The Pratt Slide fossil site of Brown County, Nebraska, has yielded the largest and most diverse Clarendonian North American Land Mammal Age (late Miocene; approximately 10.5–9.5 Ma) snake fauna thus far known. The overall composition of the snake fauna consists of extinct taxa from older Miocene times, temporally isolated taxa, and modern taxa. More specifically, the fauna includes at least two extinct erycine boids, 14 colubrids of which five genera are extinct, and possibly three extant viperid genera. The fossil site yielded additional evidence (three vertebrae) of the unique late Clarendonian erycine Tregophis brevirachis and vertebrae of a new distinctive tiny natricine colubrid. Additional vertebrae of a Paleoheterodon–Heterodon snake(s) further support the suggestions by others that these two xenodontine colubrids are indistinguishable at the vertebral level. Overall, the composition of the Pratt Slide snake fauna provides supportive evidence of a North American late Miocene transition from an archaic to a modern snake fauna with most of the modernization taking place during Clarendonian to Hemphillian time (approximately 10–6 Ma).