Fossil pharyngeal teeth of grass carp group and their implications for evolution, temporal and spatial distribution pattern, and paleoenvironment

Grass carp (Ctenopharyngodon idella) is a cyprinid species, widely living in the freshwater on the plain area of East Asia. The fish has characteristic comb-like pharyngeal teeth, an adaptation to their typical vegetarian feeding habit. This paper deals with fossil pharyngeal teeth that related to grass carp, found from about 20 localities, ranging from the lower Oligocene to Holocene. Based on the morphology of the teeth, we established two new genera and species related to the genus Ctenopharyngodon (Dezaoia saintjaquensis, Eoctenopharyngodon liui) and two new species of Ctenopharyngodon (C. xiejiaensis and C. orientalis). The first recognized C. idella pharyngeal teeth were from the lower Pliocene of Yushe, Shanxi, and many others from 11 Pleistocene to Holocene localities. Judged from the materials and their occurrences, the grass carp may have originated from the western part of China during the early Oligocene. Throughout the Miocene, the grass-carp-like pharyngeal teeth were discovered from a vast area in China, indicating an appropriate atmosphere for the grass carp to develop, although the climate in the western part of China was possibly drier than that in the eastern part. From the beginning of the Pliocene, as the consequence of a considerable uplift of the Tibetan Plateau, the large rivers in East Asia finally formed, created favorable conditions for the Recent grass carp to thrive. Most teeth were collected in association with fossil mammals. Therefore, we are using the associated fossil mammals as the checking reference against which to test our interpretation of the surrounding environment of the water system in which the grass carp lived.