TAPHONOMY AND PALEOBIOLOGICAL IMPLICATIONS OF MIDDLE DEVONIAN (EIFELIAN) NAUTILOID CONCENTRATES, ALASKA

Significant accumulations of microscopic nautiloids in Devonian. carbo nate grainstones are reported for the first time from the fossil recor d. Small-sized nautiloids belonging to three or four taxa are the pred ominant skeletal components in shell concentrates that are nine m thic k. The nautiloids have an average length <6 mm and an average diameter <2 mm, are densely packed, and moderately well sorted with rare evide nce of grading, imbrication, or alignment. Association with abundant m assive stromatoporoids and colonial corals indicates that the nautiloi d communities inhabited a shallow-marine reefal environment. The extra ordinary abundance of small-sized nautiloids, their high density and c oncentration, as well as the thickness and repeated occurrence of the nautiloid grainstones in. the reefal sequence suggest that a combinati on of biological (group spawning and mass mortality) processes and phy sical (storm-related) phenomena were associated with the production, c oncentration, and preservation. of an unusually high abundance of smal l adult and juvenile nautiloids. The inferred life history strategies of these Devonian nautiloids suggest significant differences from, the modern Nautilus but remarkable similarities with belemnites, ammonite s, and modern coleoids.