Five years in the life of an aucilla river mastodon

Abstract

The tusks of mastodons, mammoths, and other proboscideans are accretionary structures that record, in the fashion of a stratigraphic section, numerous time-dependent parameters relevant to the living animal that grew them. Their lack of in vivo remodeling and their frequent accumulation of growth sequences that are several decades long make them ideal archives of biological and environmental data. Methods for extracting and interpreting these data are still under development (e.g. Fisher, 1996, 2001; Hoppe et al., 1999; Fox, 2000), and we are just beginning to explore the application of these methods to proboscideans of different regions and time periods. In this spirit of exploration, we undertake examination of a portion of a tusk-record representing a brief interval in the life of a single American mastodon (Mammut americanum), from a region and a time period that are interesting in their own right. We cannot yet claim generality for the observations reported here, but they at least expand the coverage achieved in prior work. In doing so, they offer an opportunity to discover new patterns of association between environmental variability and organismal response. Our focus is on documenting the pattern of variation in rate of addition of new dentin to the tusk (i.e. rate of dentin apposition), measured normal to the surface to which it is added. Then we compare this pattern to variation in the oxygen isotope composition of phosphate in dentinal hydroxyapatite. The oxygen isotope record varies with temperature and moisture and thus registers the seasonal schedule of tusk growth. We also relate these aspects of growth rate and composition to measurements of increasing tusk length and to other isotope measurements (see Chapter 13 by Hoppe and Koch). Our goals are to use the pattern of variation in oxygen isotope composition to diagnose the scale of annual increments of tusk dentin and help to characterize the seasons that comprise the annual cycles recorded in this tusk. This information will allow us to quantify tusk growth rates for part of one animal's life and to characterize the climatic and environmental setting in which this individual lived. The tusk we analyzed is UF 150701, from collections of the Florida Museum of Natural History, at the University of Florida, Gainesville. It represents a large male mastodon, M. americanum, recovered from the Page-Ladson site, on the Aucilla River, in Taylor County, northwestern Florida. It was associated with a rich fauna of both aquatic and terrestrial vertebrates (see Chapter 8 by David Webb and Simons). Seven carbon dates from the fine-grained, organic-rich silty sand of Unit 3 yield an average age of 12,425 14C BP. This tusk is one of those analyzed and discussed in Chapter 13 by Hoppe and Koch. It is also from the same assemblage that produced evidence of Florida mastodons migrating seasonally northward into the southern Apalachians (Hoppe et al., 1999). In addition, this tusk shows apparent cutmarks, at the approximate location of the alveolar margin, interpreted by David Webb in Chapter 11 as evidence of butchery. This specimen thus stands at the confluence of several interesting issues. Beyond simply providing a record of one individual and its environment, tusk analyses have a bearing on questions such as the seasonal timing of migratory movements of mastodons, and the human procurement of mastodons.