PALEOENVIRONMENTAL AND TAPHONOMIC PARAMETERS OF AN EXCEPTIONALLY PRESERVED TYRANNOSAURUS REX

Vertebrate fossils exhibiting exceptional soft tissue preservation in the form of hair, muscle tissue, color, and skin impressions or casts have been noted in the fossil record. However, examination of the extent of preservation of these specimens beyond the gross and microscopic levels has rarely been attempted, and taphonomic and environmental parameters affecting these exceptional specimens have mostly been ignored.

An almost complete, articulated Tyrannosaurus rex skeleton exhibiting exceptional preservation of histological components was recovered from the Upper Cretaceous (Maestrichtian) Hell Creek Formation in eastern Montana in 1990. Minimal secondary mineralization characterizes some cortical bone, but no mineral infilling of trabecular bone at either meso-or microscopic scales is present; Haversian canals are open and devoid of secondary minerals. Density of trabecular bone is comparable to extant bone of similar texture and is significantly lower than well-mineralized fossil bone. Histological, biomolecular, and elemental analyses further demonstrate the extent of preservation of the T. rex bone. Cancellous bone shows little evidence of alteration and presence of degraded biogenic collagen; trabecular bone tissues contain red blood cell-like intravascular structures and degraded hemoglobin fragments.

The T. rex skeleton was found within the basal intraformational lag of a thin (<2.0 m thick) meandering fluvial channel sandstone body incised into underlying mudstone 13 m below the K-T boundary. The large size and articulated state of the skeleton, combined with lower flow regime sedimentary structures in the sandstone body suggest minimal transport within the channel and rapid burial. Lack of bone surface modification by scavenging, transport, or weathering, combined with the high degree of articulation, indicate rapid in-channel burial of an essentially intact carcass.

Abundant point contacts and floating framework grains suggest minimal sand compaction prior to cementation. Sandstone cementation occurred primarily by early clay mineral precipitation. Concretionary zones of calcite precipitation are present within the softer, clay-cemented sandstone and around localized areas of the skeleton, isolating some elements from subsequent contact with pore fluids.