ON SMALL CONTINENTAL MAMMOTHS AND DWARFISM

Abstract

In the evolution of proboscideans, the appearance of dwarf and semi-dwarf forms occurred repeatedly, in different territories and at different times, due to a lack of resources caused by geographic isolation on islands and by landscape isolation on the mainland. Despite a significant amount of information on the insular forms of mammoths and elephants, the question of the relationship between a decrease in body size and morphological changes in the dental system remains a matter of debate. Some data show that dwarfism was accompanied by a decreasing number of plates (lophs) and by tooth enamel thickening. Other data show that changes in the dental system on the islands indicate that the number of plates could either decrease or remain unchanged, or even increase. Taking into account the importance of the number of plates as a diagnostic feature in the species identification of proboscidean taxa and the lack of a consensus on the stability of the trend towards a decrease in the number of plates from large continental ancestors to insular dwarf descendants, we (1) summarize the data on the records of small tooth-mammoths of the genus Mammuthus with a reduced number of plates in sites across northern Eurasia, (2) provide new radiocarbon dates, and (3) consider the number of plates as a possible sign of dwarfization in continental mammoths. The small teeth of the last generation of M. primigenius from the coastal part of northeastern Siberia and a comparison with data from other regions show that the posterior sections of mammoth cheek teeth are the most variable and represent a reduction complex. For the woolly mammoth, reduction primarily affects that part of the crown which became more complex by the gradually increasing number of plates during the phyletic evolution in the Middle to Late Pleistocene. The rapid loss of the evolutionary achievements through reduction is not a unique feature of woolly mammoth teeth. This has been observed in other mammals as well. Similar reduction complexes are observed for the cheek teeth of proboscideans and rodents of the subfamily Arvicolinae, as is shown by a decrease in the number of serially homologous crown elements: plates in proboscideans and pairs of prisms in voles. A comparison of the number of plates with the size of the teeth of M. primigenius suggests that a decrease in the number of dental plates, while retaining other specific features of the tooth, can be used as a criterion for separating semi-dwarf and small specimens of the woolly mammoth. The largest decrease in size of mainland mammoths is observed during warming periods, which also showed significant transformations of landscapes in a reduction of available resources.