Josephoartigasia

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.^[2] This sloping enamel thickness and thin cementum layer are characteristic of the genus.^[2][1]

In J. magna, the P₄ has a root surface area of 4.97 cm² (0.770 sq in) 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 4.31 cm² (0.668 sq in), 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 6 cm² (0.93 sq in), and only the first two lophs fuse, leaving it with four external and five internal lophs. M₃ is unknown in this species.^[2]

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 24 cm² (3.7 sq in). 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.^[1]

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 8.29 cm² (1.285 sq in), measuring 3.07 cm × 2.7 cm (1.21 in × 1.06 in) length x width (mesiodistal x linguolabial).^[2] In J. monesi, only the base of the right incisor (¹I) is known, and the length of the two incisor sockets together is 6.73 cm (2.65 in). J. monesi has a prodigious incisive foramen, corresponding to the blood vessels connected to the incisor.^[1]

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.^[12]

Skull

J. monesi is the only species for which the skull has been identified. Its skull is massive, measuring 53 cm (1 ft 9 in) in length.^[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).^[13] Its skull is 65% bigger than the skull size of the previous largest identified rodent, Phoberomys pattersoni.^[14]

There is nearly complete fusion of several cranial bones, namely the

J. monesi probably had a constricted optic canal, which contains the optic nerve and ophthalmic artery, corresponding to vision.^[1]

Eumegamyines typically feature an unusual large cavity in the

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.[1]

In 2008, based on the relation between skull dimensions and body size among 13 specimens belonging to eight different hystricognath rodent genera, Rinderknecht and Blanco estimated a living weight of 468–2,568 kg (1,032–5,661 lb), for an average of 1,211 kg (2,670 lb). For comparison, the largest living rodent, the capybara, weighs about 60 kg (130 lb) on average. This estimate for J. monesi outweighed the then heaviest known rodent, Phoberomys pattersoni, which may have been 400–700 kg (880–1,540 lb), making J. monesi the largest known rodent. Based on this, rodents have the second largest body weight span of any mammalian order, aside from Diprotodontia (the most speciose order of marsupials).^[1]

Later that year, Canadian biologist Virginie Millien criticized Rinderknecht and Blanco's skull-to-body-mass equation, as those authors used individual specimens, not species averages to calculate mean values for each species, and only thirteen specimens, three of which were

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 1,000 kg (2,200 lb), but he was unable to reproduce as low a number as 272 kg (600 lb) 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.[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. He calculated significantly lower body masses: 254–576 kg (560–1,270 lb) for J. monesi and 108–200 kg (238–441 lb) for P. pattersoni. Assuming the paracondyles functioned the same as in paracana, he suggested 480–500 kg (1,060–1,100 lb) is the most likely range for J. monesi; and assuming rabbit-like condyles in P. pattersoni, 125–150 kg (276–331 lb). Though they are still the largest rodents ever discovered, he argued estimates exceeding 600 kg (1,300 lb) are unwarranted.^[17]

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.^[2]

Paleobiology

Bite

In 2012, Blanco, Rinderknecht, and Uruguayan paleontologist Gustavo Lecuona estimated the

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. This is because they initially guessed J. monesi could not grind up tough plants due to having weak chewing muscles, on account of its slender cheekbones, proportionally small grinding teeth, and small pterygoids (the pterygoids move the jaw side to side, important for grinding).^[1] They later recanted this after calculating an immense bite force for J. monesi. This would have allowed it to consume a wide variety of different foods, hard or soft. Its incisors, in addition to attacking predators, could have been used to dig up roots, much like how elephants use their tusks. The bite force of an elephant has never been measured, impeding more direct comparisons,^[18] but because African bush elephants are able to dislodge the opercula^[c] of marula fruit seeds, they must be able to exert at least 2,700 N (610 lb_f). 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").^[20]

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.^[21]

Paleoecology

Josephoartigasia has been identified from the San Jose Member of the Raigón Formation. The other animals discovered at this member include: the

The San Jose Member can be roughly dated to around the Pliocene–Pleistocene boundary. During this time, the Uruguayan climate evolved from a dry and semiarid one with sparse forest cover—coinciding with the onset of the Quaternary glaciation—to a climate that was warmer and more humid than today, effecting reforestation.^[8]

Notes