood. These observations are consistent with the advanced distribution of endosteal bone along the inner margin of the medullary cavity and associated erosional rooms, and the increasing occurrence of avascular bone on the outer regions. The conditions reported here are similar to those reported for the other basal ornithuromorph Iteravis huchzermeyeri. Yanornis and Iteravis are, however, members of two different sister clades of Ornithuromorpha. The former genus is nested with Hollanda in that it exhibits interrupted growth patterns within the same clade. We propose that a rapid uninterrupted growth strategy probably evolved independently at least twice during the evolution of ornithuromorph birds.