Abstract: We conduct histological analysis of two sexually mature fossil birds from the Lower Cretaceous Jehol Group referable to Jeholornis sp. and Enantiornithes indet. Histology confirms that in these lineages of basal birds, sexual maturity was achieved before skeletal maturity. The samples reveal structural differences from previously described relevant specimens, indicating that the ontogenetic changes in bone tissue that occur in Mesozoic birds are more complicated than previously recognized. The female specimen ofJeholornis sectioned here is more vascularized than previously described specimens, with both reticular and longitudinal canals. The enantiornithine bone tissue is most similar to that reported for the Early Cretaceous Concornis, primarily formed by parallel-fibered bone but more vascularized than other reported adult enantiornithine specimens. The bone shows a distinct decrease in the rate of bone deposition, inferred to represent the onset of sexual maturity.