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Description

A selection of extinct giant caviomorph rodents known as dinomyids and neoepiblemids, to scale.

Rodents are among the most successful groups of organisms, equating to 40% of all living mammals. Characteristically they possess a single pair of evergrowing incisors in both jaws, and are generally small in size. Living species range between the 3 gram pygmy jerboa (Tiddlers - the smallest vertebrates ) up to capybara at 66 kgs, with most weighing less than 100 grams. However the fossil record shows that rodents were capable of much bigger sizes (see Damn it - Castorids , Prickly customers - Porcupines ), with the largest being the massive Josephoartigasia at nearly 500 kgs. It was part of a lineage of super-sized rodents, split across two South American families of caviomorphs, the dinomyids and neoepiblemids. Only one living species (a dinomyid) still exists - the pacarana Dinomys branickii, which reaches a modest 15 kg. It was considered a caviod (cavies, capybara, agouti), however more recently pacarana have been shown to be an unusual chinchilloid, a group which includes the familiar chinchilla and viscacha. Dinomyids and neoepiblemids are regarded as separate families, however the relationship between the two is problematic. One suggestion is that neoepiblemids are a sister lineage to the true chinchillids, with the dinomyids being a more basal group to both. Another small extinct group, the cephalomids, may be even closer linked to the neoepiblemids although they also show some similarities to the caviids. The extinct heptaxodontid giant hutias from the Caribbean islands (Extinct Island Fauna - Caribbean 1 ) were traditionally thought to be large versions of the living hutias (octodontoids), but appear to an additional lineage of large chinchilloids. Phylogenetic issues are due to poorly defined unique features within the extinct families (fragmentary remains), and thus there is little clear morphological similarities which unite all species.

In general they all have heavily reinforced skulls (Josephoartigasia has a skull roughly the same as a large domestic cow!). The musculature and reinforced bones gave these rodents very high bite forces, equivalent to large cats and crocodilians. Several suggestions have been put forward for this: digging with teeth, feeding on hard-shelled nuts, and offense against predators. The related Telicomys had unusually wide incisors, very flat occipital (rear of skull) and deep masseteric fossae (muscle attachment adaptation for strong bite forces), while a fossil skull from Neoepiblema shows that these rodents were not particularly brainy, but possessed a good sense of smell.

Both groups date from the Early Miocene, with early forms probably resembling the much smaller potamarchines (Potamarchus, Ferigolomys). While dinomyids still survive with the pacarana, neoepiblemids didn't make it beyond the Miocene.

Missing the reference in the title? R.O.U.S. stands for 'rodents of unusual size' (see The Princess Bride book/movie).

Description

A selection of extinct giant caviomorph rodents known as dinomyids and neoepiblemids, to scale.

Rodents are among the most successful groups of organisms, equating to 40% of all living mammals. Characteristically they possess a single pair of evergrowing incisors in both jaws, and are generally small in size. Living species range between the 3 gram pygmy jerboa (Tiddlers - the smallest vertebrates ) up to capybara at 66 kgs, with most weighing less than 100 grams. However the fossil record shows that rodents were capable of much bigger sizes (see Damn it - Castorids , Prickly customers - Porcupines ), with the largest being the massive Josephoartigasia at nearly 500 kgs. It was part of a lineage of super-sized rodents, split across two South American families of caviomorphs, the dinomyids and neoepiblemids. Only one living species (a dinomyid) still exists - the pacarana Dinomys branickii, which reaches a modest 15 kg. It was considered a caviod (cavies, capybara, agouti), however more recently pacarana have been shown to be an unusual chinchilloid, a group which includes the familiar chinchilla and viscacha. Dinomyids and neoepiblemids are regarded as separate families, however the relationship between the two is problematic. One suggestion is that neoepiblemids are a sister lineage to the true chinchillids, with the dinomyids being a more basal group to both. Another small extinct group, the cephalomids, may be even closer linked to the neoepiblemids although they also show some similarities to the caviids. The extinct heptaxodontid giant hutias from the Caribbean islands (Extinct Island Fauna - Caribbean 1 ) were traditionally thought to be large versions of the living hutias (octodontoids), but appear to an additional lineage of large chinchilloids. Phylogenetic issues are due to poorly defined unique features within the extinct families (fragmentary remains), and thus there is little clear morphological similarities which unite all species.

In general they all have heavily reinforced skulls (Josephoartigasia has a skull roughly the same as a large domestic cow!). The musculature and reinforced bones gave these rodents very high bite forces, equivalent to large cats and crocodilians. Several suggestions have been put forward for this: digging with teeth, feeding on hard-shelled nuts, and offense against predators. The related Telicomys had unusually wide incisors, very flat occipital (rear of skull) and deep masseteric fossae (muscle attachment adaptation for strong bite forces), while a fossil skull from Neoepiblema shows that these rodents were not particularly brainy, but possessed a good sense of smell.

Both groups date from the Early Miocene, with early forms probably resembling the much smaller potamarchines (Potamarchus, Ferigolomys). While dinomyids still survive with the pacarana, neoepiblemids didn't make it beyond the Miocene.

Missing the reference in the title? R.O.U.S. stands for 'rodents of unusual size' (see The Princess Bride book/movie).