Small Continental Mammoths and the Phenomenon of Dwarfism

In the evolution of proboscideans, the emergence of dwarf and semi-dwarf forms occurred repeatedly, on various territories and at various times, due to a lack of resources caused by geographic isolation on the islands and landscape isolation on the mainland. Despite a significant amount of information on the island forms of mammoths and elephants, the question of the association between a decrease in the body size and morphological changes in the dental system remains controversial. According to some data, the formation of dwarf forms was accompanied by a unidirectional decrease in the number of dental plates (NDP) of the tooth and thickening of the enamel. According to other data, changes in the dental system on the islands were mosaic in nature; that is, the NDP could decrease, remain unchanged, or even increase. Taking into account the importance of the NDP as a diagnostic tool for species identification of proboscidean taxa and the absence of a consensus on the stability of a trend towards a decrease in the NDP from tall continental ancestors to island dwarf descendants, we (1) summarize data on the finds of small teeth of mammoths from the Mammuthus genus with a reduced NDP in the localities in northern Eurasia, (2) present new radiocarbon dates, and (3) consider the NDP as a characteristic of developing dwarfism in continental mammoths. The study of small teeth of the last generation of M. primigenius from the coastal part of northeastern Siberia and a comparison with data from other regions demonstrated that the posterior sections of cheek teeth in mammoths are the most variable and represent a reduction complex. For woolly mammoth, the reduction primarily affects that part of the crown that became more complex by the gradual addition of the plates during phyletic evolution in the Middle–Late Pleistocene. A rapid loss of evolutionary achievements of the ancestral forms due to reduction is not a unique peculiarity of the woolly mammoth teeth, and was also traced in other mammals. The similarity of the reduction complexes of cheek teeth in proboscideans and rodents of the Arvicolinae subfamily was demonstrated; it can be traced by a decrease in the number of serially homologous elements of the crown: plates in proboscideans and pairs of prisms in arvicolines. A comparison of the NDP with the size of the M. primigenius teeth allowed us to make the assumption that a decrease in the NDP while maintaining other species-specific peculiarities of the tooth can be used as a criterion for separating semi-dwarf and small individuals of the woolly mammoth. The size of continental mammoths decreased most intensively during warmer intervals, when significant transformation of landscapes and a reduction in resource space occurred.