It was much larger than J. magna, giant hutia or the largest living rodent, the capybara, which averages 60 kg (130 lb).

Its skull was heavily reinforced to withstand high stresses far exceeding what bite force alone could exert, so it could have been using its teeth to crack nuts, excavate large burrows, dig up roots, or self defense against predators.

Josephoartigasia lived in a forested estuarine environment, alongside toxodontids, ground sloths, glyptodonts, scimitar-toothed cats, terror birds, and thylacosmilids.

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.

[2] The genus name honors Uruguayan national hero José Artigas,[3] and magnus is Latin for "large".

[1] 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.

[5] 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.

Dinomyidae is a poorly defined family, and there is no clear morphological diagnosis that can include every member currently relegated to it.

[7][2] J. magna was not found in situ (in the spot the fossil was originally deposited in), but in 1966, Frances and Mones assumed it came from one of the older two of the three formations they defined, since eumegamyines had never been reported from the Quaternary.

Of those two, they guessed it came from the younger one, the San José Formation, since, on account of its enormous size, they surmised it must be a derived species (it had a lot of time to evolve such a specialized trait from a hypothetically minuscule ancestor).

[10] 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.

Like other dinomyids, the occlusal (biting) surface of each grinding tooth has smooth and slightly curved lophs (ridges).

[1] For comparison, in a 2007 study, a sample of 110 beef cows belonging to 9 different breeds had a maximum head length of 52.8 cm (1 ft 9 in).

Like other dinomyids, the occipital condyles (where the spine connects to the skull) has paracondyles (extra prominences which serve as attachments).

Eumegamyines additionally typically have a well developed stylomastoid foramen, which funnels the facial nerve, and a short ear canal.

This potentially over-represents the influence from capybara and introduces confounding variables if certain specimens in the dataset are much smaller than usual.

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.

He also agreed that reconstructing the body mass of enormous creatures which far exceed the size of living counterparts will always be highly problematic.

[16] 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,[a] producing a body length of 262.8 cm (8 ft 7 in), though he noted these rodents may have proportionally longer heads.

The atrophy of the socket was probably a compensatory response to the missing tooth, sharply reducing jaw height towards the back.

The compressive and tensile strengths (the stresses at which the bone would fail) of the cranium were respectively 180 and 130 MPa (26,000 and 19,000 psi) so the skull was clearly reinforced for something other than just the bite.

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.

[12] In 2008, Rinderknecht and Blanco preliminarily supposed that J. monesi ate predominantly fruits and soft plants, namely aquatic vegetation as the animal seems to have lived in an estuarine environment.

Since germination of the seeds is much more likely if they pass through the elephant digestive system, marula may have evolved to be dispersed by mega-herbivores capable of producing such high bite forces ("large-bite-force hypothesis").

The other animals discovered at this member include: the toxodontids Trigodon and Charruatoxodon, the peccary Platygonus, the ground sloths Catonyx tarijensis and Pronothrotherium figueirasi; the glyptodonts (Glyptodon, Plaxhaplous and Uruguayurus); the darter bird Giganhinga;[23][8] an indeterminate scimitar-toothed cat,[d] a terror bird (possibly Devincenzia),[26][27] the capybara Cardiatherium talicei,[8] the vampire bat Desmodus draculae, and several fishes and turtles.