Evergrowing incisors of diprotodont marsupials record age and life history

Objective

Tooth growth and wear are commonly used tools for determining the age of mammals. The most speciose order of marsupials, Diprotodontia, is characterised by a pair of procumbent incisors within the lower jaw. This study examines the growth and wear of these incisors to understand their relationship with age and sex.

Design

Measurements of mandibular incisor crown and root length were made for two sister species of macropodid (kangaroos and wallabies); Macropus giganteus and Macropus fuliginosus. Histological analysis examined patterns of dentine and cementum deposition within these teeth. Broader generalisability within Diprotodontia was tested using dentally reduced Tarsipes rostratus – a species disparate in body size and incisor function to the studied macropodids.

Results

In the macropodid sample it is demonstrated that the hypsodont nature of these incisors makes measurements of their growth (root length) and wear (crown length) accurate indicators of age and sex. Model fitting finds that root growth proceeds according to a logarithmic function across the lifespan, while crown wear follows a pattern of exponential reduction for both macropodid species. Histological results find that secondary dentine deposition and cementum layering are further indicators of age. Incisor measurements are shown to correlate with age in the sample of T. rostratus.

Conclusions

The diprotodontian incisor is a useful tool for examining chronological age and sex, both morphologically and microstructurally. This finding has implications for population ecology, palaeontology and marsupial evolution.