Belongs within: Rodentia.
Includes: Castoridae, Heteromyidae, Geomyidae.
The Castorimorpha are a clade of rodents supported by molecular data including the modern Castoridae (beavers), Heteromyidae (pocket mice) and Geomyidae (gophers).
Hanging around with beavers
Published 13 January 2026
Even by the standards of mammalian systematics, studies on rodent evolution have tended to focus closely on the skull. This should not be surprising. Rodents are a group of mammals that have become very successful largely on a diet of tough plant foods, and a number of major innovations in rodent evolution have involved developing ways to better process this challenging diet. In the very first rodents, the masseter muscle that controls chewing was attached to the zygomatic arch (ie. the ‘cheekbones’). This is the arrangement in most mammals: feel along your own cheekbones while chewing and you’ll feel your own masseter muscles pumping away. However, the amount of force that this arrangement produces is limited, and it was not long before rodents began to evolve ways of increasing their masticatory powers. This was done by moving the muscle attachments forward, anchoring them on the rostrum rather than on the cheeks. Where this gets interesting is that different rodent lineages moved the masseter muscle in different ways. In squirrels and beavers, the muscle passes underneath the zygomatic arch and is attached directly to the rostrum, bypassing the inside of the arch entirely. This condition is known as sciuromorphy (literally ‘squirrel-form’) and results in a concentration of force towards the front of the jaws, making the animal a powerful gnawer. Other rodents have at least some of the masseter muscle passing through the front of the zygomatic arch via the infraorbital foramen, and consequently are better suited for grinding food between the back teeth.
Historically, squirrels and beavers have been considered close relatives because of their shared sciuromorphy. However, inconsistencies in other features and the more recent input of molecular data have established that this is not the case (Swanson et al. 2019). Instead, beavers belong to an independent clade from squirrels known as the Castorimorpha. Sciuromorphy evolved more than once. As well as the two living species of beaver, the castorimorphs include over a hundred species of smaller rodents found in the Americas. These rodents are divided between two families, the Geomyidae and the Heteromyidae. The Geomyidae are the gophers, about forty species of robust, burrowing rodents with high cheek teeth and powerful limbs. In contrast, the Heteromyidae, pocket mice and kangaroo rats, are small, lightly built rodents with slender limbs and long tails. The pocket mice of the subfamilies Heteromyinae and Perognathinae are similar in overall appearance to true mice, though all heteromyids show at least some differentiation in length between fore and hind legs (Korth 1994). This is taken to its greatest extent in the kangaroo rats of the subfamily Dipodomyinae which, as the vernacular name suggests, are adapted for locomotion via jumping.
North America seems to have always been the centre of diversity for the castorimorphs, for which a number of fossil families are known in addition to the modern taxa (Korth 1994). As well as the beavers, still represented in the native European fauna by a single species, some of these fossil families also diversified across the Holarctic. The Eutypomyidae were large rodents related to beavers; though mostly North American, there seems to be some disagreement whether certain Asiatic taxa were eutypomyids or beavers. The remaining families are typically associated with the clade formed by the geomyids and heteromyids. These include the Heliscomyidae, some of the smallest rodents to ever inhabit North America, which had remarkably short cheek teeth and presumably fed on softer fare than their relatives. The Eomyidae were a diverse family across the Holarctic and survived in Europe until the end of Pliocene, though they went extinct earlier elsewhere. Eomyids are typically compared to modern squirrels or dormice and appear to have been diverse in habits. Two species preserved with soft body parts from the Enspel deposit of Oligocene Germany demonstrate that one was a terrestrial generalist, whereas the other was a glider with cartilage from the elbows supporting a patagium (Kimura et al. 2020).
Systematics of Castorimorpha
<==CastorimorphaMJ11 |--CastoroideaMJ11 | |--CastoridaeMJ11 | `--EutypomyidaeMJ11 | |--Mattimys Korth 1984SM93 | |--Microeutypomys tillieiS96 | |--JanimusS96 | | |--J. dawsonaeS96 | | |--J. mirusS96 | | `--J. rhinophilusP96 | `--Eutypomys Matthew 1905SM93 | |--E. acaresS96 | |--E. magnus Wood 1937S96 | |--E. obliquidensS96 | `--E. parvus Lambe 1908S96 `--Geomyoidea [Geomorpha, Saccomyina, Saccomyoidea]R06 |--HeteromyidaeHH83 |--GeomyidaeMJ11 |--Florentiamyidae [Florentiamyinae]R06 | |--FlorentiamysMHL03 | |--Ecclesimus Korth 1989SM93 | `--Harrymys Munthe 1988SM93 |--HeliscomyidaeR06 | |--PassaliscomysMJ11 | `--Heliscomys Cope 1873SM93 | |--H. hatcheri Wood 1935S96 | |--H. mcgrewiTPG96 | |--H. ostranderi Korth et al. 1991S96 | `--H. woodiTS96 `--EomyidaeHH83 |--Omegodus Pomel 1853SM93 |--Estramomys Janossy 1969SM93 |--Meteomys Kretzoi 1952SM93 |--Aulolithomys bounites Black 1965S96 |--Cupressimus barbaraeS96 |--MeliakrouniomysS96 |--CentimanomysS96 |--Zemiodontomys burkeiTPG96 |--Montanamus bjorkiTPG96 |--Metadjidaumo hendryiTPG96 |--MetanoiamysR06 |--SymplokeomysR06 |--Yoderimys Wood 1955SM93 | `--Y. stewarti (Russell 1972)S96 |--GriphomysW96 | |--G. alecer Wilson 1940W96 | `--G. toltecus Lillegraven 1977W96 |--ParadjidaumoS96 | |--P. hansonorum (Russell 1972)S96 | `--P. trilophusTPG96 |--Protadjidaumo Burke 1934SM93 | |--P. altilophusS96 | |--P. pauliS96 | `--P. typusP96 |--AdjidaumoS96 | |--A. craigiS96 | |--A. maximusS96 | `--A. minimus (Cope 1873)S96 |--Namatomys Black 1965SM93 | |--N. fantasmusW96 | |--N. fugitivusS96 | |--N. lacusS96 | `--N. lloydi Black 1965S96 `--Eomys Schlosser 1884D07 |--E. abnatensisD07 |--E. burkeiD07 |--E. craikyD07 |--E. intermediusD07 |--E. majorD07 |--E. maximusD07 |--E. minimusD07 |--E. minutesD07 |--E. orbicularisD07 |--E. orientalisD07 |--E. quercyiD07 `--E. zitteliD07
*Type species of generic name indicated
References
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