Giant Rodents of South America Roedores gigantes de América del Sur カピバラの巨大な祖先水豚巨祖先

Imagine a capybara the size of a huge, male buffalo!

The largest rodent discovered so far is the massive Josephoartigasia Monesi, who was about the size of a bull buffalo (and much bigger than a 2-year-old Hereford bull – a Hereford bull only weighs about 700 kg!) and weighed about 1000 kg, with a very powerful bite force. Isostylomys laurillardi is thought to have been nearly as large.  Arazamys castiglionii weighed about 800 – 900 kg, smaller than Josephoartigasia Monesi and Isostylomys, but larger than Phoberomys; they were almost the size of a buffalo.

For reference: Pleistocene: 2.58 million years ago – 11,700 years ago, ended with the Ice Age. Pliocene: 5.333 million years ago – 2.58 million years ago. Miocene: 23.03 million years ago – 5.333 million years ago. Oligocene: 33.9 million years ago – 23.03 million years ago. Tertiary: 65 million years ago – 2.588 million years ago. Quaternary 2.6 million years ago into the present.

Phoberomys compared to human size and capybara

Size comparison between human, capybara and the genus Phoberomys. Phoberomys only weighed about 700 kg. Josephoartigasia Monesi was much larger and weighed 1000 kg

This is an overview of some of the many species of giant rodent who roamed South America during the Miocene, Pliocene and Pleistocene epochs, which lasted from 23 million years ago until the start of the last Ice Age, 11,700 years ago. Most of these giant rodents lived between 9 – 2 million years ago.

Palaeontologists believe there may have been over 50 species of giant rodent, but when the fossil evidence consists of only one or two fragments of mandible or skull (the bone most likely to survive over millions of years), or a tooth, it can be challenging to identify a species! Teeth survive for thousands, even millions, of years because of the strength of the enamel covering the teeth which is 97% mineral making teeth stronger than bones. Teeth and fragments of skull or mandible, are often the only fossil evidence that palaeontologists have when trying to identify different species of these long extinct giant rodents. Thus the different morphology and size of the teeth, and differing features of fragmentary pieces of skull or mandible, are crucial clues when identifying long extinct species.

Rodents appear in the fossil record in South America at least 31.5 million years ago, during the very early Oligocene.  The earliest caviomorph fossils have been found in Peru and dated to the late Oligocene. Capybaras are caviomorph rodents; the Caviomorpha clade is a subgroup of hystricognath (see paragraph below) rodents. South America was separated from other landmasses during most of the Tertiary, so evidence is lacking as to how rodents reached South America. Hystricognaths originated in Asia, but the South American rodents probably arrived from Africa by raft. The ancestors of most of today’s South American rodents, including capybaras, probably appeared between the middle and late Miocene. These giant rodent lineages in South America went extinct over 1 million years ago, some millions of years ago, with only the capybara alive today as the largest extant rodent. Hydrochoerus hydrochaeris, the most numerous and well-known of the two species of modern-day capybara, weighs on average about 50 to 60 kg, and is about the size of a large dog.

The Hystricognath rodents are an Infraorder of rodents distinguished from other rodents by the bone structure of their skulls. There are 18 families within the Hystricognathi, divided into 2 Parvorders, the Caviomorpha and the Phiomorpha. Capybaras are representatives of the Family Caviidae in the Parvorder Caviomorpha. Caviomorpha are mostly found in South America, with a few species in North America and the Caribbean. The Phiomorpha are found in the Old World.

A Clade is a group believed to include all the evolutionary descendants of a common ancestor.

Rodents are the most abundant group of living mammals. They also vary enormously in size. The smallest, about the size of a pygmy mouse, weighs only a few grams, while the largest, the extinct giant rodent Josephoartigasia Monesi weighed about 1000 kg. Rodents are also one of the most diverse mammals in South America, with over 160 species in total, either living today or extinct species. Today’s rodents include arboreal (tree dwellers), gliding rodents, fossorial (adapted to digging and living primarily but not exclusively underground), cursorial (adapted specifically to run; i.e. can run fast or maintain a constant speed over a long distance) and semi aquatic rodents. During the Tertiary rodents’ range of body size and physical characteristics was wider than today.

