104 Thomas D. Beaver, George A. Feldhamer, Joseph A. Chapman 



molar, and 7 others involved both upper and lower premolars in which 

 thinning was associated with the posterior premolar in the tooth row. 

 Smith et al. (1977) also found alveolar thinning associated "almost 

 exclusively" with the last upper molars in three species of platyrrhine 

 monkeys. They attributed the condition to internal pressures associated 

 with mastication. Such thinning may predispose underlaying tissues to 

 periodontal disease, and both local and systemic factors have been 

 implicated in this process (Clark et al. 1970). 



Congenital agenesis occurred in 8 (3.9%) of the skulls. Three addi- 

 tional skulls appeared to possess this condition, however X-ray roent- 

 genograms revealed a non-erupted tooth. Agenesis was bilateral in two 

 individuals; one case involved the second lower premolars and the other 

 the second upper premolars (Fig. 6). The other six cases, all unilateral, 

 involved two different locations in the maxillary tooth row: five 

 instances occurred at the first premolar and one at the second premolar. 



Agenesis can result from delayed tooth formation and eruption, or 

 a genetically induced reduction associated with phylogenetic shortening 

 of the tooth row (LaVelle and Moore 1972). Delayed eruption was 

 noted in three instances; however, this was not the case for the nine 

 individuals in which no unerupted tooth or alveolus was present. In all 

 cases congenital agenesis involved the premolars. Hall (1940:1 18) stated, 

 "when a premolar is missing the place most often is at the anterior end 

 of the premolar series." This was the situation in only 5 of 9 individuals 

 (55.6%) in our sample; the remainder involved the second premolar. 



Supernumerary teeth were found in 8 (3.9%) of our specimens. 

 Seven cases occurred near the first upper premolar; one was posterior to 

 the last upper molar. X-rays revealed distinct alveoli for three of the 

 supernumerary premolars and the molar. In the other four cases, two 

 teeth appeared to protrude from the same alveolus, although this was 

 difficult to verify. Previous explanations for supernumerary dentition 

 include: 1) splitting of a permanent or milk tooth bud; 2) failure to 

 shed deciduous teeth; 3) atavism; and 4) "arising of tooth as a new crea- 

 ture (genetic conditioning possible)" (Pavlinov 1975:516). We regard the 

 latter as the least probable. Splitting of the tooth bud is a reasonable 

 explanation in the four examples of two teeth, virtually identical in size, 

 protruding from the same alveolus (Fig. 7). 



To attribute supernumerary upper premolars in the river otter to 

 atavism is unreasonable. The resulting five premolars exceeds the primi- 

 tive number of four — already present in the normal upper premolar 

 complement of the genus. The supernumerary premolars with distinct 

 alveoli, therefore, are more likely due to failure of the deciduous teeth 

 to shed. However, atavism is a likely explanation regarding the super- 

 numerary molar. Because teeth tend to be lost at the end of a row (Hall 



