Castoridae

Belongs within: Castorimorpha.

The Castoridae include the beavers, large Holarctic rodents. Among the modern fauna, beavers comprise only two surviving species known for their practice of building complex semi-aquatic nests and dams. These are the North American beaver Castor canadensis and the European beaver C. fiber, with the former having a shorter skull than the latter. Their vernacular names reflect their native ranges though the North American beaver has also been introduced to some parts of Europe.

Fossil species of beaver were more diverse, including burrowing as well as semi-aquatic forms. The most basal subgroup is the latest Eocene to late Miocene subfamily Agnotocastorinae, characterised by a relatively long rostrum, parallel upper tooth rows, and procumbent incisors with a smooth enamel surface.

Beaver fever

Published 7 March 2012

Beavers are one of those animals that are familiar even to people who do not live in parts of the world where you can find beavers. The two living species of beaver are semi-aquatic rodents with one species each native to Eurasia (Castor fiber) and North America (C. canadensis) (though the North American beaver has been introduced to several parts of Europe). Differences between the two are slight: the Eurasian beaver is generally larger (up to 35 kg) and has a somewhat longer skull and a less rounded tail. Beavers are best known, of course, for their construction of elaborate subaquatic nests and dams. Dams are generally about fifteen to seventy metres across, but have been recorded over 600 metres across (Rybczynski 2008). Beavers may also dig burrows connected to their dams, and construct canals over one hundred metres long (Rybczynski 2008).

Phylogenetically, beavers are somewhat remote from other rodents, and represent the last survivors of a once more diverse lineage. First known from the late Eocene, the members of the beaver family Castoridae are divided in the most recent treatments between five subfamilies (Korth 2001, 2004). The plesiomorphic subfamilies Agnotocastorinae and Anchitheriomyinae are not well known, and the Agnotocastorinae in particular may be non-monophyletic (Rybczynski 2007). The remaining subfamilies fall into two distinct lineages: one containing the Palaeocastorinae (Oligocene-Miocene), the other the Castoridinae (Oligocene-Pleistocene) and Castorinae (Oligocene-present). Of these two lineages, only the latter are known to have been semiaquatic: the Palaeocastorinae are strictly terrestrial.

The palaeocastorines, a strictly North American lineage, were specialised burrowers. Their incisors, which have rounded faces in modern beavers, became flattened and adapted for digging. Their burrows were distinctive helicoidal structures, described as trace fossils under the name of Daimonelix (‘devil’s spiral’), that could reach over 2.5 metres in depth and twenty centimetres in diameter. These burrows were constructed in ‘towns’ with multiple burrows in close proximity. Though each burrow was independent, without connections between adjacent burrows, such close positioning suggests that palaeocastorines may have had a well-developed social structure (Hugueney & Escuillié 1996). However, though the palaeocastorines were much more diverse at their apogee than the castoroidine-castorine lineage, they became extinct after a relatively short period.

The Castorinae and Castoroidinae may never have achieved the diversity at any one point in time of the palaeocastorines, they were more successful over the long haul: the more diverse of the two subfamilies, the Castoroidinae, only became extinct fairly recently. Castoroidines are commonly referred to as the ‘giant beavers’, and while not all castoroidines were giant (many, if not most, were smaller than modern beavers), the largest of them certainly were: the North American Castoroides reached an estimated size of about 100 kg, and would have been as large as a small bear. Whether the giant Castoroides produced similarly gigantic dams, however, is uncertain. Evidence of wood-chopping behaviour like that known for modern beavers (in the form of preserved wood bearing identifiable tooth marks, in association with beaver remains) is only well supported for one fossil species, the castoroidine Dipoides (suggested evidence for wood-chopping in Castoroides is more equivocal) (Rybczynski 2008). Phylogenetic bracketing between Dipoides and modern beavers would suggest that wood-chopping arose at the base of the castoroidine-castorine clade; alternatively, the absence of direct evidence of such behaviour may suggest convergence between these two species. Also, Dipoides was a less efficient wood-cutter than modern Castor, cutting with the rounded edges of its incisors while Castor uses the flattened ends, and if it used chopped wood to construct nests then they would have probably been correspondingly more simple (beavers also use chopped wood for food, eating the leaves and bark, so wood-chopping does not automatically indicate dam-building). There are other indications that fossil beavers may not have been as specialised aquatically as the modern species: the early castorine Steneofiber, for instance, did not possess the flattened tail of Castor (a flattened tail has been indicated for Castoroides but Castor and Castoroides probably developed such tails independently) (Hugueney & Escuillié 1996).