Some scientists have suggested these giant rodents may have gone extinct because they were not fast enough to outrun predators, and too large to dig burrows to hide in. Climate change may also have contributed to their demise.

Other extinct giant rodents include: the genus Eumegamys who weighed about 800 – 900 kg, and was a similar size to a hippopotamus. Phoberomys burmeisteri, believed to be the largest of this genus. Phoberomys Pattersoni thought to have weighed about 700 kg, and Phoberomys insolita (although Phoberomys insolita may have been a sub adult Phoberomys pattersoni) thought to have weighed about 400 kg. These predate Josephoartigasia Monesi and lived during the late Miocene in modern day Venezuela. Telicomys gigantissimus, the size of a small rhinoceros, about 2 meters long and with a weight of about 500 kg. and about 70% of the size of Phoberomys pattersoni.  Neoepiblema Acreensis, weighed about 80 kg. (See below for more about these giant rodents)

J Monesi to

Artist’s impression of Josephoartigasia Monesi, who was the size of a massive bull buffalo and weighed about 1000 kg

In 1987, the almost complete skull of Josephoartigasia Monesi was discovered in modern day Uruguay on the coast of the Rio De La Plata, in the San José formation. Based on the strata of the San José formation in which the J Monesi fossil was found, J Monesi lived 4 – 2 million years ago, during the Pliocene and early Pleistocene.

J Monesi is believed to have lived in a lowland forested, wetland habitat such as a delta or estuary. By identifying the diet of an extinct species (see below) scientists are able to speculate on the type of habitat the species lived in based on the vegetation it foraged. J Monesi shared its habitat with giant ground sloths, and predators such as enormous flightless “terror birds” sparassodonts, short faced bears and sabre toothed tigers.

Another feature of J Monesi that is especially interesting are its teeth. J Monesi had enormous, incredibly powerful, chisel -shaped incisors, an adaptation which allowed these incisors to be successfully used in hierarchy battles between males to ensure access to females and breeding rights, and in defence against predators. J Monesi may also have used their enormous incisors for digging in the same way that an elephant uses its tusks. These incisors were able to resist much greater forces than the molars, powered by the masticatory muscles, could generate. The molars are believed to have been used to masticate tough vegetation with forceful bites. Josephoartigasia Monesi’s diet probably consisted of grasses, aquatic plants and the bark of trees and bushes. As large mammals they would have been able to utilise these coarse, low quality food resources which much smaller species would not have been able to digest.

Representatives of the Dinomyidae family frequently had incisors which were able to resist much greater forces than their molars.

Scientific Classification: Josephoartigasia Monesi    Kingdom: Animalia Phylum: Chordata Class Mammalia Order: Rodentia Suborder: Hystricomorpha Infraorder: Hystricognath Family: Dinomyidae Genus: Josephoartigasia Species: Josephoartigasia Monesi

It can be challenging to make an accurate identification of different extinct rodent species as palaeontologists are often working from scant fossil remains, consisting of only a few fragments of mandible or skull (the bone most likely to survive) or a tooth. Some fossils which were thought to represent different species have been reassessed as representing juveniles, sub adults and adults of the same species. Some of these mega rodents had a unique, and complicated tooth morphology which changed as they aged (as is the case with capybaras). This change accounts for some of the erroneous designations of fossils, which were initially thought to represent different species’, but are now considered to be representatives of the same species, but of different ages. With other species the size of the teeth did not grow or change as the animal developed from being a juvenile to an adult. This means that the juvenile and the adult had the same sized teeth and were not different species.

Likewise, it can be difficult to gauge the size of the animal, and make size comparisons between different extinct species, unless the identical parts of the respective bodies are found. For example, the body size for Phoberomys pattersoni is based on parts of the fore limb and hind limb, and these were compared with this species closest living relative, the Pacarana. In the case of Josephoartigasia Monesi an almost complete skull was found. Although J Monesi is known only from this 1.7 foot (just over half a meter) long skull, this provided enough fossil evidence to indicate that this was a new species and the largest rodent who ever lived. To arrive at a body mass for J Monesi this skull was compared with several other living rodent species.

