Abstract
The idea that original soft tissue structures and the native structural proteins comprising them can persist across geological time is controversial, in part because rigorous and testable mechanisms that can occur under natural conditions, resulting in such preservation, have not been well defined. Here, we evaluate two non-enzymatic structural protein crosslinking mechanisms, Fenton chemistry and glycation, for their possible contribution to the preservation of blood vessel structures recovered from the cortical bone of a Tyrannosaurus rex (USNM 555000 [formerly, MOR 555]). We demonstrate the endogeneity of the fossil vessel tissues, as well as the presence of type I collagen in the outermost vessel layers, using imaging, diffraction, spectroscopy, and immunohistochemistry. Then, we use data derived from synchrotron FTIR studies of the T. rex vessels to analyse their crosslink character, with comparison against two non-enzymatic Fenton chemistry- and glycation-treated extant chicken samples. We also provide supporting X-ray microprobe analyses of the chemical state of these fossil tissues to support our conclusion that non-enzymatic crosslinking pathways likely contributed to stabilizing, and thus preserving, these T. rex vessels. Finally, we propose that these stabilizing crosslinks could play a crucial role in the preservation of other microvascular tissues in skeletal elements from the Mesozoic.
Highlights
Hollow, pliable, and transparent vessel-like structures have been recovered from skeletal elements of multiple fossil vertebrates, including non-avian dinosaurs[1,2]
In this work, our goal was to identify and test for the possible contribution of an explicit set of transition metal-catalysed crosslinking mechanisms to the preservation of vessel-like structures recovered from the compact bone of a Tyrannosaurus rex (USNM 555000 [formerly, MOR 555]), to lay a possible foundation for additional studies of preservation mechanisms for other soft tissues recovered from Mesozoic or more recent fossils
Prior studies on aged collagenous tissues[35], glycated fibrillar collagens[25,34,35,36], and demineralised tissues of Mesozoic fossils[4,9] all reported this same spectral feature to varying relative intensities, which can result from Fenton-type reactions in the immediate vicinity of peptide sequences[36], leading to peptide crosslinking and the subsequent formation of aldehydic carbonyls[37], or immature, intramolecular crosslinks
Summary
Pliable, and transparent vessel-like structures have been recovered from skeletal elements of multiple fossil vertebrates, including non-avian dinosaurs[1,2]. We proposed that these proteins could be detectable in some form if the structures investigated in this work were remnant dinosaur vessels, with chemical signatures diagnostic of their current preservation state Both elastin and collagen are identifiable by certain hallmark features constrained by their structure and molecular composition. The presence of reducing sugars contributes to the formation of carbonyl-containing glycation products (see Fig. S1), which mature into advanced glycation end products via subsequent reaction mechanisms (i.e., AGEs or Maillard reaction products)[24,25] Because these pathways are non-enzymatically driven, they can continue after death. The existing biomedical and materials engineering literature shows that the accumulation of these non-enzymatic crosslinks between or within structural proteins significantly reduces their susceptibility to common degradation pathways, because as these crosslinks accumulate, vessel walls increase in stiffness[12,17,26] and become more resistant to biological turn-over[12] and/or enzymatic degradation[27]
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