Brontornis

''Brontornis'' is an extinct genus of giant bird that inhabited Argentina during the Early to Middle Miocene. Its taxonomic position is highly controversial, with authors alternatively considering it to be a cariamiform, typically a phorusrhacid (terror bird) or an anserimorph.

Taxonomy

The first fossils of ''Brontornis burmeisteri'' were described by paleontologists Francisco Moreno and Alcides Mercerart in 1891, the fossils being a left femur, tibiotarsus, fibula, and tarsometatarsus all from the same individual found in the Lower-Middle Miocene strata of the Santa Cruz Formation in Santa Cruz Province, Argentina.Brodkorb, P. (1967). ''Catalogue of fossil birds: part 3 (Ralliformes, Ichthyornithiformes, Charadriiformes)''. University of Florida. In the same paper, two distal tibiotarsi from the same area were referred to ''Brontornis'' as well.Moreno, F. P., & Mercerat, A. (1891). ''Catálogo de los pájaros fósiles de la República Argentina conservados en el Museo de La Plata''. Taller de Publicaciones del Museo. Unknowingly, Moreno & Mercerat also named a species now seen as synonymous with ''Brontornis burmeisteri, Rostrornis floweri,'' that includes: various bill and skull fragments, seven vertebrae and fragments, femoral head, lower part of left femur, ectocondyle of right tibiotarsus. Some pedal phalanges and a trochleae of tarsometatarsus were also referred to the type, but they haven't been described in detail. These fossils were found in the lower Miocene Monte León Formation, one of the two formations the taxon is definitively known from. Some additional fossils, including a quadrate, were referred to ''Brontornis'' by Moreno & Mercerart in 1891, but these were dubiously referred. Mercerat named another taxon synonymized with ''Brontornis'', ''Stephanornis princeps'', in 1893, though the fossil is now lost so this assessment is unverifiable. Moreno's rival, Florentino Ameghino, also found fossils of ''Brontornis'' which he named ''Brontornis platyonyx'' in 1895 based on 4 pedal phalanges found at Monte Leon. The species has been synonymized with ''Brontornis.'' Ameghino also named ''Liornis floweri'' based on a distal tibiotarsus, distal tarsometatarsus, and 2 pedal phalanges of a juvenile that was synonymized with ''Brontornis'' in 1967.Buffetaut, E. (2016)
A reassessment of the giant birds Liornis floweri Ameghino, 1895 and Callornis giganteus Ameghino, 1895, from the Santacrucian (late Early Miocene) of Argentina.
''Palaeovertebrata'', ''40'', e3. ''Liornis minor'' was also synonymized in 1967, but it has since been synonymized with ''Phorusrhacos.'' ''Callornis giganteus'', another Ameghino taxon, may also be a synonym of ''Brontornis'' or a distinct close relative. Recently, the affinities of some ''Brontornis'' specimens has come into question due to the uncertain and debated phylogenetic position of the genus and its synonyms.

Evolution

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.

''Brontornis'' is a genus of birds (Aves), which is sometimes referred to its own family Brontornithidae. These are very large, flightless birds from the Miocene of South America and are known from very fragmentary material. The exact systematic assignment of ''Brontornis'' and the Brontornithidae is very uncertain and in a constant flux, which is largely due to the fragmentary nature of the remains. Originally placed on the base of the Phorusrhacidae ("terror birds"),Alvarenga, H. M., & Höfling, E. (2003)
Systematic revision of the Phorusrhacidae (Aves: Ralliformes).
''Papéis Avulsos de Zoologia'', ''43''(4), 55-91. there are several different views on its classification todau. After several cladistic studies, the Brontornithidae have been found as basal geese relatives.Agnolin, F. (2007). Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina. ''Revista del museo argentino de ciencias naturales nueva serie'', ''9''(1), 15-25. On the other hand, they also form the sister group of the Cariamiformes, which combine today's Seriema and the Phorusrhacidae. Since the introduction of the Brontornithidae as a family group, this has mostly been considered monogeneric or monospecific. Some authors were considered as possible relatives, but was mostly regarded as a synonym of ''Brontornis'' or ''Phorusrhacos''. However, a Tibiotarsus of a large bird similar to ''Brontornis'' was found in Oligocene strata in Salla-Luribay, Bolivia, making it the oldest known Brontornithid fossil known.

