Structure and composition of the incisor enamel of extant and fossil mammals with tooth pigmentation

The inclusion of iron compounds in teeth, which impart a red to orange colour to them, is a phenomenon shown by several groups of vertebrates in different periods of their evolution. Incisors from fossil and extant shrews and from extant rodents were sectioned and studied with the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to compare their structure and the distribution of Fe. The enamel in white- and red-toothed soricids has three layers; two of them are divided into two zones in the red-toothed species. However, the most external layer varies among taxa; it is well defined in Sorex but difficult to identify in the Early Pleistocene genera Beremendia or Dolinasorex. In the arvicoline rodent Terricola, only two layers can be defined, the outer of which is divided into two zones depending on the presence or absence of Fe. The Fe proportions in the larger soricids reach up to 45%, but in rodents only up to 10% (weight % with respect to Fe + Ca + P). The STEM study shows that in a fossil soricid the Fe phases form clusters of nanometric particles of very poor crystalline oxides or hydroxides surrounding the apatite crystals that form the enamel