Josephoartigasia is an extinct genus of enormous dinomyid rodent from the Early Pliocene to Early Pleistocene of Uruguay. Its closest living relative is the pacarana. Josephoartigasia is named after Uruguayan national hero José Artigas. It contains two species: J. magna, described in 1966 based on a left mandible, and J. monesi, described in 2008 based on a nearly complete skull. Both are reported from the San José Member of the Raigón Formation by the Barrancas de San Gregorio along Kiyú beach.
The skull of J. monesi measures , similar to a beef cow skull, equating to a full body length of —though this is likely an overestimate—and a weight of about . This makes J. monesi the biggest rodent ever discovered. It was much larger than J. magna, giant hutia or the largest living rodent, the capybara, which averages . J. monesi also had a massive bite force of approximately at the incisors (on par with large carnivores) and at the third molar (rivaling large crocodilians). 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 defend itself against predators.
Josephoartigasia lived in a forested estuarine environment, alongside toxodontids, ground sloths, glyptodonts, scimitar-toothed cats, terror birds, and thylacosmilids. Like other giant extinct rodents, Josephoartigasia predominantly ate C3 plants, such as leaves or fruits, though the extreme bite force of J. monesi would have permitted it to consume a wide variety of different plants if necessary.
Discovery and etymology
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, and magnus is Latin for "large".
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
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.
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). Nonetheless, they could not entirely rule out Kiyú provenance. The San Jose Formation contains fluvial deposits (carried by rivers and streams), and features gray-greenish sandstones and conglomerates, overlain by clayish sandstone with loess-intercalated carbonate pockets.
In 1966, Uruguayan geologists Héctor Goso and Jorge Bossi defined the Raigón Formation, which they subdivided into the San José Member (the same as Francis and Mones' San José Formation) below and the San Bautista Member above. 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.
Grinding teeth
The walls of the molars are concave, and the molars are tightly packed together. Like other dinomyids, the occlusal (biting) surface of each grinding tooth has smooth and slightly curved lophs (ridges). The back lophs have a decently thick enamel coat, whereas the front lophs have next to no enamel. This, alongside a sparsity of interprismatic (between crystallic prisms of enamel) cementum, would suggest that the lophs are separated only by a thin layer of enamel. This sloping enamel thickness and thin cementum layer are characteristic of the genus.
In J. magna, the P4 has a root surface area of and has five lophs; the first and widest is almost an oval with a sharp, thin edge on the labial surface, and a thickened, rounded edge on the lingual surface; the other four lophs are almost quadrilateral with rounded edges on either surface. M1 is smaller at , and the first three frontward lophs fuse into a single loph towards the edge, leaving it with three external and five internal lophs. M2 is somewhat bigger at , and only the first two lophs fuse, leaving it with four external and five internal lophs. M3 is unknown in this species.
In J. monesi, the upper left premolar (P4), left first molar (M1), right second molar (2M), and both third molars (M3) are preserved. The grinding teeth are all about equal size, each having a grinding surface area of about . They each have five lophs, but the back three fuse on the lingual (tongue) side, leaving them with three lingual lophs. M3 has six lophs, with the front three fusing. The molar series of J. monesi is proportionally shorter than that of J. magna.
Incisors The incisor is long and broad. In J. magna, the only known incisor (the lower left, I1) is broken into two pieces. At the base, the edge of the internal face proceeds at a sharp right angle, and the external face at a rounded acute angle. It is triangular, but bevels at the tip, leaving a concave, almost triangular, surface at the tip and base of the incisor. In J. magna, the surface area is , measuring length x width (mesiodistal x linguolabial). In J. monesi, only the base of the right incisor (1I) is known, and the length of the two incisor sockets together is . J. monesi has a prodigious incisive foramen, corresponding to the blood vessels connected to the incisor.
The incisor of J. monesi at the level of the root had a high section modulus (a measure of an object's ability to resist bending) on account of its extreme incisor procumbency (the incisors were angled instead of pointing straight down), since moving force through a curved body (the incisor) would subject it to much higher bending stresses than if it were straight.
Skull J. monesi is the only species for which the skull has been identified. Its skull is massive, measuring in length. For comparison, in a 2007 study, a sample of 110 beef cows belonging to 9 different breeds had a maximum head length of . Its skull is 65% bigger than the skull size of the previous largest identified rodent, Phoberomys pattersoni.
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).
J. monesi probably had a constricted optic canal, which contains the optic nerve and ophthalmic artery, corresponding to vision.
Eumegamyines typically feature an unusual large cavity in the ectotympanic, which holds the eardrum in place. The pacarana has several small cavities (sinuses) in the ectotympanic, so it may be that in many eumegamyines, these cavities enlarged and combined with each other. It is unclear if these sinuses serve a function. Eumegamyines additionally typically have a well developed stylomastoid foramen, which funnels the facial nerve, and a short ear canal. Josephoartigasia, instead, lacks this ectotympanic cavity, and the ear canal is quite long, extending well into the backend of the skull. Its auditory apparatus converges more with those of chinchillids (chinchillas, viscachas, and relatives). Josephoartigasia also has a small auditory bulla, which encloses the middle and inner ear bones.
Body mass