Description and osteohistology of two early immature enantiornithines (Aves: Ornithothoraces) from the Early Cretaceous Jehol Biota

The Enantiornithes is the dominant clade of Cretaceous land birds and the most diverse recognized clade of Mesozoic birds. More than half of this diversity is from the Lower Cretaceous Jehol deposits in northeastern China, and numerous late immature and mature specimens have been sectioned for histological analysis. These specimens reveal thin cortices with low amounts of vascularization and variably-present growth lines. An inner circumferential layer is typically observed, but rarely a distinct outer circumferential layer. Here we describe the morphology and histology of two early immature enantiornithines and explore ontogenetic changes in skeletal morphology and bone formation. These specimens help to fill in a crucial ontogenetic gap between the previously sectioned embryonic specimen of Gobipteryx and purportedly mature or near mature specimens. In support of interpretations regarding the precocial onset of flight in Enantiornithes, the proportions of the forelimb cortical thicknesses relative to the hindlimb in IVPP V15575 is very similar to mature enantiornithines. Opposite neornithines, the bone tissue of the humerus is more mature than the tibia. Precocial development of the femur is considered plesiomorphic to Aves, thus the shift towards greater maturity in the forelimb relative to the hindlimb observed in IVPP V15575 probably reflects the apomorphic evolution of super-precocial flight in enantiornithines. Osteohistological traits in IVPP V15686 and V15575 resemble those of extant chicks on the more altricial end of the developmental mode spectrum, but individuals from later growth stages. Since ossification indicates these two specimens are very immature, this highlights the unique osteohistological development of enantiornithines. Differences between these two individuals are potentially indicative of intertaxonomic variation in enantiornithine growth strategies. However, developmental plasticity in stem birds means that anatomical and osteohistological maturity are decoupled and that ossification patterns can vary even within a taxon. As such, significantly more data are required to fully understand observed differences and extract patterns regarding variation in developmental strategy among enantiornithines.