Modern Vertebrate Track Taphonomy at Lake Manyara, Tanzania

Fossil vertebrate tracks have a potential for providing valuable paleobiologic data to complement investigations of body fossils. Before tracks can be used for this purpose, it is critical to establish precisely what biologically significant signals are encoded in them. It is equally important to know how those signals are affected by the environmental conditions existing at the time and site of track formation and the subsequent taphonomic processes affecting the track. We studied modern tracks at Lake Manyara, Tanzania, to document the environmental and taphonomic controls on track preservation. This closed basin, saline lake closely resembles the depositional setting of many ancient track-bearing strata. Two study sites (an alkaline mudflat and a delta floodplain) were established to monitor substrate conditions and physicochemical/biological processes of importance to track formation, preservation and survivorship probability. Physicochemical factors of importance to tracks included substrate composition and texture, daily wave and seiche reworking, seasonal lake level fluctuations, groundwater table fluctuations, surficial drying, wind deflation and salt crust development. Textural, compositional and moisture content differences between sites regulate the probability of initial track registration and depth of penetration. The various flooding and drying phenomena also generate cycles of track reworking and cracking and stimulate invertebrate bioturbation. Salt crusts on the Alkaline Flats strongly influenced the distribution and preservational quality of smaller tracks. Biological factors included both invertebrate (insect) bioturbation and secondary vertebrate trampling. Surficial turnover rates were estimated at 0.5-1 cm/yr (locally up to 20 cm/yr) for invertebrates and 2.5-3.7 cm/yr for vertebrates. Track and trackway survivorship was monitored both qualitatively (to document the sequence of track deterioration) and quantitatively (to determine rates of deterioration). A systematic and direct relationship exists between track survivorship and distance of the track from shoreline. Nearshore track assemblages persist over time periods measured in hours whereas at distances of >100 m inland, tracks may persist for months. This results both directly from wetting/drying cycles and indirectly from induced invertebrate bioturbation. An abrupt threshold of increased bird track survivorship correlates with the landward limit of significant nightly groundwater discharge. A strong shoreline-parallel zonation of environmental variation correlates with differences in track preservation style. Zone 1, a landward zone of dry, salt encrusted sediments, is dominated by large ungulate tracks which are deformed primarily by baking, cracking and deflation. Tracks here have low probabilities of initial formation but high survivorship. Zone 2, a strandline zone of saturated sediments, contains a mix of large and small mammal and bird tracks. Tracks formed here display the best morphologic definition. Both track formation and reworking rates are higher here than in zone 1. Zone 3 is a subaqueous zone where liquefaction and increasing water depth again limit tracks to those of larger mammals and prevent good track definition. Shoreline parallel taphonomic zonation of tracks should be useful in defining paleoenvironments for ancient track sequences. Preburial track survivorship can be modelled as the interaction between three variables: 1) strain susceptibility of a substrate prior to track formation; 2) track loading stress; and 3) secondary reworking rate. Survivorship isograds (surfaces of equal survivorship probability) can then be plotted in this 3-D space to investigate their relative importance for track preservation. Apparently thresholds or discontinuities exist in track preservation space, where isograds are bunched together. These correlate with points along environmental gradients where extremely rapid transitions in track taphofacies occur. Time averaging is important in the formation of track assemblages but operates over a much briefer time scale than for body fossils. By quantifying the shapes of survivorship isograds ichnologists will ultimately be able to directly estimate trackmaker mass and velocity, substrate compactibility and water content at the time of track formation, as well as quantify time averaging of track assemblages.