Wear stages and crown heights in prehistoric bighorn sheep (Ovis canadensis) lower molars in eastern Washington state, U.S.A.

Journal of Archaeological Science: Reports 15 (2017) 40–47

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Wear stages and crown heights in prehistoric bighorn sheep (Ovis canadensis) lower molars in eastern Washington state, U.S.A.

MARK

R.Lee Lyman Department of Anthropology, 112 Swallow Hall, University of Missouri, Columbia, MO 65211, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Bighorn sheep Mandibular molars Molar crown heights Molar crown widths Molar wear stages Ovis canadensis

Bighorn sheep (Ovis canadensis) remains are often found in western North American archaeological sites. Determination of ontogenetic age of zooarchaeological individuals would allow assessment of season of procurement, herd demography at the time of procurement, and perhaps differential processing. The known dental eruption schedule for bighorn is imprecise for purposes of estimating season of death, and all teeth are erupted when an individual is ~42 months old, precluding detailed analysis of herd demography. The tooth wear sequence established by Sebastian Payne for Old World domestic sheep (Ovis aries) extends to near the end of an individual's lifespan and is applicable to New World bighorn based on a zooarchaeological sample of 105 mandibular molars. Crown heights and crown widths of the zooarchaeological bighorn molars correlate with one another and with wear stages, suggesting study of a modern sample of bighorn mandibular dentitions from individuals of known ontogenetic age would repay the effort.

1. Introduction

Archaeological dentitions of these species have not been as intensively and extensively studied as those of bison, though some work with pronghorn dentitions has proven archaeologically informative (e.g., Lubinski, 2001; Lubinski and O'Brien, 2001). The relative rarity of studies of non-bison ungulate dentitions likely results from the rarity of kill site-related bone concentrations; pronghorn kill sites are known, for instance (e.g., Fenner, 2009), but I am unaware of, on one hand, deer kill sites. Bighorn sheep kill sites or closely related deposits such as camp sites where remains from one or more kills of (multiple?) individual bighorn have been accumulated and deposited are, on the other hand, known (e.g., Driver, 1982; Fisher and Valentine, 2013; Frison, 1985; Grayson, 1988; Hughes, 2004; Rapson, 1990; Thomas and Mayer, 1983). Detailed studies of zooarchaeological bighorn sheep dentitions may be rare because little is known about tooth eruption schedules and wear patterns for this taxon and its North American congeners (e.g., Deming, 1952; Hemming, 1969). Detailed study of Holocene (last 11,700 years) bison dentitions recovered from North American archaeological deposits has revealed much about tooth development, eruption, and wear in this taxon that was not previously known among wildlife biologists (see Todd et al. (1996) for a brief history). This suggests similarly detailed study of prehistoric remains of other ungulate species may be equally revealing. In this paper I describe age-related dental phenomena observed in a large collection of prehistoric bighorn sheep teeth recovered from a deposit in eastern Washington state. Although the collection is in some ways less than ideal, the same could be said for the first collections of

Paleozoologists have long examined teeth because dentitions comprise one of the most taxonomically diagnostic skeletal parts and also because teeth tend to preserve well in the fossil record (Hillson, 2005). Further, teeth often reveal details about the ecology (e.g., Fortelius and Solounias, 2000; Rivals et al., 2007) and demography (e.g., Kurtén, 1983) of the represented species. Zooarchaeologists, those who study animal remains recovered from archaeological deposits, have found, like wildlife scientists, that knowing the relationship between the dental eruption schedule or stage of tooth wear and reproductive season of a species will indicate the season when prehistoric people hunted (and killed) a species (e.g., Stiner, 1991; Todd et al., 1990, 1996). In North America, a great deal of the latter kind of research has involved bison (Bison spp.), a taxon the remains of which are common in many archaeological deposits, particularly those in the Plains states and provinces where locations known as kill sites contain remains of tens and sometimes hundreds of individual bison (e.g., Frison, 1973, 1974; Reher and Frison, 1980; Speth, 1983; Wilson, 1988). Kill sites often produce large samples of dentitions conducive to detailed study of numerous attributes of teeth (e.g., Todd et al., 1996). The remains of non-bison ungulates are also often found in archaeological deposits across North America (e.g., Frison, 2004). Species represented by zooarchaeological remains include deer (Odocoileus virginianus, O. hemionus), elk or wapiti (Cervus canadensis), pronghorn (Antilocapra americana), and bighorn sheep (Ovis canadensis).

