Potential for powered flight development in microraptorine dromaeosaurids, bird-like ‘raptor' dinosaurs from the Cretaceous period

Abstract

The evolution of birds from non-flying theropod dinosaurs is a fantastic example of flight evolution and has been studied for over 150 years. Unfortunately, a deeper understanding of early theropod flight has been frustrated by gaps in the fossil record and by disagreements on the relationships between birds (avialans) and their closest theropod relatives. This presentation focuses on the implications a new, larger, more resolved evolutionary hypothesis has on the potential that powered flight developed in microraptorine dromaeosaurids, a subclade of early diverging ‘raptor’ theropods from the Cretaceous of East Asia and North America. Recently, a larger, more resolved evolutionary hypothesis of coelurosaur theropods - carnivorous dinosaurs including T. rex as well as early birds and their closest relatives - was produced by an automated analysis pipeline tailored for large morphological datasets. This corroborates the basic composition of the Paraves – a clade of bird-like dinosaurs that groups dromaeosaurids + troodontids (Deinonychosauria) as the sister taxon to birds. The hypothesis also supports the recovery of the controversial Late Jurassic anchiornithid paravians as the earliest diverging subclade of birds. These results have important implications on the sequence of flight-related evolutionary changes that occurred along the theropod lineage to modern birds. This includes the potential of flight in non-avialan paravians that have not traditionally been thought of as flying animals, such as the early diverging microraptorine dromaeosaurids. Microraptorinae includes the tiny species Microraptor which possesses potentially flight-relevant anatomy like asymmetrically-vaned feathers on their long arms and legs as well as the potential for non-powered flight as indicated by physical, computer and equation-based aerodynamic modelling. Tianyuraptor is the largest and earliest diverging microraptorine with another relatively large-bodied microraptorine, Zhenyuanlong, recently recovered as the next earliest diverging microraptorine for the first time. In the context of the revised microraptorine portion of the phylogeny, forelimb length and body size, traits traditionally important in considering flight in birds, both appear to decrease along the microraptorine lineage. Ancestral state reconstruction analysis of wing loading and specific lift, proxies for powered flight capability in modern birds, identifies Microraptor as having the potential for powered flight. Taken together, the context provided by the new phylogeny raises the possibility that the potential for powered flight decreased along the microraptorine lineage. This has motivated intensive on-going study of the dozens of known microraptorine specimens, including laser-based imaging of otherwise invisible aerodynamically-related preserved soft tissues. The way these new data will be used to the refine the aerodynamic modelling needed to further interrogate this new hypothesis of microraptorine powered flight development will be discussed.