''Mesembriornis'' is a genus of intermediate-sized phorusrhacids that grew up to in height. They represent a well-distinct lineage of terror birds, differing from the massive large groups and the smaller

Psilopterinae Phorusrhacids, colloquially known as terror birds, are an extinct clade of large carnivorous flightless birds that were one of the largest species of apex predators in South America during the Cenozoic era; their conventionally accepted tempor ...

. In general proportions, they most resembled the Patagornithinae which flourished somewhat earlier, mainly to the south of the range of ''Mesembriornis''. Fossils of the terror bird have been found in Montehermosan deposits of the

Monte Hermoso Formation Monte may refer to: Places Argentina * Argentine Monte, an ecoregion * Monte Desert * Monte Partido, a ''partido'' in Buenos Aires Province Italy * Monte Bregagno * Monte Cassino * Montecorvino (disambiguation) * Montefalcione Portugal * Monte ...

in Argentina.''Mesembriornis''
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...

.org

Etymology

The genus name, ''Mesembriornis'', means "southern bird" after its discovery in the southern Argentina, while the specific name is after French paleontologist Alphonse Milne-Edwards.

Taxonomy and discovery

''Mesembriornis'' was first described by Argentine paleontologist Francisco Moreno during the "Argentine Bone Wars" between him and Florentino Ameghino in 1889 based on a cervical vertebral centrum along with the proximal section of a right tibiotarsus and fibula ( MLP-140-142), the species name being ''Mesembriornis milneedwardsi.''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. In the same paper, Moreno erected a new genus and species of what he thought to be a fossil

stork Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family called Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other families, such as herons an ...

, ''Paleociconia australi''s, based on a distal left

tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) and meta ...

. This species has since been synonymized with ''Mesembriornis milneedwardsi,'' and Moreno also unknowingly assigned a femur of ''M. milneedwardsi'' to another one of his taxa, ''Driornis pampeanus''. All of the fossils were collected from the

strata of the town of

Monte Hermoso Monte Hermoso is a town located on the Atlantic coast of Argentina, some east of the city of Bahía Blanca, in the south of the Province of Buenos Aires. It is the administrative seat of the partido of Monte Hermoso. Founded at the beginning ...

Buenos Aires Province Buenos Aires (), officially the Buenos Aires Province (''Provincia de Buenos Aires'' ), is the largest and most populous Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of th ...

, Argentina.Brodkorb, P. (1967). ''Catalogue of fossil birds: part 3 (Ralliformes, Ichthyornithiformes, Charadriiformes)''. University of Florida. 2 years later in 1891, Moreno and his colleague Alcides Mercerart described two more ''Mesembriornis'' species that are now seen as synonymous with the large Phorusrhacid '' Phorusrhacos'', the species being ''Mesembriornis studeri'' and ''Mesembriornis quatrefragesi'', the former species' type specimen notably included skull and mandible material. It wasn't until 1914 that additional fossils of ''Mesembriornis'' were described by Gaetano Rovereto, who believed that the genus name ''Mesembriornis'' was a

nomen nudum In taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published with an adequate descr ...

and should be replaced by the genus name ''Hermosiornis'', even creating the family Hermosiornidae for the genus and the two species he assigned to it, ''Mesembriornis milneedwardsi'' and ''Mesembriornis'' (''Paleociconia'') ''australis.''Rovereto, C. (1914). Los estratos araucanos y sus fósiles. An. del Mus. ''Nac. Hist. Nat. Buenos Aires'', ''25''. Rovereto referred a nearly complete skeleton lacking the skull to ''M. milneedwardsi'' (

MACN The Bernardino Rivadavia Natural Sciences Argentine Museum ( es, Museo Argentino de Ciencias Naturales Bernardino Rivadavia) is a public museum located in the Caballito, Buenos Aires, Caballito section of Buenos Aires, Argentina. History and ov ...

-5944) that may actually be from the same individual as the holotype (MLP-140-142). Rovereto also believed that "''Hermosiornis''" was the ancestor of the modern '' Cariama'' and the descendant of '' Psilopterus'' (''Pelecyornis''), though this has since been disproven. The only other valid species of ''Mesembriornis'' was described in the same paper as ''Prophororhacos incertus'' based on a dorsal vertebra, partial right hindlimb, and assorted postcranial elements found in the Upper Miocene to

Lower Pliocene Lower may refer to: * Lower (surname) * Lower Township, New Jersey *Lower Receiver (firearms) * Lower Wick Gloucestershire, England See also *Nizhny Nizhny (russian: Ни́жний; masculine), Nizhnyaya (; feminine), or Nizhneye (russian: Ни ...

strata of the Andalgala Formation in Catamarca Province, Argentina. Several more fragmentary postcranial elements of an individual originally ascribed to '' Procariama'' also belong to ''Mesembriornis incertus.'' Skull material of ''Mesembriornis'' wasn't described until in 1946, Uruguayan paleontologist Lucas Kraglievich named another species of "''Hermosiornis", Hermosiornis rapax,'' based on a nearly complete skeleton (MMP-S155) including a mandible and partial skull but missing parts of the limbs. The fossils described by Rovereto and Kraglievich were found in the

Chapadmalal Formation Chapadmalal compound is an official retreat of the President of Argentina. It is located on the southern coast of Mar del Plata, in the Buenos Aires Province, and serves as a summer residence. It has a hotel complex nearby. The compound was buil ...

