SYNCHROTRON X-RAY PHASE-CONTRAST IMAGING OF MAMMOTH DENTIN

Dentin grows by accretion, and daily growth increments are thought to be present in the dentin of all mammals. In addition, long-period (~7-day) growth increments and annual growth increments have been observed in mammoth tusks. Such increments are used to determine individual age and season of death, and to provide temporal calibration for dentin compositional (e.g., stable isotope) series. Because the clarity of daily growth increments varies in thin sections imaged with conventional light microscopy, it has been difficult to verify the expected relationship between supposed daily and annual growth increments (~365 daily growth increments per annual increment) by direct counting. Alternatives to continuous accretion marked by daily increments include the possibility of periods of the year during which dentin accretion might have stopped, or the possibility that mammoths living in the arctic, like some other arctic vertebrates, exhibited a lack of circadian organization under the constant-light conditions of the arctic summer.

Using synchrotron X-ray phase-contrast imaging, we are able to observe daily growth increments throughout sections of arctic mammoth tusks. Each daily increment is approximately 14 to 23 microns in thickness. There are about 365 “daily” increments within the “annual” growth increments that have been analyzed, suggesting that the observed features do represent daily and annual features. This also shows that dentin accretion is continuous throughout the year. Furthermore, the consistent formation of daily growth lines may indicate that arctic mammoths maintained circadian organization throughout the year despite the lack of 24-hour light/dark cues in the summer and winter. It is also possible that the analyzed annual growth increments were actually formed when these mammoths were living south of the Arctic Circle, exposed to year-round 24-hour light/dark cues.