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jasrep.2017.07.003 Received 26 May 2017; Accepted 5 July 2017 2352-409X/ © 2017 Elsevier Ltd. All rights reserved.

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Zooarchaeological Laboratory and are curated there. The systematic paleontology and taphonomy of the collection is described elsewhere (Lyman, 1995). Briefly, > 2500 bones and teeth of bighorn sheep make up the majority of the collection. This is an order of magnitude more bighorn remains than in any other zooarchaeological collection in the region (Lyman, 1995; Parks, 2000). The regionally uniquely large collection of MCC bighorn remains contains a few dental pathologies and a single skeletal pathology, suggesting the represented herd was relatively healthy (Lyman, 2010). Dental enamel hypoplasias were observed in some of the permanent lower molars and will be described in detail elsewhere. It suffices here to note that of 121 permanent mandibular molars in the collection (m1 + m2 + m3), 46 (38%) display hypoplasias. The calendric age or ages of the bighorn remains from MCC are unknown given the mixed condition of the sediments. Based on temporally diagnostic projectile point styles (Chatters et al., 2012; Lohse and Schou, 2008) recovered from the same disturbed sediments as the bighorn remains, the latter date to the Holocene epoch or last 11,700 years. Given the human occupational history of eastern Washington, it is likely that the majority of the bighorn remains date to the last 4000 years or so when the human population was relatively high and occupation of the area within a 100 km radius of the cave is well documented (Ames et al., 1998; Andrefsky, 2004; Chatters, 1995; Chatters et al., 2012; Prentiss et al., 2006). Regardless of the likelihood that particular bighorn specimens are of different calendric ages, they likely all derive from the same local genetic lineage. The known prehistory of bighorn in eastern Washington indicates the taxon has been present there at least since 10,500 years ago (Lyman, 2009). Early suggestions as to the distribution of bighorn prior to colonization of the area by Europeans (e.g., Buechner, 1960) are now known to be inaccurate based on zooarchaeological evidence (Lyman, 2009). Bighorn in Washington were originally thought to represent two subspecies, Rocky Mountain bighorn (O. c. canadensis) and California bighorn (O. c. californiana) (Cowan, 1940; Johnson, 1983). Following recent suggestions regarding the taxonomy of North American bighorn (Loehr et al., 2006; Ramey, 1999; Wehausen and Ramey, 2000), Holocene-age bighorn in eastern Washington likely were Rocky Mountain bighorn. Determination of the exact number of individual bighorn represented by the MCC materials is difficult given the temporally mixed nature of the specimens (Lyman, 1995, 2009, 2010). The inventory of mandibular molars in the collection indicates a minimum of 48 individual bighorn (Table 1). Some variability in the size of the MCC teeth and bones might be attributed to sexual dimorphism (larger males, smaller females) (Krausman and Shackleton, 2000; Shackleton, 1985). No sufficiently complete horn cores or innominates are in the collection to assess sex ratios. Cowan's (1940) early observations on the ontogenetic age of individuals when teeth erupted became a bit more detailed over the

