Over 11,000 skeletal elements of snakes from a series of fossil deposits ranging from the Lower Pliocene to the Recent from Kansas and adjacent areas were studied. Each local fossil fauna is relatively distinct, containing as few as one and as many as 15 species of snakes. Glacial and interglacial faunas differ in kind, number, and size of snakes present. Many, but not all, of the Pliocene and Interglacial snake species occur in Kansas today. The greatest overlap of the ranges of all of the species of snakes represented in the fossil deposits occurs today in southwestern Texas, in the Balconian Biotic Province. In contrast to the mammal fauna and other herpetofaunas, the snake faunas from the high plains have no extinct forms in the Pleistocene deposits (though extinction or change in two forms occurred in the Mid and Upper Pliocene) and further indicate that most genera and species of Recent snakes go back, almost unchanged, to at least the Upper, and in some cases Lower Pliocene.
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Bayard H. Brattstrom (1967) studied this question.