DIGITAL PALEOBIOLOGY AND TAPHONOMY OF AN ORDOVICIAN LAGERSTÄTTE: BEECHER’S TRILOBITE BED

Beecher’s Trilobite Bed, a unit of the Ordovician Frankfort Shale of New York, contains a diverse array of pyritized soft-bodied and mineralized organisms.  Although much work has been done on the trilobite Triarthrus eatoni, difficulties in extracting pyritized specimens from shale, together with outcrop accessibility issues, have hampered further analysis of the deposit.  X-radiographic microfocus computed tomography (microCT) and back-scattered electron (BSE) microscopy of ~130 new Beecher’s specimens provide renewed insight into the paleobiology, taphonomy, and paleoecology of this deposit.

BSE analyses of T. eatoni reveal framboidal pyrite replacement of soft-tissues, supporting previous hypotheses that sulfate-reducing bacteria mediated soft-tissue preservation.  Additionally, BSE micrographs show trimodal mineral replacement along T. eatoni’s dorsal surface, with layers of ankerite and barite overlying pyrite framboids.  Using microCT to exploit compositional contrast between these dense minerals and the surrounding shale, we were able to render 106 isosurface models of T. eatoni for morphometric analysis.

T. eatoni comprises 85% of the organisms sampled; other taxa include graptolites, branching algae, brachiopods, and problematica.  Examined T. eatoni specimens have constant cephalic length:width ratios, yet distinct instars were not recognizable.  Volume-based analysis of the extent of T. eatoni mineralization suggests that most specimens have  ~45-65% of available soft-tissue surfaces mineralized.  Whereas larger specimens are characterized by thicker mineralized surfaces, there is no correlation between size or specimen completeness and the spatial extent of mineralization or the preservation of appendages.  Soft-tissue preservation is spatially stereotypical, though – five modes of mineralization occur and can be grouped into semiquantitative Trilobite Preservation Indices (TPI).  TPI correlate with volume-based mineralization analyses, and are amenable to 2D analysis.  These observations have the potential to inform future geochemical analysis of soft-tissue mineralization pathways in this deposit, and to evaluate Triarthrus’ hypothesized relationship with chemoautotrophic bacteria.