TAPHONOMIC PROCESSES IN EARLY JURASSIC RIFT LAKES OF THE HARTFORD BASIN: PHYSICAL AND BIOCHEMICAL CONTROLS ON PRESERVATION OF BONY FISHES

Early Jurassic lake beds of the Hartford Basin are noteworthy for abundant well-preserved fishes representing the holostean genera Semionotus, Redfieldius, and Ptycholepis, and a large coelacanth, Diplurus. Driven by Milankovitch climate cycles, rift lakes in the Hartford Basin expanded and contracted with a periodicity of about 21ky. During high stands, deep lakes occupied much of the basin, creating ideal conditions for diverse and substantial populations of ray-finned and lobe-finned fishes that, in turn, supported unexpectedly large numbers of carnivorous theropod dinosaurs prowling the shorelines.

Upon mortality, some fishes sank beneath the hypolimnion, coming to rest in sulfide-rich benthic sediments. Anoxic bottom waters excluded the typical array of scavengers, and fish carcasses were subject to: 1) biochemical dissolution, and 2) physical decomposition, each taphonomic process producing specific styles and trends of preservation.

Fish remains lying in profundal offshore waters with low sedimentation rates were exposed to lengthy periods of microbially mediated dephosphatization that progressively reduced robust skull, shoulder-girdle, and scale elements to thin, but fully articulated, carbonate coatings or carbonaceous films. Soft fish parts containing organic P and bony parts largely composed of hydroxyapatite (HAp) undergo diagenesis simultaneously, with the organic P being released into solution and metastable HAp being degraded on a timescale of days to months depending on water redox conditions. Paleogeographic trends in dephosphatization correlate well with relative rates of sedimentation predicted by our depositional models.

Fishes lying at the sediment-water interface were slowly decomposed and mechanically disarticulated leaving fragmentary remains in situ. In some specimens, the buildup of decomposition gases in the gut and cranial cavities caused the carcasses to burst.

These two taphonomic trends reveal important information regarding: A) paleogeography of the ancient lake basins; B) geochemistry of lake waters and bottom sediments; C) lake ecosystem dynamics, particularly the microbial ecology; and D) phosphorus cycling and partitioning in stratified and non-stratified lakes.