bison (e.g., Reher, 1974) and pronghorn (e.g., Nimmo, 1971) dentitions studied by archaeologists. The large number of bighorn dentitions discussed here reveals patterns in several ontogenetic variables that might either not be apparent or be considered idiosyncratic in samples of smaller size. The sample also allows assessment of whether certain dental variables demonstrated to correlate with ontogenetic age in other artiodactyls also correlate with age in bighorn sheep. This paper represents an early step toward learning about bighorn tooth eruption and wear as variables that will be analytically useful to paleozoologists and perhaps also wildlife biologists. One might argue that the best means to establish ontogenetic dental criteria is to inspect numerous dentitions of modern bighorn of known ontogenetic age. Although such an approach merits serious consideration, there are three reasons to at least initially take the different approach followed here. First, two of four metric attributes commonly recorded on zooarchaeological specimens can only be taken on loose teeth, that is, teeth no longer firmly set in mandibles. Virtually all collections of modern bighorn dentitions are made up of teeth firmly embedded in mandibles, and curators typically do not want those mandibles cut apart to expose teeth. Second, as will become clear, wear stages evident on the occlusal surface of a tooth are at best ordinal scale measures of ontogenetic age, whereas crown heights (the recording of which requires the tooth to be separate from the mandible) seem to more closely approximate interval-scale measures of ontogenetic age. Crown heights must be measured on teeth not embedded in mandibles. Third, mandibular tooth eruption schedules established for modern bighorn indicate all permanent teeth are fully erupted when an individual attains 36–40 months of age (see below). Tooth wear is the only variable that varies more or less continuously until the animal dies (at 10–15 years of age) and one technique for recording wear is nondestructive—record the pattern of dentine exposure on the occlusal surface. And though it seems logical to suppose that wear stages, crown heights, and molar lobe widths (another metric variables) will covary directly with ontogenetic age and each other because they do in other artiodactyls, this covariation has not been established for bighorn. My goal here is to explore the relationships between these variables on the presumption that if they are correlated, then it will be worthwhile to gather data on them in large samples of modern bighorn mandibular dentitions of known ontogenetic age. If the variables studied here do not seem to reflect ontogenetic age, there would seem to be little urgency to study them among modern specimens of known ontogenetic age. 2. Materials and methods Bighorn sheep remains were collected from deposits originally within Moses Coulee Cave (official state site number: 45DO331; hereafter MCC) in central Washington state (northwestern USA). The cave is located near the mouth of a 200+ m deep, steep walled erosional canyon (=coulee) fluvially carved into Miocene basalt bedrock during the late Pleistocene when glacial dams catastrophically failed and several hundred cubic km of water swept out of western Montana through northern Idaho and then flowed southwest through eastern Washington (Baker et al., 1987). The cave is an erosional feature of the turbulence caused by the plunge pool of water created by the flow off of the cliff above the cave. Once flood waters drained, seasonal wind and rainwater deposited fine sands, silts and clays within the cave; freeze–thaw cycles contributed roof-fall rocks to the deposit. A large portion of the sediments within the cave were mechanically removed in 1932 and deposited outside the cave by the landowner at that time to provide shelter within the cave for his cattle. Fifty-five years later two avocational archaeologists screened the relocated sediments using 1/4 inch (0.635 cm) mesh hardware cloth. Artifacts and faunal remains they recovered were donated to Archaeological and Historical Services, Eastern Washington University, Cheney. Faunal remains were subsequently turned over to the University of Missouri-Columbia,

Table 1 Inventory of bighorn sheep mandibular molar dentitions recovered from Moses Coulee Cave (45DO331). Note: premolars not included. Tootha

N of left specimens

N of right specimens

Total

Isolated m1 Isolated m2 Isolated m3 m1, m2 (associated) m1, m3 (associated) m2, m3 (associated) m1, m2, m3 (associated) ∑ m1 ∑ m2 ∑ m3

8 0 39 6 0 6 3 17 15 48

9 1 33 4 0 1 4 17 10 38

17 1 72 10 0 7 7 34 25 86

a “Isolated” indicates the tooth was not set in a mandible. “Associated” indicates the listed teeth were embedded together in a mandible fragment.