There are two methods scientists use to identify the diet of prehistoric species. Where fossil evidence is available scientists can investigate the preserved contents of the gut and faeces, both inside the body (cololites) and outside the body (coprolites). Scientists also study the chemical isotope in the fossil bones and teeth of these long extinct species, and compare this information with the carbon isotopes of different varieties of plants, to identify the animal’s diet. The isotopic signature of the food eaten by the animal is incorporated into the fossil bones and teeth, and remain stable over tens of thousands, even millions, of years. Teeth are often the only fossil remains of an extinct species which palaeontologists find, as teeth, being harder due to their enamel cover, survive better than bones. Palaeontologists also used the condition of Josephoartigasia Monesi’s teeth, the degree of wear and worn edges, pits and scratches, to assess the type of diet this species relied on.

The almost complete skull of J Monesi provided new information about the anatomy of extinct giant rodents of the Dinomyidae family who are primarily known from scant fossil evidence. The mandible is the largest and strongest facial bone, and forms the lower jaw, acting as a receptacle for the lower teeth. Together with the temporal bone, the mandible articulates on either side forming the temporomandibular joint. Scientists compared the mandible of J Monesi to that of its closest living relative, the Pacarana, to help them rebuild J Monesi skull. To simulate how J Monesi would bite at different locations along the jaw, scientists used a 3-D scan of this Pacarana mandible. J Monesi had an extremely powerful bite, estimated to be 3 times more powerful than a tiger’s bite, and comparable to modern day large crocodilians. The maximum bite force of its incisors has been estimated at close to 500 kg. The molars could only exert a maximum force of about 150 kg (about 300 lbs); thus the incisors were able to exert a much more powerful force.  J Monesi’s incisors were extremely strong and could resist greater forces than the masticatory muscles of the molars could generate, leading scientists to speculate that the primary role of these incisors was use in hierarchical combat, or defence against predators. The molars were primarily used to chew tough vegetation.

Palaeontologists are also able to estimate the bite force and how the stress of chewing might have affected the skull, by using an engineering technique, Finite Element Analysis. The maximum bite force of Josephoartigasia Monesi has been estimated at a maximum of 4165N, which is 3 times as powerful as a tiger.

Josephoartigasia Monesi’s closest living relative is the Pacarana, a critically endangered species at high risk of extinction, and the last surviving member of “their” family, the Dinomyidae. The Pacarana, Dinomys branickii, is a rare hystricognath rodent found only in small areas in South America. They are the second largest rodent species alive today; second only to the capybara, with a body length of about 80 cm (31 inches) and weighing about 15 kg (32 lbs). Pacaranas have a large head and a thick, furry tail. The name comes from the Tupi word meaning “false paca”, named because they look superficially similar to the paca. Due to the Pacarana’s low population levels, scientists had believed the animals were extinct. Like many species around the world today, they are vulnerable to human predation and habitat loss caused by human activities.

The Dinomyidae are a family of hystricognath rodents native to South America. Several extinct members of this family include the largest rodents who have ever lived, including the largest so far discovered, Josephoartigasia Monesi. The Dinomyidae are believed to have been large grazing mammals, the largest representatives of this family disappeared after the formation of the Isthmus of Panama about 3 million years ago, connecting North and South America. There were once many species in this group but now only the Pacarana survives.

The earliest fossils of the Dinomyidae family date from the middle Miocene, and fossils of this family have been found almost throughout South America. The clade reached its highest diversity (i.e. the greatest number of species of this family) in the late Miocene. The family is subdivided into 4 or 5 subfamilies: Potamarchinae (which includes the oldest known representatives of the family, possibly from the middle Miocene to the early Pleistocene). Gyriabrinae (late Miocene to late Pliocene). Dinomyinae (including the only living representative, Dinomyinae branickii – Pacarana). Eumegamyinae (late Miocene to the late Pliocene, and includes some of the largest members of the family). Tetrastylinae (late Miocene to late Pliocene. May be a subgroup of Eumegamyinae or Dinomyinae). It is not possible to determine the anatomy of these animals as there is insufficient fossil evidence: just a few fragments of mandible or skull, or a tooth.

