Spondyloarthropathy has clearly been documented as not limited in occurrence to humans. Transmammalian in nature, it is of interest to understand the antiquity, and perhaps the origins, of this disorder in animal groups sufficiently represented in the skeletal record.Fossil and recent skeletons of perissodactylae from North America were systematically examined to determine the occurrence and population frequency of spondyloarthropathy.Spondyloarthropathy was the most common form of arthritis recognized in the extant and fossil records. Common in extinct families such as Brontotheriidae and Chalicotheriidae, a progressive increase in the frequency of spondyloarthropathy was observed through geologic time in Equidae and Rhinocerotidae.Erosive arthritis of the spondyloarthropathy variety is now documented as not only persisting in Perissodactyla, but as actually increasing significantly in frequency (3-6 fold). Given the unusual evolutionary penetrance of this "disease," the possibility must be considered that its persistence provides evidence for some unknown benefit to the affected host.