PATHOLOGIC GIGANTISM IN RAYONNOCERAS SOLIDIFORME CRONEIS FROM THE LOWER FAYETTEVILLE SHALE (CHESTERIAN-MISSISSIPPIAN), NORTHWESTERN ARKANSAS

The Chesterian Series, Mississippian System, in the southern Ozarks represents a single third-order eustatic cycle. Maximum flooding occurred in the lower Fayetteville black shale reflected by anoxic to disoxic seafloor conditions. In contrast, the upper Fayetteville Shale was deposited as a high stand succession that developed a well oxygenated seafloor. Lower Fayetteville fossil assemblages are dominated by cephalopods to the virtual exclusion of other organisms. Both nautiloids and ammonoids exhibit "hot spot" occurrences characterized by horizons with abundant individuals of similar size separated by barren intervals suggesting semelparous accumulations. The actinoceratoid nautiloid Rayonnoceras solidiforme Croneis is a common element in those assemblages. Large numbers of mostly fragmentary, but with some complete, specimens collected over a period of nearly 100 years suggest a mature length of about one meter, and a semelparous reproductive strategy. In 1963, the lower Fayetteville produced a specimen with a length of approximately 2.5 m. That specimen was regarded as a pathologic giant, but it remained a unique occurrence until the discovery of a slightly longer (3 m) specimen in January, 2003, apparently the largest documented nautiloid ever recovered. While slightly longer, the first specimen is only about three-quarters as wide as the early specimen. If the modern Nautilus can be used for comparison, it is possible that these Fayetteville specimens are sexual dimorphs; the wide specimen representing a male, and the narrow specimen a female. The most unusual feature of the two pathologic giants is that they are both missing the distal .5 m of the phragmocone. Loss of that portion of the shell occurred in life and is not the result of problems with preservation or excavation. Pathologic gigantism may occur in individuals from semelparous populations that fail to reach sexual maturity. The giants may live considerably longer as a consequence, and the older parts of their shells may have become degraded and lost as a reflection of that longevity.