Isostylomys laurillardi.

Artist’s impression of Isostylomys laurillardi, who was almost as large as Josephoartigasia Monesi

Isostylomys laurdillardi, is one of the largest species of giant rodent who lived during the Miocene epoch, 9 – 6.8 million years ago. This species is believed to have weighed almost 1000 kg.

In 2016 scientists found an almost complete and intact skull and part of the jaw of one adult, and the complete lower jaw, with all its teeth intact, and the right heel of a juvenile, of this species. The fossils were found in southern Uruguay in the Camacho Formation of the Rio de la Plata coastal region. Uruguay’s Camacho Formation was laid down during the late Miocene epoch, 12 million years ago – 5 million years ago.

The fossils were in exceptional condition, the best preserved fossils found so far for this species; previous finds amounted to only fragments of skull and the odd tooth. This allowed the scientists to compare tooth development between the adult and juvenile of this species. This led to a new understanding of the other species in this genus, which had previously only been studied from fragmentary fossil evidence.

These fossils, which represent an adult and juvenile, raise questions regarding the classification within their genus, Isostylomys, suggesting that fossils thought to indicate different, related species may in fact represent a single species, due to the way teeth are formed as the rodent ages.

The scientists discovered that the adult tooth shape emerged quite early in the rodent’s development, growing larger as the animal matured. By evaluating earlier fossils, considered to be the tooth forms for prenatal, juvenile and adult, they learned that adult tooth forms could vary in size. This led to an understanding that fossils which had been thought to represent related species, three species of Isostylomys, were in fact just one species of Isostylomys, representing different age groups, rather than different species.

As a result of this analysis, which showed that from a very young age the giant rodents were very similar to the adults, scientists have been able to learn how the world’s largest fossil rodents grow.

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Suborder: Hystricomorpha
Infraorder: Hystricognathi
Superfamily: Chinchilloidea
Family: †Dinomyidae
Genus: †Isostylomys
Species:Isostylomys laurillardi

Arazamys castiglionii weighed about 800 – 900 kg, smaller than Josephoartigasia Monesi and Isostylomys, but larger than Phoberomys. They were almost the size of a buffalo. This species lived in the late Miocene. An incomplete skull of an adult was found in the Camacho Formation, in the San Jose Department, Uruguay. As with many other members of the Dinomyidae family, the molars are small relative to the large, powerful incisors and the estimated size of the skull.


Artist’s impression of Phoberomys pattersonii

Phoberomys pattersoni was a giant caviomorph rodent who lived during the late Miocene from about 9.0 – 6.8 million years ago. They lived in South America, in modern day Brazil, Argentina, Venezuela and Peru. Sedimentary evidence indicates they lived in coastal wetland habitats consisting of lagoons of shallow water separated from the coast by barriers of sand.

An almost complete fossil skeleton of Phoberomys pattersoni was discovered in 2000, in the Urumaco formation of the Orinoco River delta in Venezuela. The skeleton measured 3 meters (9.8 feet) with a 1.5 meter long tail (4.9 feet). This species was estimated to have weighed about 700 kg (1540 lbs). These size and weight estimates are reasonably accurate as almost an entire skeleton was discovered.

In appearance they may have looked more like a guinea pig than a rat. However, the humerus (which is part of the forelimb) gave a smaller estimate of body size than the femur (which is part of the hind limb), 436 kg as against 741 kg. This suggests that the hind limbs played a more important role in locomotion and weight support. The forelimbs may have been used to manipulate food as is the case with the Pacarama (Dinomyidae branickii) the only living member of this family.

The genus Phoberomys were herbivores, and like many rodent species they had high crowned premolars and molars indicating they were grazing animals i.e. grass eaters. The morphology of its teeth indicates that its diet was abrasive: coarse grasses and probably included aquatic grasses. Like the capybara, Phoberomys are believed to have been semi aquatic. Like Josephoartigasia Monesi, Phoberomys is a member of the family Diomyidae, whose only living relative, the Pacarana, is in danger of extinction.

