"Asiaceratops" and "Turanoceratops" are each considered "nomina dubia" and not included. Along with Dong Zhiming, You described "Magnirostris" in 2003, but to date has not included it in any of his cladograms. Ceratopsia appears to have originated in Asia, as all of the earliest members are found there. Fragmentary remains, including teeth, which appear to be neoceratopsian, are found in North America from the Albian stage (112 to 100 million years ago), indicating that the group had dispersed across what is now the Bering Strait by the middle of the Cretaceous Period. Almost all leptoceratopsids are North American, aside from "Udanoceratops", which may represent a separate dispersal event, back into Asia. Ceratopsids and their immediate ancestors, such as "Zuniceratops", were unknown outside of western North America, and were presumed endemic to that continent. The traditional view that ceratopsoids originated in North America was called into question by the 2009 discovery of better specimens of the dubious Asian form "Turanoceratops", which confirmed it as a ceratopsid. It is unknown whether this indicates ceratopsids actually originated in Asia, or if the "Turanoceratops" immigrated from North America. Unlike almost all other dinosaur groups, skulls are the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls. There is a great deal of variation between and even within ceratopsian species. Complete growth series from embryo to adult are known for "Psittacosaurus" and "Protoceratops", allowing the study of ontogenetic variation in these species. Significant sexual dimorphism has been noted in "Protoceratops" and several ceratopsids. "Psittacosaurus" and "Protoceratops" are the most common dinosaurs in the different Mongolian sediments where they are found. "Triceratops" fossils are far and away the most common dinosaur remains found in the latest Cretaceous rocks in the western United States, making up as much as 5/6ths of the large dinosaur fauna in some areas. These facts indicate that some ceratopsians were the dominant herbivores in their environments. Some species of ceratopsians, especially "Centrosaurus" and its relatives, appear to have been gregarious, living in herds.