Enamel ultrastructure of lower molars of the red squirrel Sciurus vulgaris (Rodentia, Mammalia) from different populations in Ukraine

The second lower molars of the red squirrel Sciurus vulgaris from different regions of Ukraine were examined in order to search for additional features to study the variability and clarify the subspecies status of its individual populations. Generally accepted methods were used to study the enamel ultrastructure. It is established that the tooth enamel of the red squirrel has a layered structure and is represented by different types (radial, HSB and PLEX), which are also characteristic for other rodent species. The arrangement of crystalline prisms of the radial layer in the IPM matrix is the main distinguishing feature of the morphological variability of enamel on main tooth conids. In various regional populations of the species, they act as an indicator of the adaptability of the teeth and their functionality as a single structure. This was reflected in differences in the thickness ratio of the radial layer and HSB, especially between the metaconid and the entoconid. The development of HSB is usually associated with greater functional load. Squirrel populations in the northern part of Ukraine have a relatively thick HSB layer, while those from central and southern parts of the country have a more strongly developed radial layer. According to these features, the Crimean populations are close to those from southern Ukraine and slightly differ from the Altai subspecies Sciurus vulgaris exalbidus, which was introduced into the Crimea in the first half of XX century. It is assumed that the enamel ultrastructure as a morphological character has changed in the process of adaptation of squirrels to a new environment. The distinguishing character of the enamel structure of Altai populations of the Teleut squirrel is that prisms do not fill cells of the radial layer on the hypoconid and protoconid. The tooth enamel ultrastructure of the Eurasian red squirrel cannot be an independent morphological character for their intraspecific differentiation. However, it can be successfully used in combination with other characters (e.g., fur coloration, craniometrical data), as well as the results of special molecular studies