Bison Aging Techniques

Jamie Toennies, 2003


The following information was gained while completing my M.Sc. degree from the University of Iowa, Geoscience Deptartment. I studied bison collected from the Dow's Assemblage. In March 1974, bison remains were discovered in peat-bearing spoil piles left by the construction of I-35 near Dows, in north-central Iowa. All told, the sample included remains at least 23 individuals that lived less than 10,000 years ago. The Dows locality (figure 1) is located under and on both sides of Interstate Highway 35 at the Dows Interchange, near Franklin Co. Road C-47 at Sec. 28 and W1/2, Sec. 27, T91N, R22W (Hudak, 1984). Bison, elk, deer, amphibians, reptiles, birds, mollusks, and plant macrofossils were found in the spoil piles of peat removed and uncovered by the highway construction and grading.


Figure 1- Dow's bison mandible 44775AA with the teeth appropriately labelled. Figure 2- Dow's bison mandible 44775AA with the teeth appropriately labelled. P1 is not shown on this image but would be located before P2.


Dental elements are the most useful skeletal element for aging bison (Klein and Cruz-Uribe, 1983). Bison ages are chiefly estimated by dental eruption schedules and tooth wear. Eruption schedules and tooth wear provides a system of determining the age, in years, of a specimen based on which molars and premolars have erupted and/or the degree of tooth wear. Tooth wear is evident by the amount of dentine on the tooth. As a bison ages it's teeth wear down which results in more dentine showing. For bison under the age of five it is possible to determine the age to the month based on the eruption schedule because it is systematic (Frison, 1982). For mature animals, age five or older, it is only possible to estimate the age to the year based on wear patterns. Another way to determine the age of mature bison is based on metaconid height of the lower molars. This method works in almost all bovids due to the relationship between the crown height and age (Klein et al, 1981), although the statistical relationship between age and crown height is not always linear because of individual variation in ontogeny and diet (Lyman, 1987). This method can only be used after the molars are in full wear (Todd et al., 1996). The measurement is made with a sliding caliper from the cemento-enamel junction to the crown measured parallel on the mesial side, figure 3 and can be used on all three permanent molars, M1, M2, and M3, figures 1 and 2. It has been shown that sample size will affect the age profiles (Klein et al., 1981). Lyman determined that at least thirty specimens are desirable for determining correct age profiles (1987).

Steps to Aging Bison Teeth:

1. Arrange the mandibles based on both the eruption sequence and wear patterns. It is best to refer to the actual work from the authors but I have designed a summary chart of their work, see below. See Todd et al, 1996; Frison and Reher, 1970. It might help to draw the occlusal surfaces of the teeth when arranging them, figure 3.


    0.5 years M1 erupted and level with other teeth. M2 sometimes seen inside mandible.
    1.5 years M1 shows some wear. M2 not level with other teeth and unworn. M3 sometimes seen inside mandible.
    2.5 years M1 and M2 show wear. M3 just starting to erpupt. M1 style is worn. (See figure 4 to identify the style.)
    3.5 years M1 and M2 show wear. M3 erupted and unworn.
    4.5 years M1, M2, and M3 showing wear. Style of M2 is worn.
    5.5 to 9.5 years Styles of M1 and M2 look like a loop. M3 style is worn to a circle. M1 worn to half it's original size and roots start to show.
    10.5 to 14+ M3 style worn to a loop. All teeth worn to roots.

    Figure 5- Top view of a mandible showing the styles for M2 and M3. M2's style is past the point of being a loop and M3's style is a loop.  


# 1 = 4.5 to 5 years old
# 2 = 5 to 6 years old
# 3 = 5 to 6 years old
# 4 = 5 to 6 years old
# 5 = 5 to 6 years old
# 6 = 5 to 6 years old
# 7 = 6+ years old
# 8 = 6 to 7 years old
# 9 = 6 to 7 years old
# 10 = 6 to 7 years old
# 11 = 8 to 9 years old
# 12 = 8 to 9 years old
# 13 = 8 to 9 years old
# 14 = 9 to 10 years old
# 15 = 9 to 10 years old
# 16 = 10 t0 11 years old
# 17 = 12+ years old

Figure 3- Occlusal surface drawings of the Dow's right mandibles. They are arranged in sequential order. The gray represents the dentine. Mandibles under the age of 4.5 years are not included in these drawings since they can be aged by the eruption sequence. On the left is a list of the associated ages.



2. Measure the metaconid height to determine exact ages for mature bison, see pictures below.


    Figure 4 - Metaconid height measurement (M). Source: Todd et al., 1996.
    Figure 6- This image shows how to measure the metaconid height. Notice that the caliper is measuring from the enamal border to the top of the cusp (which is very worn in this specimen).
    Figure 7- This specimen's enamel border is located at the gum line.

    3. Compare your metaconid heights with ones from other studies. They should group together into distinct age groups.

    Age (years)
    M1 metaconid height
    Example of how the metaconid heights should break into distinct age groups.


    Frison, G. C., The Agate Basin Site: A Record of the Paleoindian Occupation of the Northwestern High Plains. Academic Press, New York, 1982

    Frison, G. C. and C. A. Reher, Age Determinations of Buffalo by Teeth Eruption and Wear. In The Glenrock Buffalo Jump, 48CO304, edited by G. C. Frison. Plains Anthropologist Memoir 7:46-50, 1970.

    Hudak, C. M., Paleoecology of an Early Holocene Faunal and Floral Assemblage from the Dows Local Biota of North-Central Iowa, Quaternary Research 21, 351-368, 1984.

    Klein, R.G. and K. Cruz-Uribe, The Computation of Ungulate Age (Mortality) Profiles from Dental Crown Heights, Paleobiology 9:70-78. 1983.

    Klein, R. G., C. Wolf, L.G. Freeman, and K. ALlwarden, The Use of Dental Crown Heights for Constructing Age Profiles of Red Deer and Similar Species in Archaeological Samples. Journal of Archaeological Science 8: 1-31, 1981.

    Lyman, R. L., On the Analysis of Vertebrate Mortality Profiles: Sample Size, Mortality Type, and Hunting Pressure, American Antiquity 52(1), pp. 125-142, 1987.

    Todd, L. C., D. J. Rapson, and J. L. Hofman, Dentition Studies of the Mill Iron and Other Early Paleoindian Bison Bonebed Sites. In The Mill Iron Site, edited by George C. Frison, pp. 231-237. University of New Mexico Press, Albuquerque. 1996.

    Toennies, J. L. Dows Holocene Fossil Bison Assemblage, Franklin County, Iowa: Its Application To Conservation, Interpretation, and Outreach,University of Iowa, Department of Geoscience, M.Sc. thesis, 2003.

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