Soroavisaurus

Soroavisaurus is an extinct genus of enantiornithine birds that lived in South America during the Late Cretaceous period, about 71–69 million years ago. The only known species, Soroavisaurus australis, was named in 1993; the generic name means "sister to Avisaurus" (another enantiornithine believed to be its relative) while the specific name means "southern". All specimens confidently assigned to this species are fossilised leg bones collected from the Lecho Formation of El Brete, a locality in the province of Salta, Argentina. Some authors have proposed that either Enantiornis or Martinavis, which are known from wing bones found at the same locality, may represent the same animal as Soroavisaurus. However it is difficult to confirm which (if either) of these truly is the same species.

The body length of Soroavisaurus has been estimated at about 40 cm (16 in), but the disparity in size among known specimens suggests that some individuals grew larger than others. This animal is characterised by its tarsometatarsus, a leg bone formed by the fusion of the tarsal and metatarsal bones; specifically, there is a gap in the upper part between two of the metatarsals, which is not known in any other enantiornithine. Soroavisaurus is a member of the family Avisauridae, but its exact position within this family has been debated; the North American forms Avisaurus and Gettyia were originally thought to be its closest relatives, but other South American forms like Intiornis were later proposed to be closer to it, and some authors place Soroavisaurus as an early-diverging member of the family.

Soroavisaurus would have been a carnivore, using its sharp, curved talons to carry prey while flying in a manner reminiscent of modern birds of prey, though its foot structure suggests that its exact ecological niche differs from that of its living counterparts. The Lecho Formation dates to the Maastrichtian age and was deposited on a warm, humid coastal plain with tropical forests, ponds and lagoons. In this habitat, Soroavisaurus would have lived alongside many animals, including other species of enantiornithine birds and non-avian dinosaurs.