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relocated) deposit. Nevertheless, crown heights of sufficient specimens could be measured to allow the degree of correlation between tooth wear stage and crown height to be assessed. Crown heights were measured as two dimensions. First, crown height was measured as the dimension extending from the apex of the mesial column or anterior lobe (lobe 1) on the lingual side (metaconid) to the root–enamel juncture (REJ) proximal to the metaconid (e.g., Byerly, 2007; Todd et al., 1996). This dimension is referred to as the metaconid height (Fig. 4b). I followed Todd et al. (1996) and also measured from the REJ on the lingual side of the second column (lobe 2) to the apex of the second column (entoconid); this is the entoconid height (Fig. 4b). Several analysts recommend the crown height of a lower molar be measured from the lingual side of the metaconid to the groove between the roots of molars (e.g., Steele and Weaver, 2012; Upex et al., 2014). This recommendation is made on the basis that the inter-root groove “provides a constant point on all teeth from which to measure, overcoming any problems associated with locating a consistent reference point on the variable REJ” (Upex et al., 2014:84). The inter-root groove of the MCC bighorn molars is, however, often obscured by cementum and its apex is not always accessible to calipers. Because cementum could often be removed and the REJ exposed on the MCC bighorn molars, the REJ was chosen as the landmark from which to measure crown heights. This mimics the precedent in North America which has been to measure crown heights of bison lower molars from the REJ (e.g., Todd et al., 1996), which of course does not necessarily make it right. I note, however, that crown heights thusly measured on bison lower molars have proven revealing of individual ontogenetic ages. Other measurements taken on the MCC bighorn lower molars were width at the occlusal surface of the mesial column or anterior lobe (lobe 1), and width of the distal column or posterior lobe (lobe 2) (after Todd et al., 1996) (Fig. 4a). This dimension increases with age at least during early wear stages among some ungulates (Taber, 1963), and I wanted to determine if these width dimensions were correlated with crown height and wear stage. Insofar as width of either lobe correlates with crown height of the appropriate lobe, it likely records ontogenetic age, and could be used as a substitute for crown height when the latter cannot be measured, as was the case with some of the broken and anatomically incomplete mandibular molars from MCC. It is a given that crown height decreases with age as wear of the occlusal surface of a tooth progresses. Crown heights may provide interval-scale data on ontogenetic age when those heights are tightly correlated with age in years or months (e.g., Gifford-Gonzalez, 1991; Klein et al., 1981, 1983). The occlusal wear stages for domestic sheep are ordinal scale, but have been used to estimate interval-scale ages of domestic sheep represented archaeologically by teeth (Payne, 1973, 1987). Because Payne's (1973) wear stages vary in duration, they are best treated as ordinal scale and analyzed using appropriate statistics (e.g., Spearman's rank-order correlation, or rho). Both measures of crown heights and both measures of lobe widths are continuous and metric and interval scale, and therefore analyzed using appropriate statistics (e.g., Pearson's r). Recall that my goal is to determine the nature of the relationships between wear stages, crown height variables, and molar lobe widths. Recall as well that it is logical to presume that each of these variables is correlated with ontogenetic age of bighorn sheep given the correlations between them observed in other ungulate species. To avoid circularity of argument with respect to the MCC bighorn remains, however, I cannot conclude that simply because any two of these variables are correlated in a predictable manner, they both reflect ontogenetic age. Rather, I assume that because tooth-wear stages, crown heights, and lobe widths all independently correlate with ontogenetic age in other ungulates, they correlate with age in bighorn sheep. This assumption can only be tested with bighorn dentitions from individuals of known ontogenetic age.

Table 2 Mandibular molar eruption schedules for wild North American bighorn sheep. If the listed tooth is within parentheses, that tooth is being replaced or erupting. After Taber (1963) and Dimmick and Pelton (1994). Age mos 0–1 6 12 16–24 30 36 42

