CANCELLOUS BONE IS A VIABLE SOURCE OF FOSSIL SOFT TISSUES: EVIDENCE FROM AN EDMONTOSAURUS BONEBED IN THE CRETACEOUS HELL CREEK FORMATION

Osteocytes, vessels, and fibrous bone matrix have recently been recovered from a diverse suite of geologically ancient taxa, including dinosaurs, turtles, and mammals. In the earliest reports, it was assumed that dense cortical bone likely constitutes a more protective repository for long-term preservation of soft tissues. As a result, investigations of cancellous bone remain rare. We report the first recovery of osteocytes, vessels, and fibrous matrix from the genus Edmontosaurus, including the first recovery of these tissues from the cancellous bone of fossil ossified tendons. Fragments of 12 fossil specimens were collected via sterile methods from the Standing Rock Hadrosaur Site in the Maastrichtian Hell Creek Formation in Corson County, South Dakota. Demineralization in 0.5 M ethylenediaminetetraacetic acid pH 8.0 yielded abundant microstructures that are morphologically consistent with vertebrate osteocytes, blood vessel fragments, and fibrous bone matrix. Osteocytes were recovered from every specimen; fragments of vessels and fibrous matrix were also commonly encountered. Despite their lack of dense cortical tissue, phalanges comprised of cancellous bone yielded numerous osteocytes and a few fragments of fibrous matrix. Morphologic consistency of the demineralization products with modern analogs and an absence of any such structures from demineralization of entombing sediments support an endogenous source for the cells and tissues. A previous taphonomic survey of the site supports the hypothesis that retention of soft tissues may have been aided by burial within low-permeability mudstone and rapid cementation of portions of the sediment by siderite precipitation. Further, energy dispersive x-ray analyses indicate the tissues are rich in iron, suggesting they may have been stabilized during early diagenesis through a recently supported, free-radical driven, natural-fixation process. Although dense cortical bone may likely preserve a greater abundance and/or diversity of soft tissues, our recoveries from a vertebra and ossified tendons demonstrate that porous, biomineralized tissues are also viable targets for soft tissue and biomolecular analyses.