PERMIAN WATER MASSES AND AMMONOID BIOGEOGRAPHY

There are two models of how marine biogeographic patterns are controlled. The simpler model invokes latitudinal gradients in ocean surface temperatures, and the north-south orientation of the continents as major biogeographic barriers. The ‘Large Marine Ecosystem unit’ model incorporates biologically relevant factors such as salinity and productivity fronts, and has been proposed as a more sensitive framework for understanding paleobiogeographic patterns. Such water mass maps of the geological past have been compiled using climate-sensitive sediments, and provide a framework for paleobiogeographic distributions of marine fossils.

This study tested the utility of the water masses in the fossil record using a spatial database of 923 occurrences of Permian ammonoids compiled from the literature. These occurrences were restored to their original paleolatitudes and paleolongitudes and plotted on published Permian paleogeographic maps, which incorporated water mass information. Thus information on both the paleolatitude and water mass type was available, allowing investigation of the role of these factors in controlling Permian ammonoid paleobiogeographic patterns.

Preliminary analysis of the distributions of the ammonoids indicated that 44 genera were cosmopolitan with respect to water mass types. Another 5 genera were confined to the temperate water masses. The remaining 88 genera were restricted to tropical water masses. However, many of the endemic Tethyan genera that appeared in the Late Permian were restricted to the tropical water masses. The origination of these genera rapidly inflated the overall number of Permian genera that were restricted to the tropical water masses, relative to the cosmopolitan genera. A related finding is the fact that the ammonoid genera affected by the Permo-Triassic event were confined to warmer water conditions. This contrasts strongly with the Earliest Triassic ammonoids, which were highly cosmopolitan in their geographical distribution. This information may help to constrain extinction mechanisms proposed for the Permo-Triassic event, and demonstrates that an understanding of water masses can enhance our investigations of biotic change in the marine fossil record.