Dp2 Dp2 Dp2 Dp2 (Dp2) (Dp2) p2

Dp3 Dp3 Dp3 Dp3 (Dp3) (Dp3) p3

Dp4 Dp4 Dp4 Dp4 Dp4 (Dp4) p4

(m1) m1 m1 m1 m1 m1

(m2) m2 m2 m2 m2

(m3) (m3) m3

subsequent twenty years (Taber, 1963), but the following 30 years saw little change in what we know (Dimmick and Pelton, 1994) (Table 2). A detail of potential use concerns the relationship between life expectancy of bighorn in the wild and tooth wear. Hanson (1980:59) reports that the amount of tooth wear “probably begins to affect the animal's health at about 8 to 10 years of age, because they are unable to masticate food properly.” Modern bighorn in Washington (some of which are descendants of bighorn transplanted into the state in the middle of the twentieth century) seem to have a life expectancy of 10 to 12 years (Johnson, 1983). To date, wildlife biologists have focused on annual growth increments visible on horns to ascertain the age of individual sheep (Taber, 1963; Dimmick and Pelton, 1994). Horns are, however, seldom recovered from archaeological excavations; typically teeth are recovered and occasionally horn cores. Mandibular molars were selected for study here because they are more abundant than maxillary teeth in the MCC collection and, more importantly, there is greater knowledge of the eruption history of mandibular teeth than maxillary teeth of North American native Ovis (e.g., Deming, 1952; Hemming, 1969). Some of the MCC mandibular molars were loose and others were firmly set in a mandible. Bone was cut away to expose teeth still embedded in mandibles and, as necessary, cementum was carefully removed to expose the root-enamel juncture (REJ). Records were kept of all teeth in a single mandible in the hope that some degree of correlation between eruption stage, wear stage and crown heights could be established for the teeth of a single individual. If so, this would facilitate interpretation of the much more abundant isolated teeth. However, because few mandibles with erupting teeth occur in the collection, this research avenue could not be explored in any detail. Although tooth eruption schedules are known for bighorn, wear stages are not. As a result, wear stages of the MCC lower molars were recorded according to both Payne's (1973, 1987) realistic occlusal wear stages and his schematic wear stages of exposed dentin as documented for domestic sheep (O. aries) (Fig. 1). Several wear stages documented by Payne were not observed among the MCC specimens, either because of differences in internal tooth structure between domestic sheep and bighorn or because those wear stages simply are not represented in the available sample of teeth. I argue below that the former factor seems to be at work among the MCC materials. I assigned Grant's (1982) alphabetical labels for wear stages observed on domestic sheep teeth to the MCC molars (Fig. 2). Finally, for analytical purposes, wear stages observed in the MCC sample were assigned a separate wear-stage number (MCC 1, MCC 2, etc.) such that each stage included several specimens (Fig. 2). I emphasize that the wear-stage numbers I use are not meant to supplant Payne's more detailed and more elegant system, but are only used here to facilitate analysis and discussion. Morphometric details were recorded using standard dental terminology for the cusp pattern of the occlusal surface (Fig. 3). Crown heights could not be recorded for many specimens because cusps were often broken, likely as a result of the history of the (mechanically

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Fig. 1. Payne's (1973) standard wear stages, both realistic and schematic, in Old World domestic sheep m3. A left m3 is shown. Note the ‘u’ symbol signifies intact enamel and no exposed dentin on a cusp. Also note the change and disappearance of the fossettes in the first and second columns (lobes 1 and 2). Payne's (1973) wear stages for an m1 and an m2 are the same as shown here, but without the posterior fifth cusp. Reproduced with permission of Cambridge University Press.

This stage (stages “g” and “h” of Grant (1982)) is the most frequently represented in the MCC sample (Fig. 2). Importantly, Payne (1973:289) reports that the “sequence [of wear stages] is of course not invariable, but is usually fairly closely followed.” This seems to also be the case in the MCC bighorn sheep; the majority of the lower molars seem to follow the same pattern. The second notable exception to the general sequence of dentine islands becoming connected as documented by Payne (1973) for domestic sheep concerns Grant's (1982) stage “f”, or Payne's (1973) wear stage 9 (MCC stage 4; Fig. 2) in the general sequence. Payne's (1973) general sequence indicates the dentine islands of the protoconid and the hypoconid should become connected before those of the hypoconid and hypoconulid become connected (Fig. 2). This appears to be the case in two of the m3 bighorn specimens from MCC, but in seven of the MCC m3 specimens, the dentine islands of the hypoconid and hypoconulid join before the dentine islands of the protoconid and hypoconid join. As with the fossettes, this likely has more to do with the structure of the teeth than with how the teeth wear. Nevertheless, the overall standard pattern of wear documented by Payne (1973) for Old World domestic sheep works rather well for describing tooth wear stages in native North American bighorn sheep.

3. Results 3.1. Ordinal-scale wear stages The ordinal-scale wear stages of Payne (1973, 1987) were recorded for 28 m1s, 22 m2s, and 55 m3s; wear stages could not be reliably recorded for 6 m1s, 3 m2s, and 31 m3s. Teeth for which wear stages could be recorded revealed that the MCC bighorn sheep m2 and m3 teeth wear in a sequence similar to domestic sheep, with two notable exceptions. One exception concerns the fossettes. As documented by Payne (1973, 1987; see also Grant, 1982), the prefossette (lobe 1) and postfossette (lobe 2) of domestic sheep m2s and m3s are each initially isolated single entities. As wear increases both the prefossette and postfossette become dual fossettes, the anterior member of each pair disappearing before the posterior one of the pair is worn away (Fig. 1, Payne's wear stages 11–15). No dual fossettes were observed in any of the MCC m1s, m2s, or m3s. Given that these wear stages are illustrated by Payne (1973, 1987) (but not as thoroughly or completely by Grant (1982)), I suspect either they are of brief duration on an interval scale, or the fossettes of bighorn sheep teeth do not bifurcate rootwards. The high frequency of lower molars (∑m1 + m2 + m3 = 145) and lack of any indication of bifurcated fossettes among the MCC molars suggests the latter is the more likely, that bighorn fossettes do not bifurcate rootwards. Payne's (1973:285) sequence of wear stages depends on “the extent to which the enamel has been worn away to expose dentine.” As enamel wears away, dentine is initially exposed as isolated “islands” separated by enamel ridges (Fig. 1). With further wear, the dentine islands are eventually united such that they approximate a figure 8 pattern in the lower molars (ignoring the third column of the m3). This wear stage “is the mature wear-state of the tooth” and “lasts a relatively long time” compared to the other wear stages according to Payne (1973:285, 289).

