The observable differences in preservation across the images in Figure 1 are a preliminary example of how biomolecular histology can potentially be used to screen specimens for degree of DNA and protein sequence preservation. The extant B. taurus fibrils (Figure 1a) show the prominent ~67-nm banding pattern characteristic of type-1 bone collagen (Boatman et al., 2019; Gottardi et al., 2016; Lin et al., 1993; Rabotyagova et al., 2008; Tzaphlidou, 2005). The permafrost YG 610.2397 M. primigenius fibrils (Figure 1b) also show this banding, but it is potentially less distinct/ordered. In contrast, the temperate M. columbi MOR 91.72 fibrils (Figure 1c) show a general absence of this banding pattern, suggesting degradation of type-1 collagen protein sequences. This would be consistent with MOR 91.72 being recovered from a temperate region, unlike the permafrost YG 610.2397 innominate fragment. These differences are consistent with the difference in number of proteins recovered between the permafrost (269) and temperate specimens (35 and 19) of Cappellini et al. (2012) (Cappellini et al., 2012). While this comparison is strictly suggestive because it is between different, although relatively comparable, specimens, it demonstrates the potential of biomolecular histology as a proxy of ancient DNA and protein sequence preservation. Because the collagen fibrils shown in Figure 1 are the direct morphological manifestation of type-1 collagen sequences, any observed degradation to this morphology is hypothesized to correspond to type-1 collagen sequence degradation and with further testing may be usable as a proxy.

4 LITERATURE EXAMPLES HIGHLIGHTING THE POTENTIAL OF BIOMOLECULAR HISTOLOGY AS A PROXY

A few previous studies have examined fossil/subfossil biomolecular histology in a manner that can be linked to preservation potential for molecular sequences. A discussion of some findings relevant to the correlation of biomolecular histology with degree of sequence preservation follows.
A 2007 study by Schweitzer et al. used light and electron microscopy to survey the biomolecular histology of bone specimens ranging from modern day through to the Triassic (Schweitzer, Wittmeyer, & Horner, 2007). The study reported that the biomolecular histology (especially the “collagenous” matrix) of specimens with dates exceeding ~100–600 Ka was substantially altered relative to specimens of younger timepoints. Light microscopy was herein used to replicate and reevaluate reported data for three of the 2007 study specimens (data not shown), the M. columbi femur (MOR 91.72, ~14–15 Ka in calibrated years, ~12.5 Ka in radiocarbon years, Lindsay/Deer Creek, Montana, United States; Hill & Davis, 1998; Hill & Schweitzer, 1999), the M. columbi skull (MOR 604, ~100–600 Ka, Doeden gravel beds, Montana, United States; Hill & Schweitzer, 1999; Schweitzer et al., 2002; Wilson & Hill, 2000), and the M. pacificus skull (MOR 605, ~100–600 Ka, Doeden gravel beds, Montana, United States, species recently reassigned from M. americanum; Asara et al., 2007; McDonald et al., 2020; Wilson & Hill, 2000). This was done according to the same demineralization protocol reported by the 2007 study (Schweitzer, Wittmeyer, & Horner, 2007).
“Collagenous” matrix of the mid-Pleistocene MOR 604 and MOR 605 specimens was highly fragmented and brittle, supporting substantial degradation. Histological structures resembling blood vessels readily broke free of the degraded matrix and were easily isolated. Both specimens exhibited evidence of exogenous, orange-brown mineralization across portions of structure surfaces even after hydroxyapatite was removed via acid demineralization. In contrast, the late Pleistocene MOR 91.72, also from the temperate region of Montana, U.S.A. (Schweitzer, Wittmeyer, & Horner, 2007), preserved a structured, relatively intact collagenous matrix. Emphasis is placed on the term relatively, as Figure 1 demonstrates the collagenous matrix of MOR 91.72 itself is still somewhat degraded relative to extant specimens. No evidence for exogenous mineralization of MOR 91.72 was detected with light microscopy.