, dating to the

Chapadmalalan The Chapadmalalan age is a period of geologic time (4.0–3.0 Ma) within the Pliocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Montehermosan and precedes the Uquian age. Fossil content F ...

of the late Pliocene. ''Mesembriornis'' lived on the

pampa The Pampas (from the qu, pampa, meaning "plain") are fertile South American low grasslands that cover more than and include the Argentine provinces of Buenos Aires, La Pampa, Santa Fe, Entre Ríos, and Córdoba; all of Uruguay; and Brazil ...

of eastern Argentina, from the Late Miocene to the Late Pliocene, roughly 5.3 to 4 million years ago. Together with the

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

n giant '' Titanis walleri'', it was among the last terror birds alive.

Description

''M. incertus'' looked very much like '' Patagornis'' and ''

Andalgalornis ''Andalgalornis'' is a genus of flightless predatory birds of the extinct family Phorusrhacidae (often called "terror birds") that lived in Argentina. The type and only species is ''A. steulleti''. Taxonomy ''Andalgalornis'' is known from an in ...

'' in terms of construction (about 1.4 meters high). ''M. milneedwardsi,'' on the other hand, was at least 20% larger and heavier. The weight of this species is calculated at 70 kg and height of the back between 1.1 and 1.2 meters. The head would, if it was kept up, would be held at an altitude of almost 1.7 meters. The upper maxilla bone is relatively low, especially in the middle, and rostral extended. The symphysis of the lower jaw (Symphysis mandibulae) is short and quite low. On the Tibiotarsus, the top of the Condylus Internus (lump) is pronounced and bent on the proximal side, so that a sharper angle is formed with the diaphysis. The genus is the lankiest of all the Phorusrhacids, where the Tarsometatarsus reaches a length of 80 to 85% of the length of the tibiotarsus. The middle trochlea is spread at the distal end, with a width equal to or larger than the smallest diameter of the diaphysis.

Paleobiology

''Mesembriornis'' habits

A study called "Terror Birds on the Run" measured how fast ancient terror birds could run in 2005. The study calculated the speeds of the phorusrhacids ''Patagornis'', ''Mesembriornis,'' and a giant indeterminate phorusrhacine from the Quaternary of Uruguay. The paper found an estimate of (27 m s∼97 km h), one of the highest speeds calculated for a terrestrial vertebrate, the speed being comparable to that of the modern

spotted cheetah The cheetah (''Acinonyx jubatus'') is a large cat native to Africa and central Iran. It is the fastest land animal, estimated to be capable of running at with the fastest reliably recorded speeds being , and as such has evolved specialized ...

. This estimate is further supported by the strength of the preserved fossils, the width of the middle section of the tibial diaphysis being 3.2 cm. However, the authors of the paper pointed out that the estimate could be inflated and that the use of the limbs was instead for kicking prey. A speed as high as the one estimated could be very beneficial in the environment ''Mesembriornis'' lived in, as there was a large number of carnivorous mammals and birds like ''

Borhyaena ''Borhyaena'' is an extinct genus of South American metatherian, living between 17.5 and 15.5 million years ago in Patagonia, Argentina ( Santa Cruz and Sarmiento Formations) and Chile ( Río Frias Formation).Phorusrhacos'', and '' Cladosictis.'' There are two main theories about how ''Mesembriornis'' hunted: ;"Crushing Kicks" Based on anatomical and ecological similarities to the modern secretary Bird, phorusrhacids may have used kicks to kill prey or defend kills. If it also attained the speed first thought as well as this kick, it could not have been forced off kills as easily as cheetahs in Africa. The kick also may have been used for self defense as based on modern ratites. Blanco ''et al.''. (2005) also stated that the large, curved, and laterally compressed pedal ungues of ''Mesembriornis'' are similar to those in modern carnivorous birds.Campbell, B., & Lack, E. (1985). A dictionary of birds. 670 pp. ;"Cheetah of the Tertiary" This school of thought suggests ''Mesmbriornis'' may have lived akin to a modern-day cheetah, eating the smaller notoungulate mammals of the time (Miocene) using its speed to outrun the beasts. Its top speed is a matter of debate, but estimates go up to . Some other scientists scale down the predator's speed to 85, 80, 75 or even the average phorusrhacid speed of 70 km/h.

Diet and predation

Due to the uncertainty on the habits and paleobiology of ''Mesembriornis'', much of the inferred ecology depends on the former. Following the "kicking" hypothesis, ''Mesembriornis'' was capable of delivering a kick with a force 3.5 times that of the body weight, a force capable of breaking the bones of modern medium-sized mammals like the springbok, Thomson's gazelle, and

chamois The chamois (''Rupicapra rupicapra'') or Alpine chamois is a species of goat-antelope native to mountains in Europe, from west to east, including the Alps, the Dinarides, the Tatra and the Carpathian Mountains, the Balkan Mountains, the Ril ...

. This also means that ''Mesembriornis'' and other phorusrhacids could've accessed the

bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...

inside bones, a behavior done in the extant bearded vulture which has been observed breaking bones by dropping them from high altitudes. This implies that ''Mesembriornis'' could've filled its own ecological role, akin to that of the extant bone-breaking hyena in Africa.Kruuk, Hans (1972). The Spotted Hyena: A Study of Predation and Social Behaviour. University of California Press. ISBN 978-0226455082.

Classification

The following phylogenetic tree shows the internal relationships of Phorusrhacidae under the exclusion of ''Brontornis'' as published by Degrange and colleagues in 2015, which recovers ''Mesembriornis'' as a member of a large clade that includes ''Procariama'' and ''Llallawavis''.

References

{{Taxonbar, from=Q2556351 Phorusrhacidae Bird genera Extinct flightless birds Pliocene birds of South America Montehermosan Neogene Argentina Fossils of Argentina Fossil taxa described in 1889