Origins of spondyloarthropathy in Perissodactyla.

OBJECTIVE: 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. METHODS: Fossil and recent skeletons of perissodactylae from North America were systematically examined to determine the occurrence and population frequency of spondyloarthropathy. RESULTS: 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. CONCLUSION: 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.

Single-Crystal 40Ar/39Ar Dating of the Eocene-Oligocene Transition in North America.

Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) (40)Ar/(39)Ar dates on tephra from key magnetostratigraphic and fossilbearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal "Age" boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian boundary.

Mid-oligocene extinction event in north american land mammals.

Interest in extraterrestrial causes for the apparent 26-to 32-million-year periodicity of mass extinctions has focused on the terminal Eocene event and older events, although there is now evidence of a mid-Oligocene event near the early/late Oligocene boundary, or about 32.4 million years ago. An abrupt (200,000 years or less) mid-Oligocene extinction event appears in the record of North American land mammals, which results in the selective disappearance of archaic members of the fauna and later diversification of other taxa. The selective nature of the extinctions suggests climatic and ecological causes rather than an extraterrestrial catastrophe. Increased mid-Oligocene glaciation, worldwide cooling, a major regression event, and abrupt changes in the flora are probably the immediate causes, and these may have resulted from changes in global oceanic circulation.