3.2. Interval-scale crown heights and widths The number of specimens that could be measured varied from dimension to dimension as a result of the fragmented condition of many of the MCC mandibular molars (Table 3). Considering m3s because they provide the greatest amount of metric data, several things can be noted. First, both metaconid height (range = 18.0–53.1 mm) and entoconid height (18.5–51.5 mm) display variability at least some of which is likely related to age, a topic discussed more thoroughly below. Second, both lobe 1 (6.48–11.74 mm) and lobe 2 (5.0–11.88 mm) widths 43

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Fig. 4. Tooth dimensions measured on lower m3. a, occlusal surface of a left m3; b, lingual side of a right m3. Table 3 Inventory of bighorn sheep mandibular molars for which crown height and crown width was measured. Note, given fragmentation of many teeth, all four dimensions could not be measured on all teeth. Dimension: tooth

m1

m2

m3

Metaconid height Entoconid height Lobe 1 width Lobe 2 width

23 20 24 28

17 15 22 19

38 33 54 55

surprisingly, metaconid height and entoconid height are correlated (r = 0.985, p < 0.0001), indicating one can stand in for the other in teeth that are anatomically incomplete as a result of fragmentation. Payne (1973) originally distinguished 16 wear stages for the m3 (Figs. 1 and 2). Grant's (1982) wear stages were a bit different than Payne's (Fig. 2). Both metaconid and entoconid crown heights of the MCC m3s vary considerably relative to wear stages when more than one specimen could be measured (Fig. 5). Further, sample sizes of measured crown heights and occlusal widths are small for some MCC wear stages (Fig. 2). These facts plus the unknown sequence of the non-standard wear stages relative to the standard wear stages suggested the prudent course was to lump various of Payne's (1973) original 16 wear stages and the MCC non-standard stages together for statistical analysis. Payne's stages 2 and 3 represent MCC stage 1; Payne's stages 4, 5 and 6 represent MCC stage 2; Payne's stages 7 and 8 represent MCC stage 3;

Fig. 2. Frequencies of occurrence of wear stages of bighorn sheep m3s from Moses Coulee Cave. All wear stages are illustrated schematically on a left m3. Left-most column, number labels for Payne's wear stages assigned by Lyman. Payne Schem. (schematic) column, schematic of standard wear stages defined by Payne (1973) (see Fig. 1). Grant column is lettered stages after Grant (1982). MCC-N column, frequency of m3 specimens from Moses Coulee Cave showing the standard wear stage of Payne (1973). MCC Nonstandard column, schematic non-standard wear stages and frequencies (in parentheses) thereof among the Moses Coulee Cave specimens. Right-most column, wear stages assigned to Moses Coulee Cave specimens for analysis (see Fig. 5).

display variability at least some of which also is likely related to age. Crown heights and crown widths are correlated for both lobe 1 (Pearson's r = − 0.862, p < 0.0001) and lobe 2 (r = −0.797, p < 0.0001). Given that crown height is known to decrease with age, it seems crown width increases with age and the latter could be used when the former cannot be measured but the latter can be. Finally, not

Fig. 3. Schematic of the occlusal surface of a left m3 with landmarks mentioned in text identified. The occlusal surface of a left m1 or m2 would look very similar but lack the posterior (third) column and hypoconulid.

