Variations and anomalies in rodent teeth and their importance for testing computational models of development

In rodents, the comparative anatomy of the dentition is a key criterion to determine the specific affiliation of extant and fossil specimens because development is usually well canalized. The number, arrangement and shape of teeth are usually set up during embryonic development, only to be further modified by wear or injury. Morphological variations and anomalies observed on some wild specimens can indicate modifications to standard dental development. Previous studies already showed that studying pathological specimens (‘monsters’) can be performed to better understand developmental constraints (see Alberch 1989). Here, we consider relatively mild pathologies such as modifications of dental formula or tooth shape, as well as intraspecific variations of the dentition, considering that they can also give some information on the development constraints. Dental development has long been studied (especially in mice, but also in rat, voles and guinea pigs; see, for example, Hunt and Paynter, 1963; Lester, 1969; Witter et al ., 1996) and various authors proposed computational developmental models of the whole tooth-row proportions (see Van Valen, 1962; Osborn, 1978; Kavanagh et al ., 2007) or the crown pattern (see Jernvall, 2000; Osborn, 2008). Morphological variations and anomalies challenge computational developmental models as they enlarge the range of developmentally possible morphologies. The study of variants can be considered as a test of the strength of models: a new shape should be reproduced by tuning parameters. In this chapter, we first document some cases of number and shape dental anomalies. We then discuss the possible developmental and/or evolutionary origins of these anomalies. We then present new data on intraspecific variations on molar morphology performed on different rodent species. The variations observed on the occurrence of supplementary cusps are then used to test the robustness of development hypotheses concerning patterning of dental cusps, which are the basis of computational developmental models. Occurrences of dental anomalies in rodents Anomalies in number of teeth Many cases of mammals with supernumerary or missing teeth (that failed to develop) have been reported in the literature, including rodent cases (presented in Table 15.1).