that the fusion of the first three neck vertebrae, such as seen in the mature specimen MOR 571, might have been a paleopathology, an instance of the disease spondyloarthropathy, but in 1997 it was concluded that it was more likely a normal ontogenetic trait, the vertebrae growing together to form a so-called "syncervical" to support the heavy head. All three main known specimens have syncervicals consisting of three fused neck vertebrae; the trait could have been inherited from a smaller ancestor using a stiffer neck for burrowing or food acquisition. Social behavior It has been claimed that ceratopsian dinosaurs were herding animals, due to the large number of known bone beds containing multiple members of the same ceratopsian species. In 2010, Hunt and Farke pointed out that this was mainly true for centrosaurine ceratopsians. Horner assumed that the horned dinosaurs at Landslide Butte lived in herds which had been killed by drought or disease. Dodson concluded that the fact that the Achelousaurus bone beds were monospecific (containing only one species) confirmed the existence of herds. Metabolism There has long been debate about the thermoregulation of dinosaurs, centered around whether they were ectotherms ("cold-blooded") or endotherms ("warm-blooded"). Mammals and birds are homeothermic endotherms, which generate their own body heat and have a high metabolism, whereas reptiles are heterothermic ectotherms, which receive most of their body heat from their surroundings. A 1996 study examined the oxygen isotopes from bone phosphates of animals from the Two Medicine Formation, including the juvenile Achelousaurus specimen MOR 591. δ18O values of phosphate in vertebrate bones depend on the δ18O values in their body water and the temperature when the bones were deposited, making it possible to measure fluctuations in temperature for each bone of an individual when they were deposited. The study analyzed seasonal variations in the body temperature and differences in temperature between skeletal regions, to determine whether the dinosaurs maintained their temperature seasonally. A varanid lizard fossil sampled for the study showed isotopic variation consistent with it being an heterothermic ectotherm. The variation of the dinosaurs, including Achelousaurus, was consistent with them being homeothermic endotherms. The metabolic rate of these dinosaurs was likely not as high as that of modern mammals and birds, and they may have been intermediate endotherms. Paleoenvironment Achelousaurus is known from the Two Medicine Formation, which preserves coastal sediments dating from the Campanian stage of the Late Cretaceous Period, between