Perhaps the primary enigma among fossil beavers is the European Pleistocene Trogontherium. Although also referred to as a ‘giant beaver’, and often implied to be a European parallel to Castoroides, Trogontherium was a quite different animal. Fostowicz-Frelik (2008) argued that leg proportions and other features indicate that Trogontherium was a more terrestrial, cursorial animal than other beavers (in particular, its narrowed rather than flattened toe bones suggest that it lacked the webbed feet of modern beavers). The phylogenetic analysis of beavers by Rybczynski (2007) placed Trogontherium as closely related to Castoroides, but certain plesiomorphies in its tooth morphology lead Rybczynski to suggest that this position was probably an artifact of convergences due to large size, and that Trogontherium should perhaps be in a much more basal position.

Systematics of Castoridae

<==Castoridae
    |  i. s.: ProcastoroidesG88
    |--AgnotocastorinaeK04
    |    |--Neatocastor hesperusK04
    |    |--HystricopsK04
    |    `--Agnotocastor Stirton 1935SM93
    |         |--A. aubekeroviX96
    |         |--A. coloradensis Wilson 1949X96
    |         |--A. galushai Emry 1972X96
    |         `--A. praetereadens Stirton 1935X96
    `--+--Anchitheriomyinae [Anchitheriomyini]K04
       |    |--+--Miotheriomys Korth 2004K04
       |    |  |    `--*M. stenodon Korth 2004K04
       |    |  `--OligotheriomysK04
       |    |       |--O. magnusK04
       |    |       `--O. primusK04
       |    `--+--Propalaeocastor kumbulakensisK04, X96
       |       `--Anchitheriomys Roger 1898K04
       |            |--A. fluminisK04
       |            |--A. nanus Korth 2004K04
       |            |--A. senrudiK04
       |            `--A. stoutiK04
       `--+--+--Steneofiber Geoffroy 1833R08, SM93
          |  |    |--S. pansus Cope 1874C77
          |  |    `--S. viciacensisC77
          |  `--Castor Linnaeus 1758L58 [CastorinaeK04]
          |       |--C. accessorG88
          |       |--C. canadensisFS15
          |       |    |--C. c. canadensisB75
          |       |    `--C. c. missouriensis Bailey 1919B75
          |       |--C. fiber Linnaeus 1758L58
          |       |    |--C. f. fiberUSDI77
          |       |    `--C. f. birulaiUSDI77
          |       `--C. moschatus Linnaeus 1758L58
          |--CastoroidinaeK04
          |    |  i. s.: EucastorK04
          |    |         Dipoides intermediusTH03
          |    |--TrogontheriiniK04
          |    |    |--Trogontherium Fischer von Waldheim 1809P04
          |    |    `--Euroxenomys Samson & Radulesco 1973K04
          |    |         `--E. wilsoni Korth 2001K04
          |    `--CastoroidiniK04
          |         |--Castoroides ohioensisFS15
          |         `--Monosaulax Stirton 1935K04
          |              |--*M. pansusK04
          |              |--M. baileyi Korth 2004K04
          |              `--M. skinneriK04
          `--Palaeocastorinae [Capacikalini]X96
               |  i. s.: Pseudopalaeocastor Martin 1987X96
               |           `--P. barbouri (Peterson 1905) (see below for synonymy)X96
               |--+--Palaeocastor Leidy 1869X96
               |  |    |--P. nebrascensis (Leidy 1856) (see below for synonymy)X96
               |  |    `--P. peninsulatus (Cope 1881) [=Steneofiber peninsulatus]X96
               |  `--Capacikala Macdonald 1963X96
               |       |--C. gradatus (Cope 1879) [=Steneofiber gradatus]X96
               |       `--C. parvus Xu 1996X96
               `--+--+--Nannasfiber Xu 1996X96
                  |  |    |--*N. ostellatus Xu 1996X96
                  |  |    `--N. osmagnus Xu 1996X96
                  |  `--Fossorcastor Martin 1987X96
                  |       |--F. brachyceps Martin 1987 [=Capatanka brachyceps]X96
                  |       |--F. fossor (Peterson 1905) (see below for synonymy)X96
                  |       `--F. greeni Martin 1987X96
                  `--+--Capatanka Macdonald 1963X96
                     |    |--C. cankpeopi Macdonald 1963X96
                     |    `--C. minor Xu 1996X96
                     `--Euhapsis Peterson 1905X96
                          |--E. breugerorum Maritn 1987X96
                          |--E. ellicottae Martin 1987X96
                          |--E. gaulodon Matthew 1907X96
                          |--E. luskensis Xu 1996X96
                          `--E. platyceps Peterson 1905X96

