Osteohistology of the lower cretaceous yixian formation ornithuromorph (Aves) iteravis huchzermeyeri

Abstract

We describe for the first time the histology of an ornithuromorph bird from the Lower Cretaceous Jehol Group, revealing the bone structure of one of the oldest members of this derived clade. The newly discovered Sihedang locality of the Yixian Formation in northeastern China preserves the oldest ornithuromorph dominated avifauna, with all collected birds referable to a single taxon, Iteravis huchzermeyeri. These specimens are all preserved in a relatively greater degree of three-dimensionality compared to Jehol specimens from other localities. We sampled a specimen of Iteravis in order to test the hypothesis that this aggregation of birds may represent a breeding colony. Although medullary bone is known to facilitate three-dimensional preservation, this bone tissue was not present in the histological samples. The specimen is nearly adult with regards to skeletal fusion, and histology indicates medullary expansion had occurred and an inner circumferential layer had already formed. However, lines of arrested growth are absent. Overall the bone histology is comparable to Ichthyornis; this is consistent with the derived phylogenetic placement inferred for this new taxon relative to previously sampled basal ornithuromorphs (Patagopteryx, Hollanda), which all indicate they required more than one year to reach adulthood. This suggests that the modern avian growth strategy evolved in the Early Cretaceous in non-ornithurine birds.