Brontornis

Size comparison of Brontornis (red), Kelenken (yellow) and a human

Brontornis had an estimated height of 2.8 m (9 ft 2 in) and its body mass is estimated between 350 and 400 kg (770 and 880 lb), making it the heaviest terror bird but slightly behind Kelenken in height and skull length. It may have been about 1.75 m (5 ft 9 in) 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.[2]

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.[2]

Life reconstruction

Fossils of the terror bird have been found in the Santa Cruz and Monte León Formations in Argentina.[3] 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.

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.[4][5]

However, other analyses have also argued that Brontornis exhibits thoracic vertebrae diagnostic of phorusrhacids, supporting its placement within that group.[6]