Hindlimb feathers in paravians: Primarily wings or ornaments?

Recently a specimen of Sapeornis was described with feathers around the ankle joint, the first basal bird with metatarsal feathers. The discovery of several groups of paravians with feathered distal hindlimbs has been used as evidence that flight in Aves evolved through a tetrapteryx stage. However, most specimens of Sapeornis do not preserve feathers around the feet, and these feathers are clearly absent in at least one exceptional specimen preserving skin and feather impressions. This suggests that the presence of ankle feathers in Sapeornis may alternatively be interpreted as ornamentation. Basal birds show more ornamental tail morphologies than aerodynamic ones suggesting that sexual selection played a dominant role in shaping tail plumage. Feathers on the forelimbs initially evolved for a purpose other than flight and we propose aerodynamics was not the driving force in hindlimb feather evolution. We suggest that feathers as a means of visual communication played a very important role in the evolution of paravian plumage.