Josephoartigasia

The rodent was first described based on material collected from the Barrancas de San Gregorio, Uruguay, a series of sea cliffs in the San José Department by Kiyú beach. The enormous fossils, catalogue number 28.VI.65.1 SPV-FHC, comprise a left mandibular (lower jaw) fragment which preserves the bottom part of the incisor, the premolar, the first two molars, a cavity corresponding to the third molar, and the ramus (the portion of the lower jaw that comes up to connect to the skull). In 1966, Uruguayan paleontologists Julio César Francis and Álvaro Mones made this the type specimen of a new genus and species, Artigasia magna. They also identified a paratype, 26-XI-64-29 SPV-FHC, the tip half of the lower left incisor. The genus name honors Uruguayan national hero José Artigas, In 2007, Mones renamed the genus as Josephoartigasia because the previous name was a junior homonym of Artigasia, a genus of nematodes named by parasitologist Jesse Roy Christie in 1934 (that is, the name Artigasia was already taken). In 2008, Uruguayan paleontologist Andrés Rinderknecht and Uruguayan physicist Rudemar Ernesto Blanco described another species, J. monesi, based on a massive and nearly complete skull also from the Barrancas de San Gregorio. The name honors Mones for his work on South American rodents. The skull itself was actually recovered in 1987 and donated to the National Museum of Natural History, Uruguay, by fossil collector Sergio Viera, but sat in their repository until Rinderknecht (who worked as a curator) came across it. ==Classification==

(above) is the closest living relative of Josephoartigasia Josephoartigasia is a member of the family Dinomyidae, a group of hystricognath rodents native to South America, most commonly identified in Argentina, Colombia, Venezuela, and Uruguay. The only living member of Dinomyidae is the pacarana, one of the largest living rodents at . The family is typically divided into four or five subfamilies: Potamarchinae (which contains the oldest members of the group, to the middle Miocene), Gyriabrinae, Dinomyinae (which only houses the pacarana), Eumegamyinae (which contains the biggest genera), and sometimes Tetrastylinae, which can be merged into Dinomyinae or Eumegamyinae. Josephoartigasia is classified in Eumegamyinae. Dinomyidae is a poorly defined family, and there is no clear morphological diagnosis that can include every member currently relegated to it. This is because most dinomyid species are known by fragmentary remains of teeth and mandibles, obfuscating how different species are related to each other. ==Age and taphonomy==

In 1965, Frances and Mones stratigraphically separated the Barrancas de San Gregorio into the Kiyú Formation at the base, the San José Formation above, and the Arazatí Formation at the top. They biostratigraphically dated them to the Montehermosan, Chapadmalalan, and Pampean (Ensenadan) Ages, respectively, according to the South American land mammal age geological timescale. On the international geologic time scale, the older two would correspond to the Upper Pliocene, and the youngest to the Pleistocene. In 1988, Mones identified Lower Pleistocene levels in the San José Member. In 2002, American geologist H. McDonald and Uruguayan paleontologist Daniel Perea suggested the formation may represent a wide timespan from the Montehermosan all the way to the Ensenadan. J. monesi was recovered in situ from a boulder originating in the San José Member. The boulder is made up of siltstone, claystone, and medium-grained and medium-to-conglomeratic psammite (a type of sandstone) intercalated with siltstone. == Description ==