Fossorcastor fossor (Peterson 1905) [=Steneofiber fossor, Palaeocastor fossor; incl. P. (Capatanka) magnus Martin 1987, P. major Romer & McCormack 1928, Steneofiber simplicidens Matthew 1907, P. simplicidens]^X96

Palaeocastor nebrascensis (Leidy 1856) [=Steneofiber nebrascensis; incl. Capacikala sciuroides Macdonald 1963]^X96

Pseudopalaeocastor barbouri (Peterson 1905) [=Steneofiber barbouri, Palaeocastor barbouri; incl. Pa. milleri Olson 1940]^X96

*Type species of generic name indicated

References

[B75] Bowles, J. B. 1975. Distribution and biogeography of mammals of Iowa. Special Publications, The Museum, Texas Tech University 9: 1–184.

[C77] Cope, E. D. 1877. Report upon the extinct Vertebrata obtained in New Mexico by parties of the expedition of 1874. Geographical Surveys West of the One Hundredth Meridian 4 (2): i–iv, 1–370.

[FS15] Faurby, S., & J.-C. Svenning. 2015. A species-level phylogeny of all extant and late Quaternary extinct mammals using a novel heuristic-hierarchical Bayesian approach. Molecular Phylogenetics and Evolution 84: 14–26.

Fostowicz-Frelik, Ł. 2008. First record of Trogontherium cuvieri (Mammalia, Rodentia) from the middle Pleistocene of Poland and review of the species. Geodiversitas 30 (4): 765–778.

[G88] Gray, J. 1988. Evolution of the freshwater ecosystem: the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 62: 1–214.

Hugueney, M., & F. Escuillié. 1996. Fossil evidence for the origin of behavioral strategies in early Miocene Castoridae, and their role in the evolution of the family. Paleobiology 22 (4): 507–513.

Korth, W. W. 2001. Comments on the systematics and classification of the beavers (Rodentia, Castoridae). Journal of Mammalian Evolution 8 (4): 279–296.

[K04] Korth, W. W. 2004. Beavers (Rodentia, Castoridae) from the Runningwater Formation (Early Miocene, Early Hemingfordian) of western Nebraska. Annals of Carnegie Museum 73 (2): 1–11.

[L58] Linnaeus, C. 1758. Systema Naturae per Regna Tria Naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Laurentii Salvii: Holmiae.

[P04] Popov, V. V. 2004. Pliocene small mammals (Mammalia, Lipotyphla, Chiroptera, Lagomorpha, Rodentia) from Muselievo (north Bulgaria). Geodiversitas 26 (3): 403–491.

Rybczynski, N. 2007. Castorid phylogenetics: implications for the evolution of swimming and tree-exploitation in beavers. Journal of Mammalian Evolution 14: 1–35.

[R08] Rybczynski, N. 2008. Woodcutting behavior in beavers (Castoridae, Rodentia): estimating ecological performance in a modern and a fossil taxon. Paleobiology 34 (3): 389–402.

[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.

[TH03] Tedford, R. H., & C. R. Harington. 2003. An Arctic mammal fauna from the Early Pliocene of North America. Nature 425: 388–390.

[USDI77] United States Department of the Interior. 1977. Endangered and threatened wildlife and plants—republication of list of species. Federal Register 42: 36420–36431.

[X96] Xu, X. 1996. Castoridae. In: Prothero, D. R., & R. J. Emry (eds) The Terrestrial Eocene–Oligocene Transition in North America pp. 417–432. Cambridge University Press.