Tooth be told: A new method for dental microwear analysis in bison

For the past few years, I have committed, heart, body and soul to the pursuit of my graduate degree in archaeology. I know many people in pursuit of their degrees would choose to study a fascinating subject, with the potential to change the world; but being the go-getter that I am, I chose the blood-racing world of dental microwear analysis. My focus, specifically, was in applying the study of dental microwear to bison from sites in southern Alberta to determine which seasons those sites were occupied (the site’s seasonality).

Albus, helping me observe the seasonal trends in grass growth on one of my regular visits to Glenbow Ranch Provincial Park.

What is dental microwear?

The analysis of microscopic patterns on teeth has been useful to several fields, including archaeology. The method can provide information about an animal’s diet immediately before death, allowing researchers to reconstruct past environments and determine the season that an animal died. This is based on the recognition that different types of food produce identifiable features on the enamel of teeth.

In faunal archaeology, the most common distinctions made based on microwear are between herbivores who either graze or browse. Grazing animals that mainly eat grasses and other low-lying plants, such as cattle, tend to use a grinding motion when chewing, which drags food across the enamel. This produces features referred to as striations. In contrast, the diets of browsing animals, like moose, that mainly eat vegetation such as leaves, stems, and bark, use more of a chopping motion, which means their teeth come into direct contact with one another. This causes the formation of features called pits.

These microscopic features are usually replaced after a couple of days, which means that the method is particularly useful in reconstructing a narrow window of diet before death. Specific inferences can therefore be made about animal remains recovered from archaeological sites. For example, microwear can be used to reconstruct the season(s) a site was occupied, based on animals who switch between browsing and grazing during different times of the year.

Unfortunately, determining seasonality based on microwear has been limited to making assumptions based on seasonal transitions between grazing and browsing. As you can see, the method would not be relevant in determining the season at death for bison, and therefore the seasonality for many Plains archaeological sites, because bison graze almost exclusively year-round.

This was the inspiration for this research. My aim, specifically, was to develop a new method for dental microwear analysis which looked beyond the simple features which were gouged into enamel. Instead, I focused on every aspect of microscopic dental wear – from those very same features, to the texture of the surface around it, and even the brightness of the enamel surface itself. Collectively, these are referred to as dental polish.

The purpose of this research was to assess the dental polish left by different compositions of fresh and dry grass, and by variations in the content of grit (windblown sediment) present on the grass. Both variables were anticipated to change throughout the year in a predictable pattern within southern Alberta. Ideally, dental polish would tell me the season bison from archaeological sites had died based on these variables, and therefore which seasons those sites were occupied.

Observing Dental Polish

This involved taking different proportions of fresh and dry grasses, both with and without grit, and rubbing them against the chewing surfaces of cow teeth for 30 minutes to simulate consumption and the seasonal variations in these variables.

Some of the main patterns I found included:

  • Fresh grasses made a brighter polish and dry grasses made it duller.
  • Fresh grasses produced a smooth surface while dry grass produced a rough surface. When grit was included, it always produced a rough surface, regardless of the type of grass.
  • Pits were only produced when grit was included. Scratches were present without grit and were more common from dry grass than fresh, but were always frequent when grit was included.

So, at this point, I understood the differences in polish produced by variations in the moisture content of grass and the presence versus absence of grit. Now, I needed a way to correlate this polish to seasonal patterns in southern Alberta. That meant that I had to set out to observe seasonal trends in grass growth for myself. To do this, I made observations of fresh versus dry grass compositions at regular intervals at Glenbow Ranch Provincial Park (near Cochrane, AB) for one full year. Grit was also extracted from samples taken during these visits.

Based on the trends in grass growth that I observed and in the grit extracted, seasonal categorizations were made and were expected to correlate to specific polish signatures. These seasonal categorizations included:

  • Fall/Winter: early/mid-October – early/mid-May (dry grass and low grit)
  • Early Summer: mid/late May – mid/late June (fresh grass and low grit)
  • Mid-Summer: late July – early/mid August (fresh grass and high grit)
  • Late Summer/Early Fall: mid/late August – late September/early October (mixed grass and high grit)

These categorizations were then used to analyze tooth polish on bison specimens from two archaeological sites in Alberta, with the goal of assessing the season that they died.

