K-strategy and adaptative specialization in Steneofiber from Montaigu-le-Blin (dept. Allier, France; Lower Miocene, MN 2a, ±23 Ma): first evidence of fossil life-history strategies in castorid rodents

The fresh-water limestone deposits of Montaigu-le-Blin yielded a small pocket containing almost exclusively cranial and post-cranial elements of the castorid Steneofiber. A full inventory of these skeletal remains makes it possible to recognize an assemblage of two adults and eight juveniles of two different ages. It corresponds well to a family group of the extant beaver, usually consisting of an adult breeding pair together with a variable number of yearlings and kits; consequently this pocket is interpreted as a fossil burrow containing the catastrophic death-assemblage of a Steneofiber family, and we infer a K-strategy model of reproductive pattern for this genus. Moreover, a minute fossil ungual phalanx corresponds well to the “combing-claw” of the extant beaver and suggests the development of nonwettable fur, an important adaptation to a semiaquatic way of life. The shape of the fossil caudal vertebrae, without the wing-like transverse processes of Castor, indicates that the tail of Steneofiber was cylindrical, however.