Experimental taphonomy of fish bone from warm and cold water species:testing the effects of amino acid composition on collagen breakdown in modern fish bone using thermal maturation experiments

Decay experiments have the potential to provide useful analogues in the interpretation of archaeological remains. Previous studies have focused on how physical properties or processing methods can influence fish bone distributions within archaeological sites. However, the means by which intrinsic chemical properties of fish bone, such as baseline collagen type I [‘collagen (I)’] chemistry, may affect both biomolecule and whole bone degradation has not been the focus of any prior study. The variation of facies and resulting impact on taphonomy is not a new concept, but an understanding of the discrete relationship between temperature and the breakdown of collagen (I) in bone material has not been well explored. Here, modern fish bone powder is subjected to differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and water-immersed heat experiments. This is to test whether taxa with less thermally stable collagen (I) configurations, such as cold-water species with reduced proline and hydroxyproline concentrations (Pro+Hyp), will generate collagen breakdown products (TOC and TN) more rapidly than those with more thermally stable arrangements, such as warm-water fish with typically increased Pro+Hyp. Our results show that bone collagen (I) in the cold-water fish species in this study (cod and herring with lower Pro+Hyp concentrations) display significantly increased decomposition rates than collagen (I) from the warm-water fishes in this study (amberjack and tilapia with higher Pro+Hyp concentrations), given the same experimental conditions (heating in water at 75 °C for up to eight days). Initial reaction rate estimates, based on TOC and TN product concentrations, suggest that cod bone (15.6% Pro+Hyp) reacts ~9 times faster than tilapia bone (20.3% Pro+Hyp). We suggest that the primary influencer of collagen (I) stability in bone is the concentration of Pro+Hyp residues and not a function of physical bone structure. Our results suggest that a reduction in collagen (I) stability is likely to lead to a decrease in whole bone stability following deposition, due to the intimate association between organic and inorganic phases of bone. Therefore, species composition based upon bone remains may vary in archaeological and palaeontological sites, as a function of the thermal stability of collagen (I).