Soft-Tissue, Rare Earth Element, and Molecular Analyses of Dreadnoughtus schrani, an Exceptionally Complete Titanosaur from Argentina.

Abstract

Simple SummaryAlthough many analytical techniques have shown that organic material can be preserved in fossils for millions of years, the geochemical factors that allow this preservation are not well understood. This is partly because paleomolecular studies often do not include geochemical analyses of the fossil or burial environment from which it came. We conducted in-depth geological, geochemical, and molecular analyses of a specimen of Dreadnoughtus schrani, an immense dinosaur from Argentina. We reviewed physical aspects of the sediments in which Dreadnoughtus was deposited, then characterize the following features: the structural integrity of the bone microstructure; the amount and type of external mineral that infiltrated the bone; the concentration of elements that are rare in the Earth’s crust (REEs) throughout the bone; the preservation of soft-tissue structures (e.g., bone cells and blood vessels); the preservation of bone protein using antibodies that specifically recognize collagen I. Our data show that original bone microstructures and protein are preserved in Dreadnoughtus, and that after burial, the specimen was exposed to weakly-oxidizing conditions and groundwaters rich in “light” REEs but experienced little further chemical alteration after this early stage of fossilization. Our findings support the idea that fossils showing lower concentrations of REEs are well suited for molecular analyses.Evidence that organic material preserves in deep time (>1 Ma) has been reported using a wide variety of analytical techniques. However, the comprehensive geochemical data that could aid in building robust hypotheses for how soft-tissues persist over millions of years are lacking from most paleomolecular reports. Here, we analyze the molecular preservation and taphonomic history of the Dreadnougtus schrani holotype (MPM-PV 1156) at both macroscopic and microscopic levels. We review the stratigraphy, depositional setting, and physical taphonomy of the D. schrani skeletal assemblage, and extensively characterize the preservation and taphonomic history of the humerus at a micro-scale via: (1) histological analysis (structural integrity) and X-ray diffraction (exogenous mineral content); (2) laser ablation-inductively coupled plasma mass spectrometry (analyses of rare earth element content throughout cortex); (3) demineralization and optical microscopy (soft-tissue microstructures); (4) in situ and in-solution immunological assays (presence of endogenous protein). Our data show the D. schrani holotype preserves soft-tissue microstructures and remnants of endogenous bone protein. Further, it was exposed to LREE-enriched groundwaters and weakly-oxidizing conditions after burial, but experienced negligible further chemical alteration after early-diagenetic fossilization. These findings support previous hypotheses that fossils that display low trace element uptake are favorable targets for paleomolecular analyses.