Predators would have included large, very powerful birds like Brontornis and Kelenken, giant crocodilians. Sabre toothed marsupials would have preyed on the young.

As mentioned previously, reclassification sometimes occurs following further research, due to the challenges of identifying a species based on scant fossil remains, possibly only a few bones or teeth.

As an indication of this: the genus Phoberomys was thought, at one time, to consist of seven species, including: Phoberomys burmeisteri, Phoberomys praecursor, Phoberomys insolita, Phoberomys lozanoi and Phoberomys minima. However, a more recent study of Phoberomys fossils found in the Entre Rios province in Argentina, and dated to the late Miocene epoch, concluded that these were in fact just one species: Phoberomys burmeisteri. The study concluded that the differences among the fossils reflect different ages and stages of development of a single species. This gives an idea of just how difficult it is to accurately identify species from a very sparse fossil record.

Recent research also suggests that Phoberomys pattersoni and Phoberomys insolita may in fact be the same species. Phoberomys insolita had been estimated to be a little larger.

Scientific Classification Phoberomys pattersoni:    Kingdom: Animalia   Phylum: Chordata   Class: Mammalia   Order: Rodentia   Suborder: Histricomorpha   Family: Dinomyidae;   Genus: Phoberomys   Species: Phoberomys pattersoni

Phoberomys burmeisteri is believed to be the largest member of the Phoberomys genus who lived in the late Miocene epoch. The remains of this species was found in the Ituzaingó formation in Entre Rios province in Argentina.

Eumegamys is an extinct genus of giant rodent, about the size of a hippopotamus, of the family Dinomyidae. They lived during the late Miocene and Pliocene. Their fossil remains have been found in the Solimoes Formation in modern day Brazil, the Urumaco Formation in Venezuela, and the Ituzaingó formation in Argentina. It’s skull was about half a meter long (1.65 ft).

Telicomys gigantissimus lived in South America during the late Miocene and early Pliocene epochs (11.2 million – 5.3 million years ago). They were about the size of a small rhinoceros, about 2 meters long, and are thought to have weighed about 500 kg. They also were a member of the family Dinomyidae related to the Pacarana.

Eumegamys paranensis is an extinct species of giant rodent of the family Dinomyidae, who lived during the late Miocene and Pliocene in modern day Brazil, Venezuela and Argentina. It’s skull was 50 cm long. Evaluation of its cheek teeth, and the complexity of the crown, indicate that its diet consisted of coarse vegetation and demanding food items, and it was able to process more food each time it chewed (i.e. each masticatory cycle). Eumegamys paranensis fed on a varied diet, foraging close to the ground. It was probably a wide-ranging species, living near water and in gallery forests (narrow bands of forested area lining rivers) as would have been found in the Mesopotamic area of what is now the Paraná River system in north-east Argentina. This genus also lived in Brazil and Argentina.

Neoepiblema Acreensis is an extinct giant rodent, a relative of chinchillas, who weighed about 80 kg and lived about 10 million years ago in present-day Brazil in South America.

Using computerised tomography to look inside fossil skulls, scientists estimate that the brain of Neoepiblema Acreensis weighed just 47 g, meaning this rodent’s brain was unusually small compared to its body size. Two Neoepiblema Acreensis fossil skulls were studied, which gave an encephalisation quotient of 0.20 for one and 0.33 for the other. South American rodents alive today have an average encephalisation quotient above 1.05.

To compare the brain sizes of different animals of varying weights, scientists calculate a species’ “encephalisation quotient”, which measures the difference between the expected brain size based on body size, and the actual brain size for an animal of a given weight. Any value under 1 means that an animal’s brain is smaller than expected. The ratio between the size of the brain and the size of the body is thought to indicate intellectual ability.

This has led scientists to speculate that Neoepiblema Acreensis was not the brightest rodent. If this was indeed the case, the reason may have been that there were few predators to outwit so a larger brain was not worth the “cost” of maintaining a larger brain.