Discussion and Taxonomy

Even though Martin (1987) noted that dental characters are probably not the best tool for understanding beaver taxonomy, they are considered in particular here in this study, because beaver teeth or fragmentary mandibular and maxillary material with few teeth are generally found more often than complete skulls or fragmented ones with many taxonomically relevant features preserved.

The flattened incisors have been noted as characteristic for fossorial beavers by Stirton (1935). Euhapsis and Palaeocastor clearly show flat incisors, but Capacikala has semi-flat ones, and species assigned to "Capatanka" vary between flat and semi-flat (SDSM 53421, LACM 17692). Even the semiaquatic Recent Castor is somewhat difficult to classify; lower incisors are more flat-faced than upper ones which are semi-flat, and there are some changes with ontogeny.

The observed para- and metastriae/iids or rather notches are very few. The presence of striae/iids has been used as a characteristic feature for beavers by Stirton (1935). However, in his diagnosis for Palaeocastor he did not include other striae/iids besides hypostria/striid and mesostria/striid, and later authors (Martin 1987; Xu 1996) did not comment on the presence of striae/striids. As they only appear in very early stages of wear, in most material it is difficult to comment on. For “Capatanka”, which is mainly represented by worn teeth, it is unclear whether short striae/ids or notches other than the hypostriae/iids and mesostriae/iids occur. The lack of extra notches is comparable to Miocene beavers where Steneofiber eseri and S. castorinus show few very short striae in early stages of wear, and the later S. deptereti shows longer striae (Stefen 1997).

Comparisons in size of the teeth assigned to Capacikala, "Capatanka" and Palaeocastor respectively show that those of "Capatanka" cankpeopi are generally larger and those of Capacikala smaller. However, a detailed interpretation of length x width data of teeth is complicated by the different sized samples and wear stages present. The length x width data of m1, 2 (Figure 6.3) indicate that probably not all teeth assigned to Capacikala gradatus belong to this species as some little and medium worn teeth overlap in small size. But a clear- cut differentiation in two groups is also difficult as some little-medium worn teeth overlap with both possible size groups. For upper P4 Capacikala parvus, generally distinguished by smaller size (Xu 1996), falls well within the size range of C. gradatus. The size differences are only slightly clearer when teeth of the different wear stages are considered. “Capatanka” minor overlaps in size with Capacikala gradatus and, to some degree, with C. parvus. Whether this size difference alone can be considered a taxonomic distinction is uncertain because M1/2 vary markedly in size in Capacikala gradatus, which completely overlap of measurements with teeth assigned to C. gradatus and “Capatanka” minor.

The larger samples of Capacikala gradatus and Palaeocastor sp. show a variability in size comparable to other beavers of different radiations like Steneofiber eseri (Stefen 1997). Unfortunately, however, not all taxa could be compared with similar numbers of teeth in the same wear stages. Differentiation between different species of Palaeocastor is nearly impossible on the basis of tooth morphometry alone.

The DFAs indicate: Pseudopalaeocastor barbouri, "Capatanka" cankpeopi and Capacikala gradatus or rather Palaeocastor nebrascensis and "Capatanka" cankpeopi using maxillary or mandibular teeth separate well, but the other studied species are difficult to differentiate.

Comparison of the lengths of lower and upper dentitions of the studied taxa do not give a clear picture mainly due to small and different sample sizes. For lower dentitions the samples of "Capatanka" cankpeopi, Capacikala gradatus and Palaeocastor nebrascensis and Palaeocastor sp. seem to give a fairly realistic picture of the natural variation compared to the also subphysodont Steneofiber eseri. The larger size range of Recent Castor fiber is certainly due to its larger size and hypsodonty, and therefore not a proxi for the fossil palaeocastorine beavers. The few data for Pseudopalaeocastor barbouri are at the lower size range, even smaller than "Capatanka" minor mainly differentiated by its smaller size. Overall the available data on tooth row length do not contribute much to the better understanding of the studied taxa. Not much further clarity can be found concerning the taxa, but that "Capatanka" minor is similar to Capacikala gradatus, several taxa assigned to Palaeocastor overlap and "Capatanka" cankpeopi is the largest.

The few available data on skull width and length of palaeocastorine beavers (Figure 13.1) probably do not represent the natural range of variation in size. Only for Capacikala gradatus some variation is indicated. Data for Recent Castor indicate a possible range of variation (Figure 13.2), however, certainly larger than for the smaller palaeocastorine beavers. Capacikala gradatus, C. parvus, Palaeocastor nebrascensis and Pseudopalaeocastor barbouri are very close. "Capatanka" cankpeopi, Palaeocastor fossor and Euhapis platyceps are very close, but larger than the other taxa. All these taxa have a nearly square skull only slightly longer than broad.

McKenna and Bell (1997) included "Capatanka" in Palaeocastor. The morphometrics of teeth do not contribute well to the systematic status on genus level, but the slight differences on their own would not necessitate a generic differentiation of "Capatanka", Capacikala and Palaeocastor.

Judging from these discussed data alone it seems likely that C. cankpeopi and C. magnus belong to the same species, "Capatanka" minor is more likely to belong to Capacikala with slightly smaller and relatively longer skulls. Only Capacikala parvus seems clearly smaller than other taxa. The differentiation between Capacikala parvus, Capacikala gradatus and "Capatanka" minor should be reviewed as well as the species assignments in Palaeocastor. Judging from the data considered herein it seems that P. peninsulatus is smaller than P. nebrascensis. Material assigned to Palaeocastor sp. includes three size forms, and P. fossor needs to be clearly separated.