COMPARISON OF CASTOR WITH FOSSIL BEAVERS

The size range from juvenile to fully grown individuals is about 5.7 mm for incisor length in recent Castor: this is equivalent to about 57% of the maxl of the incisor (Table 1). For the length of the upper incisor of A. suevicus, the range is 7.9 mm, which is equivalent to nearly 57% of the maxl of the tooth. This is very similar to modern Castor, and thus the observed size differences in the small sample of fossil teeth of A. suevicus can well be attributed to postnatal growth, as suggested by Stefen & Mörs (2008).

The range of the length of upper incisors differs between these two species: about 55% maxl in Castor, and about 44% in Anchitheriomys (Fig. 2, Table 1). The range in length of upper and lower incisors together is 62% maxl of the incisor in S. eseri, which is even larger than in modern Castor. The few studied specimens of the larger capybara (H. hydrochaeris) show a range in the length of upper and lower incisors of over 80% of maxl.

The size ranges of mandibular tooth rows for different fossil beavers and C. fiber are illustrated in Figure 3. For all of the species studied, a noticeable variation in tooth row length can be observed: about 11 mm (30% maxl) in C. fiber, 11.8 mm (30% maxl) in A. suevicus, and 60 mm (71% maxl) in H. hydrochaeris. For the samples of the early Miocene S. eseri and S. castorinus studied, the range of the mandibular tooth row is around 4 mm and 17% of maxl. In all species, the low size ranges were observed in m1/2 and M1/2 lengths and widths, respectively.

In recent Castor, no pattern in the form and distribution of enamel folds and islands on the chewing surface, which could be used as clear age indicators, were recognizable (Fig. 4). There are only a few features, like the closing of the para-, meso-, or metafossette, and, at the latest, hypofossette, that can be easily determined.