Teeth The dental formula of Josephoartigasia is , with one incisor (I1), no canines, one premolar (P4), and three molars (M1, M2, and M3) in either half of either jaw. As a rodent, the teeth grew continuously throughout the animal's life, there is a gap (diastema, and a rather long one) between the incisors and the grinding teeth (premolars and molars), and the grinding teeth are pushed far forward in the mouth ahead of the eye sockets. Skull J. monesi is the only species for which the skull has been identified. Its skull is massive, measuring in length. Its skull is 65% bigger than the skull size of the previous largest identified rodent, Phoberomys pattersoni. (below), showing their paracondyles (teal for the pacarana)Scale = There is nearly complete fusion of several cranial bones, namely the nasal and frontal bones; they are poorly differentiated and the shape and size of each one is difficult to observe, most especially the lacrimal bones in the eye socket. Fusion of the frontal and parietal bones created a mass of bone projecting laterally (out to the side). There is a tall temporal crest arcing across the top of the skull on either side, which join at the midline to form a short sagittal crest. The temporal fossa is narrow but deep. J. monesi has the deepest insertion point for the masseter muscle (which closes the mouth while biting down) of any rodent. It is similarly shaped to that of the large capybara, which is either due to heterochrony (because Josephoartigasia is closely related to the capybara) or allometry (because both Josephoartigasia and the capybara became big). The pterygoid fossa is small, conferring to a reduced medial pterygoid muscle (also important for biting). The zygomatic arches (cheekbones) of J. monesi are unexpectedly slender given how fortified the skull is. Like other dinomyids, the occipital condyles (where the spine connects to the skull) has paracondyles (extra prominences which serve as attachments). Body mass J. monesi is the first dinomyid whose near complete skull has been discovered; as other dinomyids are known only by highly fragmentary remains, J. monesi presented the first opportunity to estimate the living size of a dinomyid. By absolute measure, it is much larger than J. magna. Blanco disagreed with Millien's methods. He pointed out that, while the J. monesi skull may have been unexpectedly long in her dataset, it was not inconsistent with the proportions of its closest living relative, the pacarana. She also estimated the measurements from published photos rather than taking them from the specimen itself, which could confound the results. Blanco also pointed out their average estimates are rather close, about , but he was unable to reproduce as low a number as that Millien reported as her lowermost bound. Blanco nonetheless conceded his preliminary estimates were not the most meaningful, especially considering the high error margin, as his body mass estimates were not meant to be so high-resolution, rather to give a general idea of the creature's gargantuan nature. He also agreed that reconstructing the body mass of enormous creatures which far exceed the size of living counterparts will always be highly problematic. In 2022, American biologist Russell Engelman reestimated body sizes of multiple massive dinomyid and neoepiblemid rodents using the width of the occipital condyles where the skull attaches to the spine, because he had earlier demonstrated it to be a reliable metric for this purpose among several therian mammals. He also assumed J. monesi had the same head-to-body ratio as the pacarana, producing a body length of , though he noted these rodents may have proportionally longer heads. He calculated significantly lower body masses: for J. monesi and for P. pattersoni. Assuming the paracondyles functioned the same as in paracana, he suggested is the most likely range for J. monesi; and assuming rabbit-like condyles in P. pattersoni, . Though they are still the largest rodents ever discovered, he argued estimates exceeding are unwarranted. Pathology The type specimen of J. magna is missing its M3, and the tooth socket is badly atrophied. The atrophy of the socket was probably a compensatory response to the missing tooth, sharply reducing jaw height towards the back. == Paleobiology ==

Bite In 2012, Blanco, Rinderknecht, and Uruguayan paleontologist Gustavo Lecuona estimated the bite force of J. monesi at the incisors by reconstructing the major biting muscles and their strengths. They reported with a mean of , which is not entirely unrealistic given the animal's mass. The bite force is comparable to that of many large carnivores (which have a much smaller mass-to-bite-force ratio); for example, the polar bear has a bite force of roughly and the jaguar . Josephoartigasia had strong and extremely procumbent incisors, a reinforced skull, and a large diastema between the incisors and the grinding teeth. This combination is usually seen in rodents that use their incisors to either dig or process hard objects (such as nuts or wood). If the former, Josephoartigasia could have been digging up roots, or excavating large burrows to inhabit (fossoriality). Mega-burrows have been discovered in South America, but this is not conclusive evidence of fossoriality because Josephoartigasia is not the only animal that could have been carving them out. Powerful incisors were likely also used for defense against predators, contending with terror birds and borhyaenids (marsupial-like carnivores); defense against a charging predator would have subjected the incisors to variable and intense loads, which would necessitate a high section modulus to avoid structural failure. Carbon isotope analyses of the enamel of giant fossil rodents, including J. monesi, report that they ate only C3 plants, such as leaves or fruits, as opposed to C4 plants, such as grasses. The modern capybara, similarly, is known to selectively eat C3 over C4 plants, even when the latter is much more plentiful, yet they will still eat C4 plants in leaner times. This behavior may have also been exhibited in Josephoartigasia. ==Paleoecology==

s like Xenosmilus (above) inhabited Uruguay during the Plio-Pleistocene, alongside Josephoartigasia. the capybara Cardiatherium talicei, The area may have been a forested estuarine environment. ==Notes==