[4] In the same paper, Moreno erected a new genus and species of what he thought to be a fossil stork, Paleociconia australis, based on a distal left tarsometatarsus.

[1] The only other valid species of Mesembriornis was described in the same paper as Prophororhacos incertus based on a dorsal vertebra, partial right hindlimb, and assorted postcranial elements found in the Upper Miocene to Lower Pliocene strata of the Andalgala Formation in Catamarca Province, Argentina.

On the Tibiotarsus, the top of the Condylus Internus (lump) is pronounced and bent on the proximal side, so that a sharper angle is formed with the diaphysis.

The middle trochlea is spread at the distal end, with a width equal to or larger than the smallest diameter of the diaphysis.

[9] A speed as high as the one estimated could be very beneficial in the environment Mesembriornis lived in, as there was a large number of carnivorous mammals and birds like Borhyaena, Phorusrhacos, and Cladosictis.

Blanco et al.. (2005) also stated that the large, curved, and laterally compressed pedal ungues of Mesembriornis are similar to those in modern carnivorous birds.

[9][12][13] This school of thought suggests Mesmbriornis may have lived akin to a modern-day cheetah, eating the smaller notoungulate mammals of the time (Miocene) using its speed to outrun the beasts.

[9] High speed pursuit predation is potentially further supported by a close cousin Llallawavis, with CT scans of its inner ear showcasing adaptations to stabilizing the head when making quick movements.

[9][16] The following phylogenetic tree shows the internal relationships of Phorusrhacidae under the exclusion of Brontornis as published by Degrange and colleagues in 2015, which recovers Mesembriornis as a member of a large clade that includes Procariama and Llallawavis.