Brontornis

Brontornis; Temporal range: Early-Mid Miocene (Santacrucian-Laventan); ~17.5–11.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Fossil femur
	Fossil tarsometatarsus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Infraclass:	Neognathae
Genus:	†Brontornis; Moreno & 1891
Type species
	†Brontornis burmeisteri; Moreno & Mercerat 1891
Synonyms
	Genus synonymy Rostrornis Moreno & Mercerat 1891 ; Species synonymy Brontornis platyonyx Ameghino 1895 ; Rostrornis floweri Moreno & Mercerat 1891 ;

Brontornis is an extinct genus of giant flightless terror birds (Phorusrhacidae) that inhabited Argentina during the Early to Middle Miocene.^[1] 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.^[2] In 2021, a study found the genus to be closely related to mihirungs in the anserimorph order Gastornithiformes.

Description[]

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]

Discovery[]

Fossils of the terror bird have been found in the and 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.

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.^[4]^[5]

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

In a 2021 paper by Agnolin found Brontornis to a gastornithiform sister to the mihirungs.^[7]