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The saiga antelope was one of important components of the Pleistocene steppe-tundra faunal complex. Although the species is critically endangered now and inhabits several isolated regions in Central Asia, it was widely spread in the Pleistocene in Europe, across Asia up to North America. Such a broad spatial and temporal distribution caused that many authors described several morphological forms. Most often, two forms are recognized and assigned to the species rank as Saiga borealis and S. tatarica, or in the subspecies rank as Saiga tatarica borealis and S. t. tatarica. The former became extinct at the beginning of the Holocene, and the latter has survived to the present. To comprehend the morphological diversity of this genus, we conducted extensive morphometric analyses of the cranial material of saiga covering the whole region of its distribution. The study showed that S. borealis was larger in several length skull dimensions and occipital region than S. tatarica, which in turn showed larger diameters of the horncore base. Moreover, we found a significant decrease in many skull measurements of saiga since the Middle Pleistocene till modern times, which was probably associated with the appearance of unfavourable climatic and ecological conditions. The observed significant distinctions between the Pleistocene and recent forms as well as between some geographical subgroups indicate that the saiga population was subjected to temporal and spatial differentiation, the former factor being more important for variation of the saiga skull than the latter. [ABSTRACT FROM AUTHOR]