Peer Review #2 of "In situ SEM/EDS compositional characterization of osteocytes and blood vessels in fossil and extant turtles on untreated bone surfaces; different preservational pathways microns away (v0.1)"

Abstract

Osteocytes and blood vessels are the main cellular and tissue components of the bone tissue of vertebrates. Evidence of these soft-tissue microstructures has been widely documented in the fossil record of Mesozoic and Cenozoic turtles. However, all these studies have characterized morphologically and elementally these microstructures via isolation from the fossilized bone matrix where they were preserved or in ground sections, which could raise skepticism about the results due to potential cross-contamination or reagents effects. Fossil turtle bones from three different localities with distinct preservation environments and geological settings, including Mongolemys elegans from the Late Cretaceous of Mongolia, Allaeochelys crassesculpta from the Eocene of Germany, and a podocnemidid indet. from the Miocene of Colombia are studied here. Bone from two extant turtle species, Lepidochelys olivacea, and Podocnemis lewyana, as well as a commercial chicken Gallus gallus were used for comparisons. Scanning Electron Microscopy-Energy Dispersive Spectroscopy analyses performed directly on untreated fresh surfaces show that osteocytes-like in the fossil turtle bone are mostly composed of iron and manganese. In contrast, the in situ blood vessels-like of the fossil turtles, as well as those from the extant taxa are rich in elements typically organic in origin (carbon and nitrogen), which are absent to minimally present in the surrounding bone or rock matrix; this suggests a possible endogenous composition for these fossil structures. Also, the results presented here show that although originally both (osteocytes and blood vessels) are organic soft components of bone as evidenced in the extant turtles and chicken, they can experience completely different preservational pathways only microns away from each other in the same fossil bone.