Traditionally, ''Brontornis'' was usually attributed as a genus to the extinct family of the Phorusrhacidae ("Terror birds"), which was quite widespread in South America and comprises large to very large, mostly predatory birds. These are related to the seriama, sharing features such as a hook-shaped upper beak and the curved ungues, which had sharp claws and thus distinguish the birds as predatory carnivores. Within the phorusrhacidae, ''Brontornis'' was referred to their own subfamily Brontornithinae, very large and massive birds, which also included physical territories and paraphysis. The allocation to the Phorusrhacidae was mainly based of the strong lower jaw symphysis and the targeted Tarsometatatarsus at the front and back and was supported by Herculano M. F. Alvarenga and Elizabeth Höfling in 2003. A revision of the genre of Federico L. Agnolin in 2007 rejects the assignment based on these characteristics to the Phorusrhacids, since they also occur in other early large running birds, for example in Anserimorphs or within the Dromornithidae. The rotation of the middle shape of the Tibiotarsus and the construction of the square leg suggest for a closer relationship with the geese birds (aneriform) rather than Phorusrhacidae. For these reasons, ''Brontornis'' was excluded from the Phorusrhacids and moved to the base of the geese birds by Agnolin. ''Physornis'' and ''Paraphysornis'', on the other hand, have been considered close relatives of ''Brontornis'' in their own subfamily in Phorusrhacidae, Brontornithinae, though it is sometimes referred to as Physornithinae.Silva, L. G. D. (2020). Revisão sobre a biomecânica da família Phorusrhacidae. This phylogenetic position for ''Brontornis'' had originally been favored by the first descriptions of the genus, but Mathilde Dolgopol de Sáez classified it as a relative of geese and ''Gastornis'' in 1927.Buffetaut, E. (2014). Tertiary ground birds from Patagonia (Argentina) in the Tournouër collection of the Muséum National d’Histoire Naturelle, Paris. ''Bulletin de la Société géologique de France'', ''185''(3), 207-214. Subsequently, after Agnolin's revision, his analysis was met with some approval from other paleontologists, but disagreements from other.

A phylogenetic analysis of Phorusrhacidae in 2011 again concluded that ''Brontornis'' was justified in the placement with the special expression of a fragmented thoracic vertebrae. Once again, suggesting that ''Brontornis'' sat within Anserimorpha.

With other cladist analyses suggested the structure of resemble that of Anserimorphs. The lack of the striking bone footbridge (Pons Supratendineus) on the front of the lower joint of the tibiotarsus of ''Brontornis'' was also pointed out several times, which in occurs in the Phorusrhacidae. Furthermore, the short and robust construction of the mandible constrasts to the long and slim mandibles of phorusrhacids, again suggested that ''Brontornis'' isn't a phorusrhacid. According to the examinations already mentioned, the different design of the lower jaw also results in a different diet of ''Brontornis'' than Phorusrhacids.

However, a study from 2017 suggested that ''Brontornis'' belonged to sister taxon to Cariamiformes. As a result, the genus moved close to the Phorusrhacidae, but not directly in it. Suggesting that the previous study a misinterpreted the orientation of the tarsometatarsus as a central point of systematic assignment of ''Brontornis''. It was pointed out that, the previous studies only used sparse material of ''Brontornis''. However, a 2021 study once again found ''Brontornis'' to be a Gastornithiform.

Description

Size

Due to the size of the finds, a height of 280 cm is assumed. The pronounced massiveness of the bones leads to weight estimates between 350 and 400 kg or 319 to 350 kg,Degrange, F. J., Noriega, J. I., & Areta, J. I. (2012). Diversity and paleobiology of the Santacrucian birds. ''Early Miocene paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation'', 138-155. however, some finds have clear size differences that can reach up to 33% based on the tarsometatarsus. 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.Alvarenga & Höfling, 2003 These measures would make ''Brontornis'' the fifth-heaviest bird found thus far, after ''Vorombe titan'', ''Dromornis stirtoni'', ''Aepyornis maximus'', and '' Pachystruthio dmanisensis''.Alvarenga & Höfling, 2003

Paleobiology

Diet

Furthermore, there is uncertainty about the diet, due to the lack of well preserved skull material and overlap between specimens. Due to the originally placement within the "terror birds", some researchers believe that ''Brontornis'' could have been a carrion eater. Correspondingly, art and skull reconstructions depicted ''Brontornis'' with a sharp and hooked top beak, though paleontologists are unsure because the tip of the preserved mandibles is missing. On the other hand, some paleontologists have proposed a herbivorous diet for ''Brontornis''. Studies on the lower jaw indicate that it was most likely not suitable for tearing animal food. The width and strong lower jaw is similar to that of ''Gastornis'' and ''Aepyornis,'' two herbivorous large birds. Likewise, the ungues of ''Brontornis'' do not show stronger bends compared to Phorusrhacids and thus do not allow raptor-like claws for tearing meat, which suggests a more plant-based diet.Buffetaut, E. THE BRONTORNITHIDAE: NEW DATA ABOUT AN OLD PALAEORNITHOLOGICAL RIDDLE. According to the enormous body size, it is assumed that ''Brontornis'' inhabited more open landscapes, as they have also been proven for the Santa Cruz formation.Tambussi, C. P., & Degrange, F. (2012). ''South American and Antarctic continental Cenozoic birds: Paleobiogeographic affinities and disparities''. Springer Science & Business Media. The short and wide Tarsometatarsus compared to the Tibiotarsus suggests a slow locomotion adapted to the enormous body mass.

References

{{Taxonbar, from=Q2339690

Brontornithinae
Miocene birds of South America
Burdigalian life
Langhian life
Serravallian life
Laventan
Colloncuran
Friasian
Santacrucian
Neogene Argentina
Fossils of Argentina
Fossil taxa described in 1891