Material and Methods
A detailed revision of the Palaeocastorinae has not been attempted in this
report because the study of much more material than was feasible would be
required. Taxonomy of this group of beavers has not been uniformly treated in
recent literature and here I follow the classification of
Martin 1987 and
McKenna and Bell (1997). Only "Capatanka" – synonymised to Palaeocastor – is retained written in quotations marks here for clarity. The following taxa have been used, particularly Capacikala gradatus (Cope 1879), Palaeocastor nebrascensis and "Capatanka" cankpeopi
McDonald (1963) with larger samples. Less material could be studied of Capacikala parvus (Xu
1996), "Capatanka" minor (Xu 1996), "Capatanka" magnus (Romer and Cormack 1928), "C". minor
Xu 1996, Palaeocastor peninsulatus (Cope 1881), Palaeocastor fossor (Peterson 1905) (assigned to Fossorcastor
Martin 1987 by
Xu (1996)), Pseudopalaeocastor barbouri (Peterson 1905) (assigned to Nannasfiber (Xu 1996) by
Material of Palaeocastor, Capacikala and "Capatanka" was studied in the collections of the South Dakota School of Mines and Technology (SDSM), Los Angeles County Museum (LACM), Museum of Palaeontology, University of California Berkeley (UCMP), American Museum of Natural History (AMNH) and Frick Collection of Mammals in the AMNH (F:AM) and the collection of the University of California in Riverside. Material from SDSM and LACM were assigned to the taxa according to
MacDonald (1963) and
Xu (1996) where possible. Data for some species (not included in all statistical analyses) were taken from
Martin (1987) and
Xu (1996). Material of Recent Castor fiber
LinnÚ 1758, sometimes referred to in comparisons, was studied in the Museum of Zoology Dresden (MTD) and Zoological collection of the University of Halle-Wittenberg, Germany.
Teeth were measured to the nearest 0.01 mm using a digital calliper at the occlusal surface and – where possible – at the base of the tooth. As nearly all of the studied teeth were still in the alevoles, the height of the teeth was not considered. The nomenclature of teeth follows
Stirton (1935). Notes were taken on the number of flexi/fossettes in the anterior and posterior part of the teeth (the paraflexus/fossette or flexid/fossetid, respectively, represents 1 anterior "fossette" and the metaflexus/fossette or metaflexid/fossetid, respectively, represents 1 posterior "fossette") and the presence of striae/striids in unworn or few worn teeth. To avoid lengthy descriptions, the term "fossette" in quotation marks is used in generalized way to include flexus, fossette, flexid and fossettetid respectively, referring to the enamel island in a certain position of the tooth irrespectively of its closure or whether an upper or lower tooth is considered. Uppercase letters represent maxillary teeth, and lower case letters represent mandibular teeth.
Beaver teeth are known to change in the pattern of enamel islands on the occlusal surface and size from the tip to the base (Stirton 1935;
Crusafont Pairo 1948;
Stefen 1997). To make interspecific comparisons, using specimens of the same age would be ideal. In fossils, determination of the individual age is generally difficult, but a scheme to determine age is desirable. Therefore, the studied teeth were grouped according to tooth wear and thus ages into four age or wear classes. The studied species all have brachydont to subhypsodont teeth so that it is assumed that similar wear stages determined on the basis of morphology of the teeth could represent similar ages, and a similar time span can be assumed to elapse between these stages. That would be more difficult if beaver species with subhypsodont and hypsodont teeth were compared. These wear stages used here to group the material are: unworn – no wear can be observed; tooth crown usually not of full height; if these teeth are in the jaw they have not reached occlusion yet; slightly worn – little wear can be observed and the chewing surface is flat showing the typical pattern of flexids; medium worn – mesoflexus/id is closing or just closed; heavily or strongly worn – hypoflexus/id is closing or closed (Stefen 2001;
Stefen and M÷rs 2008). Comparative data of other taxa from the literature without given wear stages could not be used in the statistical analyses.
The general change of beaver teeth with wear indicates strong correlation of the measured parameters with wear and age. Therefore, a method was searched to eliminate or at least minimize the influence of wear from the data and make teeth of different wear stages easily comparable. It should also help to make it easier to compare species better represented by teeth of different wear stages. Thus, linear regressions of all the length and width measurements against age (the four wear stages) were performed and the resulting unstandardized residuals were saved and used in further tests. For teeth of undetermined wear stages, no unstandardized residuals were saved. Discriminant analyses (DFAs) of different sets of taxa and unstandardized residuals were used to see how well taxa could be differentiated. The DFAs were completed using Wilk's lambda statistics, entry of all variables at once not stepwise, with equal prior probabilities of groups and covariance within groups. A Chi2 test in cross tables (including all studied taxa) testing the significance of the number of anterior or posterior "fossettes" against wear stage was performed.
All statistical analyses were performed using SPSS 13.