1. Introduction

Ancient teeth are windows into the lives of deceased humans and animals and thus a crucial source material for archaeologists, palaeontologists, biologists and forensic scientists. In archaeological investigations, teeth are often the preferred material for analyses involving isotopic and genetic assays. For isotopic work, teeth, in comparison with bone, can yield signals from different periods of an individual's lifetime, useful in dietary reconstruction (Bocherens et al. 2007), climate research (Rosvold et al. 2010) and investigation into movement of people and animals in the past (Britton et al. 2009). Additionally, several authors have suggested that teeth preserve biomolecules (e.g. DNA, collagen) better than bone (Gilbert et al. 2005; Adler et al. 2011; Meyer et al. 2000). The assumption is that the decay-resistant enamel cover and position within the jaw protects the dentine from attack by micro-organisms and infiltrations by extraneous materials. Bioerosion will cause loss of collagen (Hedges 2002) as well as introduction of bacterial/fungal DNA and will increase porosity, leading to further contamination (Gilbert et al. 2005) and accelerated decay. Infiltration by humics from the soil may cause problems both for the extraction of collagen and DNA (van Klinken and Hedges 1995), whereas inorganic compounds such as iron and copper compounds may cause problems for DNA extraction (Alaeddini et al. 2010; Breen and Murphy 1995; King et al. 2009). However, few systematic comparative studies have been carried out (see review in Hollund et al. 2012a) and several authors have detected chemical diagenesis in tooth enamel, a dental tissue that is considered to be highly resistant to diagenesis (Sponheimer and Lee-Thorp 2006; Hinz and Kohn 2010), protecting the tooth as a whole from exogenous diagenetic agents. Furthermore, investigations into diagenetic alterations in teeth at the histological scale are scant. This is despite the importance of teeth in archaeological research and the increasing awareness of preservation issues affecting the potential for various types of analyses. An exception is a recent study by our group, comparing histological preservation in archaeological human bone-tooth pairs (Hollund et al. 2012a). The results supported the argument that teeth often preserve better than bone. However, the study also showed that this is not always the case and severe alterations can also occur in teeth. Thus, for teeth, as for bone, it is important to consider and investigate post-mortem alterations before making decisions on further analyses of dental tissue biomolecules and chemistry.

The current study aims to investigate in more detail diagenetic patterns observable at histological scale, using mainly the same examples of human teeth as in Hollund et al. (2012a). The samples were taken from a cemetery in Eindhoven, the Netherlands, dating from 1260 to 1850 AD. In addition, seven teeth, including two animal teeth, from four other sites dating from the Mesolithic to the 16th century, were available for sampling.

The aim of comparing bone and teeth in our previous paper was to answer the general question: 'Are teeth better?'. The comparative histological study pointed both to similarities and differences between dentine/cementum and bone and that this depended on taphonomic factors, including environmental conditions and fluctuations in these. The teeth were on average better, considering histological integrity and collagen preservation (Hollund et al. 2012a). Consequently, the follow-up question is: How are teeth better? Detailed discussion of this was beyond the scope of our previous study. Here we will describe diagenetic patterns observed in the teeth and include a catalogue of micrographs. There are several publications reporting histological characterisation of bone diagenesis, but most relevant as a comparison in this case is the review of bone diagenesis by Jans (2005), which includes an atlas of diagenetic features. The current article seeks to provide a comparable review and atlas of the features observed in teeth. This histological study will show patterns in severity and type of degradation in the different tissues of teeth. The assemblage consists mainly of adult human teeth, with only two children's teeth and two cattle teeth. As such it does not permit consideration of differences caused by species and biological age. However, several of the teeth contained common pathologies such as caries and attrition, which allows observations of any relationships between diagenesis and pathology.

Numerous studies of bone diagenesis at histological scale (Hackett 1981; Garland 1987; Grupe and Dreser-Werringloer 1993; Jans et al. 2004; Hedges et al. 1995), have left basic questions unanswered, including which organisms are involved in bioerosion and how fast such attack is initiated and progresses post-mortem. Detailed characterisation of microbial attack within dental tissues may throw light upon the processes behind microbial skeletal tunnelling as the process takes place in a different substrate. The characterisation would seek to address questions such as: