The finds of Miocene beavers are not rare at all. Nevertheless, in most localities these rodents are only represented by a few dental elements, so that large collections are only occasionally available to researchers. Hence, the description of sufficiently large samples, such as the recently published description of Steneofiber depereti from Hambach 6C (NW Germany, MN5) by Mörs and Stefen (2010), deserves the greatest attention. After a comprehensive description and comparison of the material, these authors conclude that previous citations of the beaver Chalicomys jaegeri from the MN4 to MN6 (early to middle Miocene) are misidentifications of the genus Steneofiber. Following Stefen (2009), these authors further question the validity of all the known Chalicomys species from the later middle Miocene (MN7+8). We agree with these authors that the occurrence of C. jaegeri in the middle Miocene is doubtful, but we disagree with their arguments for the synonymization of several species. Here we address this point and discuss the taxonomic criteria for distinguishing both genera, further reviewing the taxonomy of the genus Chalicomys at the species level.
On the distinction between Steneofiber and Chalicomys
Almost every author that has studied sufficiently complete material of Steneofiber or Chalicomys has proposed an emended diagnosis for these genera (e.g., Crusafont Pairó et al. 1948; Aldana Carrasco 1992; Hugueney 1999; Casanovas-Vilar et al. 2008; Stefen 2009). Some of the older diagnoses refer simultaneously to Steneofiber and Chalicomys (= Palaeomys), which would largely explain the incorrect assignment of some MN4 to MN6 material to Chalicomys jaegeri. Both genera are no longer considered synonymous, and recent diagnoses have emphasized the higher hypsodonty, more abundant cement and longer striae/iids in Chalicomys as compared to Steneofiber (Hugueney 1999; Casanovas-Vilar et al. 2008; Stefen 2009). In the most recent emended diagnosis Stefen (2009) considered the length of the p4 hypostriids as the main diagnostic character. According to this author, in Steneofiber the hypostriid closes before reaching the base of the crown, while in Chalicomys it remains open. Stefen (2009), however, disregards other diagnostic features, such as the more hypsodont cheek teeth in Chalicomys, or the abundant presence of cement infilling the synclines/ids. Cement may be present in Steneofiber depereti as a thin layer on the hypostria/id and/or the mesostria/id (as in IPB-HaH 6375; Mörs and Stefen 2010: fig. 6C). Nevertheless, in Chalicomys, even in the earliest species such as Chalicomys catalaunicus and Chalicomys batalleri, cement occurs abundantly in all the synclines/ids (Casanovas-Vilar et al. 2008). Stefen (2009) reports two premolars from Eppelsheim, the type locality of C. jaegeri, that are devoid of cement and on this basis questions the taxonomic validity of this character. This, however, merely seems an exception to the general rule for this species, with the presence of abundant cement apparently being a synapomorphy of a Chalicomys + Castor clade. Casanovas-Vilar et al. (2008) further mentioned the presence of a tetralophodont pattern in the cheek teeth of Chalicomys, whereas Steneofiber would retain a pentalophodont one. As pointed out by Stefen (2009), this feature disappears with moderate wear, so that in many instances it is not useful for distinguishing both genera. Nevertheless, when discernible, it clearly separates the older species of Steneofiber from those of Chalicomys, although younger species such as S. depereti already display the derived tetralophodont pattern (even though an additional anterior fosssete/id may be present in some molars). To sum up, the diagnosis of Chalicomys by Stefen (2009), which is the one followed by Mörs and Stefen (2010), ignores some important diagnostic characters. Accordingly, the synonymies suggested by these authors for certain Miocene Chalicomys species must be reconsidered.
Institutional abbreviations.—IPB-HaH, Steinmann Institut (Paläontologie), Rheinische Friedrich-Wilhems Universität, Bonn, Germany; IPS, Institut Català de Paleontologia, Bellaterra (Cerdanyola del Vallès), Barcelona, Spain.
Species of Chalicomys
According to Stefen (2009) and Mörs and Stefen (2010), the genus Chalicomys would only include the type species, C. jaegeri, and the questionable C. plassi. Their criteria therefore differ from that of Casanovas-Vilar et al. (2008), who distinguished three additional species within this genus. In particular, Mörs and Stefen (2010) reassigned C. catalaunicus to ?Eucastor (Schreuderia), transferred C. subpyrenaicus to Steneofiber, and synonymized C. batalleri with S. depereti. In the following lines we will review the species included within the genus Chalicomys. See Table 1 for a list of the species discussed and their synonymies.
Hugueney (1999) placed C. catalaunicus within the poorly known subgenus Schreuderia, which she considered a likely subgenus of the North American Eucastor. Schreuderia had been previously erected as a genus by Aldana Carrasco (1992) to accommodate S. adroveri, which strongly resembles Chalicomys but shows a markedly reduced M3. Hugueney (1999) noted that some molars of Schreuderia adroveri and Chalicomys catalaunicus show a tendency to display an S-pattern, leading her to place them into the same subgenus, which she considered to be related to castoroidines. This was later disputed by Korth (2001), who considered Schreuderia to be a castorine, probably even a subgenus of Chalicomys. Certainly, a few molars of C. catalaunicus display a tendency towards an S-pattern at particular wear stages, but generally the occlusal pattern is castorine-like. Moreover, the cheek teeth are hypsodont and the striae/iids are longer than in Steneofiber depereti, further displaying abundant cement in all the synclines/ids (Fig. 1; see also Crusafont Pairó et al. 1948: pls. 5–8). These features strongly support the inclusion of this taxon in Chalicomys as a distinct and rather small-sized species, which partially overlaps with S. depereti but tends to be somewhat larger (Fig. 2). A revision of the type species is clearly needed in order to assess the validity of the (sub)genus Schreuderia.
Mörs and Stefen (2010) considered Chalicomys batalleri a junior subjective synonym of S. depereti because they overlap in dental size and the p4 hypostriid in C. batalleri supposedly does not reach the crown base. However, the only known p4 of the latter species is still implanted in the mandible (Casanovas-Vilar et al. 2008: figs. 2, 3) and the hypostriid continues into the alveolus, so that it is not possible to known whether it closed near the base of the tooth. Size is neither a distinguishing criterion, because C. batalleri overlaps with both S. depereti and Chalicomys jaegeri (Fig. 2). On the contrary, C. batalleri differs from S. depereti by numerous features, namely: the occurrence of abundant cement in all the fossettes/ids (as in C. jaegeri); the more well-developed hypostria and mesostria in the upper cheek teeth; and the presence of three moderately developed striae on the lingual side of the P4, while only a rather short mesostria is present in S. depereti (cf. Mörs and Stefen 2010: fig. 5W; Casanovas-Vilar et al. 2008: fig. 4E, K). Both Stefen (2009) and Mörs and Stefen (2010) further question the taxonomic value of enamel crenulations, which were considered to be diagnostic of C. batalleri. The latter authors argued that the holotype (a right hemimandible) belongs to a juvenile individual, because of the presence of crenulations. Nevertheless, the holotype shows the complete definitive dentition and the cheek teeth already display a moderate degree of wear, indicating that it belongs to an adult. Histological sections of extant beaver molars have shown that crenulations do in fact occur in old specimens at advanced wear stages (Hünerman 1966; Kotsakis 1989), indicating that this feature cannot be solely indicative of a juvenile ontogenetic stage. Given all the reasons outlined above, we consider that C. batalleri is a valid species that should be retained in Chalicomys.
Concerning Chalicomys subpyrenaicus, Mörs and Stefen (2010) do not question the validity of this species, even though available specimens overlap in size with S. depereti. Nevertheless, given the fact that the hypostriids close above the crown base and that only mesostriids are well expressed on the lingual side, these authors transfer this species to Steneofiber. In spite of the correctness of the assertions above, it must be emphasized that, in C. subpyrenaicus, cement is present in all the fossettes/ids. Accordingly, we consider that it should be retained in Chalicomys.
Finally, C. plassi is known by only two teeth (the holotype, which is a p4, and a m3) from Dorn-Dürkheim (MN11, SW Germany). This species stands out because of its small size as compared to the other known Chalicomys and Steneofiber species (Fig. 2). Mörs and Stefen (2010) did not mention this material, but Stefen (2009) considered that it could be probably assigned to Chalicomys, because the hypostriid extends until the crown base. Rekovets et al. (2009) consider the specimens of Dorn-Dürkheim to be juveniles of Chalicomys jaegeri (= Palaeomys castoroides in Rekovets et al. 2009) and in accordance synonymise Chalicomys plassi with this species. Nevertheless, these authors do not discuss this issue further. Interestingly, the p4 of C. plassi overlaps in size with the dp4 of C. jaegeri, which is also present in Dorn-Dürkheim (Fig. 2). Franzen and Storch (1975) argued that this specimen could be clearly distinguished from the dp4 of C. jaegeri by the presence of three well-defined roots. Nevertheless, the dp4 of C. jaegeri does indeed show three roots exactly in the same position (see Van de Weerd 1976: pls. 15, 4a). Accordingly, we consider that this tooth is most likely a dp4 of C. jaegeri, and that C. plassi is a junior subjective synonym of the latter. Regarding the supposed m3 of C. plassi, it corresponds in size (Fig. 2) and morphology to the m3 of Euroxenomys minutus rhenanus comb. nov. from Dorn-Dürkheim. Furthermore, as in E. minutus rhenanus, the “m3” of C. plassi is devoid of cement. We therefore agree with Rekovets et al. (2009) and conclude that C. plassi is not a valid species, resulting from unfortunate misidentification of the cheek teeth of other beaver species present at Dorn-Dürkheim.
Species included within the genus Chalicomys Kaup, 1832. For each species an abridged synonymy list is provided.
The genera Steneofiber and Chalicomys are not easy to distinguish from one another. The type species C. jaegeri is clearly different from Steneofiber, but in the case of earlier Chalicomys species the distinction is not so clear. Hence, a differential generic diagnosis that only considers the characters present in C. jaegeri, such as that by Stefen (2009), inevitably results in a reassignment of the remaining species to Steneofiber. Here we propose to distinguish both genera on the basis of the occurrence of abundant cement in the fossettes/ids, which is a derived character shared between Chalicomys and the extant Castor.
We sincerely acknowledge the comments by the reviewers of this paper Marguerite Hugueney (Université Claude-Bernard, Lyon, France), Gudrun Daxner-Höck (Naturhistorisches Museum Wien, Austria), and Jérôme Prieto (Ludwig-Maximilians Universität München, Germany) which surely improved the final result. This study has been possible thanks to the support of the Generalitat de Catalunya (Grup de Recerca Consolidat 2009 SGR 754 of the AGAUR) and the Spanish Ministerio de Ciencia e Innovation (CGL2010-21672/BTE, CGL2008-00325/BTE, and RYC-2009-04533 to DMA).