INSIGHTS INTO THE PRESERVATIONAL PATTERNS OF VERTEBRATE FOSSILS IN A FLUVIAL SYSTEM USING A TWO-DIMENSIONAL COMPUTER MODEL

The three-dimensional distribution of vertebrate fossils can vary greatly from formation to formation. Some formations show preferential preservation in channel lag or fill deposits, others in lacustrine or distal floodplain environments. In addition, the taphonomic characteristics of fossil assemblages (e.g. the size-frequency distribution of elements) can also vary greatly. An understanding of the processes controlling these variations is necessary to assess the information lost from ancient communities and to determine when it is possible to validly compare fossil assemblages.

To investigate the patterns of preservation in fluvial systems, I have created a two-dimensional model of an aggrading floodplain containing a meandering channel, inhabited by a steady-state vertebrate community. Individuals in the community die, disarticulate and are transported across the floodplain according to first principles, or well established observational data. As the river migrates and the floodplain aggrades, the remains of these individuals become buried. The elements are subject to degradation based on their individual physical properties whilst in the taphonomically active zone.

By altering the parameters of the model, the influence of different properties on the distribution of fossils in a rock unit can be investigated. Altering the rate, severity or style of river flooding significantly influences the distance over which elements are transported. The balance of floodplain aggradation and river erosion is one control on the distribution of skeletal elements between channel and more distal floodplain facies. Models run without any element destruction in the taphonomically active zone provide suggestions as to the magnitude of loss of elements that must occur in even the most fossiliferous of vertebrate assemblages. The results of this model provide a preliminary hypothesis against which fossil distributions in the geological record can be compared.