Growth and population age structure in the horned dinosaur chasmosaurus

A partial skeleton of the ceratopsian Chasmosaurus mariscalensis was collected by Wann Langston Jr. for the University of Oklahoma in 1938. Although fragmentary, this previously undescribed specimen has parts of most of the major skeletal elements and allows for the first time an accurate reconstruction of the body proportions in this horned dinosaur, as well as a detailed reassessment of the population structure in the “herd” sample at the type locality. Material referable to C. mariscalensis is now known from 10 localities, all within a narrow stratigraphic interval in the lower part of the upper shale member of the Aguja Formation, and all but one locality occurs in coastal plain deposits that accumulated close to the paleoshoreline. These deposits have a distinctive pollen and wood assemblage, suggesting that the preferred habitat of C. mariscalensis was densely vegetated brackish-water marsh between areas of cypress and palm swamp. Three of the 10 known localities have yielded isolated remains of single individuals at or near full adult size. The maximum adult size (about 2500 kg) is similar to that in C. belli. Other localities yielded fragments of only very small individuals, as small as or smaller than the smallest individuals at the type locality (less than 100 kg). In contrast, the type locality is a bone bed that preserved remains of at least 20 individuals. This sample represents a mass mortality event and provides an indication of the age structure in a ceratopsian “herd.” Oddly, only one of these individuals (the type specimen) was at full adult size. The remaining individuals comprise several juvenile and subadult age classes with weights in the range of 100–300 kg, 500–900 kg, and 1200–1800 kg. The youngest animals (“hatchlings” up to about five years old) apparently lived apart from the adult-dominated groups. Sexual maturity was reached at about age 20, and most animals in the groups were 10 to 20 years old. The largest individuals of C. mariscalensis must have been at least on the order of 80 years old.