Applying Dental Polish to Archaeology

The first site chosen was the Cluny Fortified Village. Located on a floodplain along the Bow River on the Siksika First Nation about 95 km east of Calgary, this campsite was occupied about 270-280 years ago.[1][2] It has fortification features including a semicircular trench, large pits and a fence or wall structure which make it unique on the Canadian Plains.[3] The second site is referred to by its Borden designation as EgPn-440. This site, just west of Calgary, is a bison pound site used primarily to hunt and kill bison, and was used approximately 1240-1280 years ago.[4]

Within the Cluny assemblage, bison specimens were assigned to all seasonal categorizations. The most common by a significant margin was Mid-Summer, with an overall general emphasis on a spring and summer seasonality. At Cluny, other archaeological evidence that the site must have been occupied during the summer months corroborates the seasonality made evident by dental polish. Namely, that the large excavated fortification features must have been constructed during warm months when the ground was not frozen.[1]

Dental polish observed on the enamel of one specimen from the Cluny Fortified Village site. This polish was categorized as Mid-Summer because of its bright polish (indicating fresh grass) and heavy striations (indicating high grit content). The dots or pits which are obvious in the photo are enamel prisms, which are natural structures in the tooth and are prominent in young individuals. (Observed at 400X. Scale bar = 10µ.)

The EgPn-440 assemblage, in contrast, had polish signatures from bison assigned mainly to two categories: Fall/Winter, and most common by far, Late Summer/Early Fall. These results were particularly interesting because I was able to compare these to the vertical position of each artifact (which would correspond to when they were deposited relative to one another) to determine how many times the site had been used. This revealed the presence of at least three occupations at the site which were separated by depth: two in Late Summer/Early Fall, and a Fall/Winter occupation which intersected these. So, while there still might be some questions regarding the nature of occupation at Cluny and EgPn-440, dental polish was able to shed some light on the seasonality of occupation for both sites, and on the recurring use of EgPn-440.

Dental polish observed on the enamel of one specimen from EgPn-440. This polish was categorized as Fall/Winter because of its dull polish (indicating dry grass) and light striations (indicating a low grit content). (Observed at 400X. Scale bar = 10µ.)

Dental polish has the potential to be a useful tool in archaeology, particularly in assessing the seasonality of archaeological sites in southern Alberta that possess bison remains. Perhaps dental polish isn’t the most fascinating or glamorous avenue of research (although by now you’ll have to admit that it’s certainly up there), but its applications are significant and diverse depending on the context of each site. My hope is that further research can improve the accuracy of the method and extend the regions and species upon which it can be applied. While the method is still preliminary in its applications, I think this study has laid some important groundwork for the use of dental polish to assess the seasonality of archaeological sites in Alberta that contain bison.

Written By: Tatyanna Ewald, Department of Anthropology and Archaeology, University of Calgary

References

[1] Forbis, Richard G.

1977 Cluny: An Ancient Fortified Village in Alberta. Department of Archaeology, University of Calgary Occasional Papers 4. University of Calgary, Calgary.

[2] Walde, Dale, Lance Evans, Harrison Boss, James Eddy, Lynne Fulton, and Karrie Ginter

2011 The 2009/2010 Archaeological Field Seasons at Blackfoot Crossing Historical Park. Submitted to Blackfoot Crossing Historical Park. Copies available from Blackfoot Crossing Historical Park Interpretive Centre, Cluny.

[3] Walde, Dale

2008 The 2008 Archaeological Field Season at Blackfoot Crossing Historical Park. Submitted to Blackfoot Crossing Historical Park. Copies available from Blackfoot Crossing Historical Park Interpretive Centre, Cluny.

[4] Tischer, Jennifer C.

2000 EgPn-440: A Late Prehistoric Bison Pound on the Northwestern Plains. Master’s thesis, Department of Archaeology, University of Calgary, Calgary.

 

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