Tendency to afflict one part of the skeleton, rather than another, could be referred to as the osseotropism of the disease. That term would also include which part of the particular bone was affected. That, in addition to characteristics of erosions, facilitates distinguishing spondyloarthropathy from rheumatoid arthritis, calcium pyrophosphate deposition disease and gout. Spondyloarthropathy, however, is not limited to humans. Initially recognized in 20% of gorillas and rhesus macaques, it was subsequently identified in 25% of bears and 35% of rhinoceros. It is truly a pan-mammalian phenomenon, extending from marsupials and rodents to whales and as ancient as dinosaurs.