Brontornis burmeisteri

Morphological description and paleoecological considerations of Brontornis burmeisteri (Aves, Phorusrhacidae?) from the Miocene of Argentina

Brontornis burmeisteri , one of the most impressive fossil birds in the South American record, is known from remains found in Middle Miocene sediments (approximately 17 to 15 million years ago) in the Santa Cruz Formation, located in Argentine Patagonia. Initially assigned to the family Phorusrhacidae, this taxon has been the subject of systematic debate due to its unique morphological characteristics, which substantially differentiate it from other "terror birds."

Large in size, Brontornis burmeisteri reached estimated heights of up to 2.8 meters and a weight of around 350–400 kg, making it among the heaviest known flightless birds. Its postcranial skeleton reveals an extremely robust build, with short, powerful hindlimbs, broad metatarsals, and a proportionally highly developed femur. These adaptations suggest a graviportal mode of locomotion, in which speed was sacrificed in favor of stability and strength, possibly to support its body weight while walking on open terrain.

The skull, although incomplete in the fossil record, shows evidence of a large, curved beak, similar to that of other phororacidae, although less sharp, which has led to divergent hypotheses about its diet. Unlike more typical members of Phorusrhacidae, specialized in an active carnivorous diet, Brontornis could have adopted a more generalist trophic regime, possibly scavenging or even partially herbivorous. This interpretation is also supported by the morphology of its jaws and the potential low speed suggested by its anatomy.

Its phylogenetic assignment has been subject to review. Although traditionally included within Phorusrhacidae due to its size and paleobiogeographic context, more recent analyses have proposed a closer affinity with the anseriformes (a group that includes ducks and swans), based on cranial and pelvic features diverging from the classic phororhacid pattern. This reconsideration would place it closer to a line of giant terrestrial birds, not directly related to predatory phororhacids, which in turn would imply a remarkable case of convergent evolution in gigantism and the loss of flight.

From a paleoecological perspective, Brontornis burmeisteri would have occupied a niche as a large herbivore or scavenger in the open grasslands of Miocene Patagonia, sharing habitat with notoungulates, toxodonts, and other large terrestrial vertebrates. Its presence highlights the ecological diversity of South American birds during this era and reflects the unique processes of faunal evolution that occurred in the relative isolation of the subcontinent during much of the Cenozoic.

Approximate measurements of the Brontornis:

• Scale 1:35
• Length 79 mm
• Height 81 mm
• Width 30 mm
• Standing height 82 mm

• Scale 1:20
• Length 137 mm
• Height 142 mm
• Width 53 mm
• Standing height 141 mm