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PART I. INTRODUCTION History The razorback sucker, Xyrauchen texanus (Abbott), an endemic fish of the Colorado River basin of the American West (Figure 1), is a member of the sucker family (Catostomidae: Catostominae). Catostomids, a diverse and successful group of freshwater fishes, are abundant in North America (Moyle and Cech 1996). Catostomids occur in northeastern Siberia, Alaska, and from northern Canada to Mexico (Nelson 1994). The origins of North American suckers remains somewhat obscure due to an incomplete fossil record. Oldest known North American catostomid specimens come from Paleocene formations (ca. 53-65 mya; Lavender 1986). However, the Catostomidae may have occurred earlier in northern latitudes where fossil records may have been destroyed or covered by glaciers. The razorback sucker arose from an ancient form that diverged very early from the main line of catostomid evolution (Miller 1958). Xyrauchen presumably originated by Pliocene and one "well-preserved" Pleistocene specimen from Salton Sea, California has been identified as Xyrauchen texanus (Miller 1958, Minckley et al. 1986). The razorback sucker is placed in the monotypic genus Xyrauchen, one of four genera in the tribe Catostomini (Nelson 1994). The species was originally described in 1860 as Catostomus texanus by Abbott (1861), who mistakenly believed that the stuffed specimen came from the Colorado River of Texas. The fish also was described as C. cypho by Lockington (1881). Eigenmann and Kirsch (in Kirsch 1889) recognized the unusual features of this species and assigned it to a new genus Xyrauchen (literally, "razornape"; Jordan and Evermann 1896). LaRivers (1962) redescribed the species and provided a complete synonymy. He also corrected the identity of the type locality, which was the "Colorado and New Rivers" in Arizona. The New River is tributary to the Gila River in Central Arizona. The razorback sucker was known to humans in prehistoric times and used by them as food. Common names, including "tsa'xnap" and "suxyex" were given to the fish by American Indians (reviewed by Minckley et al. 1991). Other common names include "humpback sucker", "buffalo" and "buffalofish" (Jordan and Evermann 1896; Sigler and Miller 1963; Minckley 1973). Presently, the American Fisheries Society recognizes the common name of razorback sucker (Robins et al. 1991).