Brontornis

Brontornis is an extinct genus of either anseriforms or giant flightless terror birds (Phorusrhacidae) that inhabited Argentina during the Early to Middle Miocene. Certainly the heaviest of the Phorusrhacidae family, and the birds from the Americas as a whole, it is one of the largest birds that has ever existed.
Description.
"Brontornis" had an estimated height of and its body mass is estimated between , making it the heaviest terror bird but slightly behind "Kelenken" in height and skull length. It may have been
about tall at the back. These measures would make "Brontornis" the fifth-heaviest bird found thus far, after "Vorombe titan", "Dromornis stirtoni", "Aepyornis maximus", and "Pachystruthio dmanisensis".
A comparison of the tarsometatarsi of two "B. burmeisteri" specimens, FM-P13259 and MLP-91 (lectotype), both coming from the same geographical region and geological formation, shows them as not to present any anatomical differences, apart from size, where in the first is around one third smaller than the second. The idea is that they are examples of intraspecific variation, possibly sexual dimorphism. There is the possibility that they represent two species.
Discovery.
Fossils of the terror bird have been found in the Santa Cruz and Monte León Formations in Argentina. It is known from bones, mainly of the legs and feet but also portions of some skulls and backbone, found in several localities of Santa Cruz Province. It coexisted with some slightly smaller and more active phorusrhacids like "Phorusrhacos".
Classification.
Recent work has cast doubt on the hypothesis that "Brontornis" is a phorusrhacid. "Brontornis" may actually represent an anseriform, with other genera traditionally assigned to Brontornithinae ("Physornis" and "Paraphysornis") being true phorusrhacids. The subfamily containing the latter two had been proposed to be renamed to Physornithinae, with "Physornis fortis" as the type species. If these conclusions are valid, this would mean that there are three groups of giant basal anseriformes, in chronological order of divergence: the gastornithids ("Gastornis" and kin), "Brontornis", and finally the mihirungs of Australia.
However, other analyses have also argued that "Brontornis" exhibits thoracic vertebrae diagnostic of phorusrhacids, supporting its placement within that group.
In a 2021 paper by Agnolin found "Brontornis" to a gastornithiform sister to the mihirungs.