The duration and rate of molar plate formation in Palaeoloxodon cypriotes and Mammuthus columbi from dental histology

Fossil elephantids are often assigned chronological ages based on tooth eruption and wear in extant elephants. Differences in body mass are likely to be accompanied by shifts in life history strategies, however, and hypotheses about these shifts cannot be tested using relative age. A better understanding of interspecific differences in the duration of tooth formation would help test hypotheses about life history variation. In this study, periodic incremental features visible in the enamel of histological thin sections of molar plates were used to estimate the rate and duration of plate formation in an insular dwarf, Palaeoloxodon cypriotes, which is smaller than extant elephants, and Mammuthus columbi, which is larger. Polarized light microscopy and image analysis software were used to determine the daily secretion rate of enamel and plate extension rate, the rate at which the plate increases in height each day, utilizing the daily incremental features, the cross striations, and accentuated lines representing the forming front of enamel at a particular time during plate formation. Estimates were made of total plate formation time from crown height and extension rate. Histological sections were prepared from molar fragments from each species. Five sections were prepared in the same plane from the large M. columbi plate. The daily secretion rate, 2-5 μm, is similar in both elephants, but the extension rate is higher in the larger M. columbi. The initial extension rate is estimated to be 62.5 μm per day, but drops to around 32.3 toward cervical portion of the plate. In P. cypriotes, the initial extension rate is estimated to be 34.4 μm per day, dropping to 12 μm per day and then rising to 23.3 μm per day in the cervical region. Estimated plate formation time is around 10.6 years for 180.9 mm of height in M. columbi and 5.9 years for 51.1 mm of height in P. cypriotes. M. columbi thus forms a taller plate by increasing both extension rate and the duration of formation. These differences could be allometric, with higher crowned teeth forming more rapidly than low crowned teeth, or they could be related to differences in life history strategies between taxa.