A new enantiornithine (Aves: Ornithothoraces) from the Lower Cretaceous Xiagou Formation with unusually short pubes

We describe a partial skeleton of an enantiornithine from the Changma locality of the Lower Cretaceous Xiagou Formation in northwestern China. The specimen is tentatively interpreted as a gastric pellet. The specimen preserves an unusually short pubis that is subequal in length to the ischium, an autapomorphy among enantiornithines, justifying erection of a new taxon, Novavis pubisculata nov. gen., nov. sp. The abbreviated length of the pubis suggests that the abdominal cavity in this species was proportionately shorter than in all other known enantiornithines. Although several muscles attach to the pubis, the mm. pubocaudalis internus and externus were probably the most functionally impacted by the increased distance between the pubes and caudal vertebrae and subsequent elongation of these muscles. In crown avians this would result in a decrease in the contraction speed of these muscles, which play a role in tail fanning and are thus important for flight during take-off and landing. Although the underlying function of the short pubis and changes in the size of the abdominal cavity are unknown, the widespread absence of aerodynamic tail musculature and plumage in enantiornithines may have relaxed pressures constraining the length of these muscles, such that elongation of these muscles through a shortened pubis was not detrimental to the aerodynamic abilities of these birds.