Mesembriornis

Mesembriornis was first described by Argentine paleontologist Francisco Moreno during the "Argentine Bone Wars" between him and Florentino Ameghino in 1889 based on a cervical vertebral centrum along with the proximal section of a right tibiotarsus and fibula (MLP-140-142), the species name being Mesembriornis milneedwardsi. The genus name, Mesembriornis, means "southern bird" after its discovery in the southern Argentina, while the specific name is after French paleontologist Alphonse Milne-Edwards. 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. This species has since been synonymized with Mesembriornis milneedwardsi, and Moreno also unknowingly assigned a femur of M. milneedwardsi to another one of his taxa, Driornis pampeanus. 2 years later in 1891, Moreno and his colleague Alcides Mercerart described two more Mesembriornis species that are now seen as synonymous with the large Phorusrhacid Phorusrhacos, the species being Mesembriornis studeri and Mesembriornis quatrefragesi, the former species' type specimen notably included skull and mandible material. Rovereto referred a nearly complete skeleton lacking the skull to M. milneedwardsi (MACN-5944) that may actually be from the same individual as the holotype (MLP-140-142). Mesembriornis lived on the pampa of eastern Argentina, from the Late Miocene to the Late Pliocene, roughly 5.3 to 4 million years ago. Together with the North American giant Titanis walleri, it was among the last terror birds alive. == Description ==

of M. milneedwardsi M. incertus looked very much like Patagornis and Andalgalornis in terms of construction (about 1.4 meters high). M. milneedwardsi, on the other hand, was at least 20% larger and heavier. The weight of this species is calculated at 70 kg and height of the back between 1.1 and 1.2 meters. When kept up, the head would be held at an altitude of almost 1.7 meters. The upper maxilla bone is relatively low, especially in the middle, and rostral extended. The symphysis of the lower jaw (Symphysis mandibulae) is short and quite low. 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 genus is the lankiest of all the Phorusrhacids, where the Tarsometatarsus reaches a length of 80 to 85% of the length of the tibiotarsus. The middle trochlea is spread at the distal end, with a width equal to or larger than the smallest diameter of the diaphysis. == Paleobiology ==

Mesembriornis habits A study called "Terror Birds on the Run" measured how fast ancient terror birds could run in 2005. The study calculated the speeds of the phorusrhacids Patagornis, Mesembriornis, and a giant indeterminate phorusrhacine from the Quaternary of Uruguay. The paper found an estimate of , one of the highest speeds calculated for a terrestrial vertebrate, the speed being comparable to that of the modern spotted cheetah. This estimate is further supported by the strength of the preserved fossils, the width of the middle section of the tibial diaphysis being 3.2 cm. ;"Cheetah of the Tertiary" 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. Its top speed is a matter of debate, but estimates go up to . Some other scientists scale down the predator's speed to 85, 80, 75 or even the average phorusrhacid speed of 70 km/h. Diet and predation Due to the uncertainty on the habits and paleobiology of Mesembriornis, much of the inferred ecology depends on the former. == Classification ==

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. == References ==