| Summary: | Histologic studies have been utilized in paleontology to determine rates of growth for several dinosaur taxa, but members of the otherwise relatively well-known family Ceratopsidae have largely been excluded. A monodominant bonebed containing remains of the centrosaurine ceratopsid Einiosaurus procurvicornis Sampson 1995, ranging from juveniles to putative adults, has been found in the Upper Cretaceous Two Medicine Formation of northwestern Montana. This bonebed represents a drought-induced assemblage, and is hypothesized to represent a single population. The full available range of sizes of tibiae (N = 16) were sectioned and bone histology examined. Tissue types and degrees of remodeling are discussed, and growth lines are used to determine ages at time of death of the individuals. The rate of growth for Einiosaurus peaks at about 3-5 years of age, at which time growth slows, suggesting that this may be the age that reproductive maturity is reached. The nature of the bone tissue suggests that growth in Einiosaurus is still relatively rapid in even the largest specimens, indicating that a fully adult tibia has not been recovered from the studied bonebed, and this bonebed is biased toward juveniles and subadults. Since the bonebed is a snapshot of a standing herd, population dynamics of Einiosaurus, such as survivorship and behavior, are assessed. This information on growth dynamics and life histories of a species has implications for future taxonomic resolution and morphometric studies of ceratopsid dinosaurs, and marks the first study on population histology of a large-bodied herbivorous dinosaur.
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