Macroeuphractus

Macroeuphractus Temporal range: Late Miocene-Late Pliocene (Huayquerian-Chapadmalalan) ~
Skull
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Placentalia
Order:	Cingulata
Family:	Chlamyphoridae
Subfamily:	Euphractinae
Genus:	†Macroeuphractus Ameghino, 1887
Type species
†Macroeuphractus retusus Ameghino, 1887
Other species
†M. outesi Ameghino, 1908 †M. moreni Lydekker, 1894

Macroeuphractus is a genus of extinct armadillos from the Late Miocene to Late Pliocene of South America. The genus is noted for its large size, with Macroeuphractus outesi being the largest non-pampathere or glyptodont armadillo discovered, as well as its specializations for carnivory, unique among all xenarthrans.

Taxonomy

There are three currently recognised species of Macroeuphractus: M. outesi, M. retusus and M. moreni.

Osteoderms of M. retusus are distinguished from those of M. moreni by their shallower grooves and dorsoventrally (top-bottom) flatter figures (the shape of the osteoderm). M. retusus is only known from the Ituzaingó, Monte Hermoso, and Cerro Azul Formations of Entre Ríos, Buenos Aires, and La Pampa Provinces respectively. In contrast, M. moreni is endemic to the Epecuén Formation of Catamarca and Tucumán Provinces and the Cerro Azul Formation of La Pampa Province. M. moreni was described in 1894 by British naturalist Richard Lydekker as a new species of Dasypus on the basis of several osteoderms that were collected from the “Catamarca beds”.

Description

M. retusus, the type species, is known from one specimen from the Late Pliocene of Buenos Aires, Argentina. This specimen is composed of a fairly well preserved skull as well as numerous postcranial elements. It represented a considerably large species at around 100 kilograms (220 lb), although it is possible that it was actually closer to 30 kilograms (66 lb), at a little over 1 meter (3 ft 3 in) in length it would still be a decently sized predator.

The skulls of M. retusus and Dasypus share many traits, although the teeth of the former are adapted for a carnivorous diet. Unlike Dasypus, M. retusis' first pair of teeth are small, while the second pair is enlarged and canine-shaped. The third pair is also small, whereas the rest of the teeth in the maxilla (upper jaw bone) regularly increase in size posteriorly (towards the back). Unlike most armadillos, Macroeuphractus has nasal bones that widen towards the tip, creating a broad rostrum. On the dorsal surface of the skull is a large sagittal crest (a ridge of bone extending dorsally from the skull), a characteristic shared with Macrochorobates but absent in many armadillos. A complete mandible is unknown, but an incomplete Macroeuphractus ramus was unearthed from layers of the Monte Hermoso Formation. This ramus has seven alveoli (tooth sockets), of which their sizes correspond with the sizes of teeth in the upper jaw. Its skull is associated with much of the postcrania, including the presternum (breastbone), elements of the mesosternum, fragmentary sternal ribs, a rib, an incomplete pelvis, sacrum, and some anterior (front) caudal vertebrae. Overall, their traits are similar to those of Dasypus. Alongside the skull and postcranium were a collection of osteoderms, including most of the last two movable bands of osteoderms in the carapace.

Classification

Macroeuphractus is traditionally grouped with euphractines; it was named after its similarity with Euphractus itself. The most recent phylogenetic examinations confirm its status as at least a sister group to euphractines, along with Paleuphractus, Doellotatus and Proeuphractus.

Biology

Macroeuphractus is one of the few known xenarthrans to have specialised extensively for a carnivorous lifestyle. Modern euphractine armadillos (such as the modern six-banded armadillo) are fairly dedicated omnivores, but Macroeuphractus shows several features that indicate hypercarnivory:

• large, conical caniniform teeth
• an enlarged temporal fossa
• more developed muscles pertaining to the temporalis musculature (as evidenced by the more prominent muscle scars)
• a deeper rostrum
• more powerful anterior teeth (particularly in the enlarged and caniniform M2)
• a deeper and more robust zygomatic arch
• a greater moment arm of the temporalis muscle than in other armadillos

These features are unusual among xenarthrans but are more in line with predatory mammal groups.

Like most armadillos, Macroeuphractus was fossorial, and it probably could dig out small and medium-sized mammals such as caviomorph rodents, small notoungulates and argyrolagoidean paucituberculates. Species like Macroeuphractus outesi were fairly large and presumably had an apex predator status in their faunal communities.

Paleoecology

Macroeuphractus moreni was a rather widespread species, and played a role in various faunal communities in the Miocene and Pliocene epochs of South America, while the other two species had a more limited range in Argentina. The genus occurs in an epoch where sparassodonts, phorusrhacids and sebecids entered in decline, and was among the various mammal groups to exploit this ecological vacancy prior to the arrival of North American carnivorans in the Pleistocene, alongside giant opossums such as Thylophorops. Nonetheless, Macroeuphractus still co-existed with late surviving sparassodonts such as Thylacosmilus and phorusrhacids like Llallawavis.