Patterns of postcranial fusion in the emu (Dromaius novaehollandiae) and Cretaceous theropod dinosaur Troodon formosus

Skeletal fusion has been a widely used indicator of maturity for fossil vertebrate specimens, including non-avian dinosaurs. However, recent research has highlighted the potential variability in fusion patterns that can exist among closely related vertebrates. Making more justified ontogenetic interpretations for fossil specimens requires the fusion patterns of extinct and extant taxa to be assessed whenever possible. In the case of Cretaceous paravian dinosaurs, a refined understanding of postcranial fusion is lacking. Here, we describe the patterns of postcranial fusion in the extant emu (Dromaius novaehollandiae) and in a Cretaceous theropod Troodon formosus. Our results suggest similarity in fusion patterns of the postcranial axial column between the two taxa. Both taxa exhibit a bidirectional pattern of neurocentral suture closure that begins anteriorly and posteriorly before converging in the sacral region and is complete before maximum body size is obtained. This pattern contrasts with the caudocranial directionality of crocodilians and the apparent craniocaudal directionality of some neognath birds and highlights that complete neurocentral suture closure is not always an indicator of maturity. Additionally, observed fusion of the atlas, axis, cervical ribs, and sacrum/synsacrum are events that happen late in ontogeny. Appendicular fusion is an ontogenetically delayed event for both taxa as well, although it is far more extensive in the emu than in Troodon. These data expand upon how ontogenetic interpretations for extinct paravian dinosaurs should be made and emphasizes the diversity of fusion patterns among Archosauria.