REACHING DEPTH IN THE JURASSIC: EVIDENCE OF FRESHWATER ECOSPACE UTILIZATION

Previous studies have suggested that prior to the Jurassic, relatively few animals inhabited freshwater environments and that ecospace utilization was confined to surficial zones (Miller et al, 2002, Miller and Labandeira, 2002). To investigate this delayed colonization hypothesis, freshwater deposits of the Newark Supergroup were analyzed. The Newark Supergroup consists of more than 13 fault basins formed during rifting that accompanied opening of the North Atlantic and are characterized by Early Triassic - Late Jurassic fluvial and lacustrine strata interbedded with basalt flows. This study focuses primarily on trace fossil evidence from Upper Triassic - Lower Jurassic fluvial and lacustrine deposits of the Newark and Deerfield Basins in Pennsylvania and Massachusetts, respectively. Bioturbation was assessed semi-quantitatively on bedding plane surfaces and vertical exposures using pattern recognition methods (Droser and Bottjer, 1986; Miller and Smail, 1997). Bioturbation categories were assigned as follows: 1=no bioturbation; 2=1 - 10%; 3=10 - 40%; 4=40 - 60%; and 5=60 - 100%. Observations were constrained to defined sites using a 0.25 m² grid placed at random to avoid bias. Sites were spaced at least 1 m apart and limited to 50 per bedding plane. The resultant data set contains 3330 discrete observations, largely from marginal lacustrine environments (93%). Observations from Jurassic quiet water systems dominated by fine grained, clay draped and mudcracked cyclic deposits showed the greatest evidence of vertical ecospace utilization; 31% of vertical sites exhibited burrowing activity from 1-10%. Thirty percent of the bedding plane observations showed trace fossil activity greater than 10%. In contrast, 37% of horizontal observations in Triassic deposits of similar lithology showed activity greater than 10%, and only 24% of vertical exposures showed burrowing evidence. Only 14% of observations from fluvial settings (6% of total) showed evidence of either bedding plane or vertical activity. Comparison of the Triassic and Jurassic data sets demonstrate that surficial activity was prevalent in the Triassic. By the Early Jurassic, an increase in vertical site observations suggests the evolution of burrowing at depth and thus the onset of effective ecospace utilization in freshwater environments.