AMMONOID ASSEMBLAGES ASSOCIATED WITH RAYONNOCERAS SOLIDIFORME CRONEIS, LOWER FAYETTEVILLE SHALE (CHESTERIAN-MISSISSIPPIAN), NORTHWESTERN ARKANSAS

Fossil assemblages of the lower Fayetteville Shale are dominated by ammonoids and nautiloids. These are “hot spot” occurrences isolated stratigraphically and geographically; intervals with abundant cephalopods alternate with barren intervals. Occasionally, concretionary "halos" surrounding living chambers of the nautiloid Rayonnoceras solidiforme Croneis contain the ammonoids Emstites, Tumulities, Cluthoceras, Fayettevillea, and Paracravenoceras, in general descending order of abundance. Previous interpretations regard these ammonoid occurrences as either scavenging the Rayonnoceras, or disgorged as partially digested stomach contents. We suggest that these are fortuitous associations of semelparous ammonoid death assemblages.

The crushed body chamber and adjacent sediment associated with a pathologic giant Rayonnoceras contained 175 ammonoids. Those ammonoids were compared with an assemblage collected at the same horizon a few yards away, an assemblage from another locality within the type area, and an assemblage from the “halo” concretion surrounding a Rayonnoceras also from the type area. Diameters were measured for all ammonoids to evaluate potential semelparity versus catastrophic mass mortality. Average diameters and percentage contribution for the general lower Fayetteville ammonoid population comprising 596 specimens are: Emstities 9.76 mm (48.9%), Tumulities 10.46 mm (30.9%), Cluthoceras 7.08 mm (10.9%), Fayettevillea 23.22 mm (5.3%), and Paracravenoceras 27.14 mm (4.0%). Diameters of ammonoids associated with the pathologic giant were generally less than one standard deviation from the general population, and of similar abundance: Emstities 16.06 mm (45.1%), Tumulities 12.92 mm (24.8%), Paracravenoceras 31.57 mm (12.1%), Cluthoceras 8.84 mm (10.9%), and Fayettevillea 24.30 mm (6.9%). Since the individuals of each genus are mostly the same size, we contend that they are semelparous death assemblages. Lack of protoconchs and ammonitellas in the concretions of the nautiloid “halos” and in the shale and concretions from other lower Fayetteville cephalopod horizons further support semelparity.