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heights and lobe 1 crown widths (Fig. 5). This variability is especially obvious for Payne's wear stage 10 (MCC stage 5) the duration of which he notes is long, although some of that variability could be the result of the large number of measured teeth for that stage. Ecologists have long grappled with sorting data for a continuous variable, such as an ecological gradient, into discontinuous categories (e.g., Beals, 1969; Ludwig and Cornelius, 1987). Toward that end they have developed techniques for identifying discontinuities (or gaps) in gradients (Lambert, 2006 and references therein) under the presumption that discontinuities define group boundaries. Previous researchers have argued a “stepwise progression” rather than gradual and continuous decrease in crown heights would be “suggestive of sequential age groups” (Wilson, 1988:204). Steps are equivalent to gaps or discontinuities in the progression of decreasing crown heights. To determine if gaps or discontinuities exist in the MCC crown heights, I first ordered all m3 metaconid crown heights from greatest (youngest; 53.10 mm) to least (oldest; 18.00 mm). Then, I determined the range of crown heights from greatest to least (53.10–18.00 = 35.10 mm). Given 38 measurable m3 specimens in the MCC collection, there are 37 gaps of greater or lesser magnitude between the crown heights of any two teeth adjacent to one another in the ordering of all teeth. The range of crown heights means an average difference in two crown heights adjacent to one another in the ordered group should be about 0.95 mm (= 35.10/37). The actual or observed 37 differences between each two adjacent crown heights in the ordered assemblage were determined, and two major (≪0.95 mm) and three minor (< 0.95 mm) gaps were identified for which one or more differences in each two adjacent crown heights in the ordered group were found to exceed 0.95 mm (Table 4; Fig. 6). The two identified major gaps result in the distinction of three crown-height groups. The first group consists of 15 m3s with crown heights ranging from 53.10 to 46.52 mm. A major gap including four specimens with crown heights of 45.00 to 39.74 mm separates the first group from the second which consists of 14 specimens with crown heights ranging from 37.30 to 32.18 mm. The second major gap includes two specimens with crown heights of 28.20 and 26.30 mm. The third group includes three specimens of 20.66 to 18.00 mm. Two minor gaps split the first group of 15 teeth into three lesser groups of 1, 7, and 7 teeth; the third minor gap splits the third major group of three teeth into lesser groups of 2 and 1 teeth (Table 4; Fig. 6). The first major group of 15 teeth represents the youngest group (greatest crown heights) and might be thought of as the early prime age group; the second major group of 14 is perhaps a year or two older and can be thought of as the late prime group; the final major group seems to represent the past prime age group given the very short crown heights. This extrapolation of age groups from crown heights tends to fit the older half of a “living structure” mortality profile (Lyman, 1987; Stiner, 1991) given that as age increases (crown height decreases), abundances of individuals decrease. Given the potentially time-averaged nature of the bighorn tooth assemblage, it is significant that gaps of any kind were found. The two major gaps each have within them several specimens. This suggests that either a single mass kill event is represented, or bighorn were hunted during a very limited time of the year over multiple years. The latter seems much more likely given the mixed condition of the deposit from which the remains were recovered and the several thousand years of time represented by associated projectile point styles.

Fig. 5. Upper graph, dispersion of m3 metaconid crown heights relative to wear stages. Lower graph, dispersion of m3 lobe 1 (anterior column) widths at occlusal surface. Each dot represents a measured specimen from Moses Coulee Cave. Note that as wear progresses (see Figs. 1 and 4), metaconid crown height decreases while lobe 1 width increases, just as observed in other ungulate species.

Payne's stage 9 (Grant's stage “f”) represents MCC stage 4; and Payne's stage 10 (Grant's stages “g” and “h”) represent MCC stage 5 (Fig. 2). Both metaconid crown height (Spearman's rho = − 0.873, p < 0.0001, n = 32 specimens representing five MCC wear stages) and entoconid crown height (rho = −0.777, p < 0.0001, n = 29 specimens representing four MCC wear stages) are correlated with wear stage. Both correlation coefficients are negative because crown height decreases as the rank order of wear stage increases. Similarly, both lobe 1 width (rho = 0.848, p, 0.0001, n = 48 specimens representing five MCC wear stages) and lobe 2 width (rho = 0.821, p < 0.0001, n = 50 specimens representing five MCC wear stages) are correlated with wear stage. The dispersion of metric values per wear stage for each of the four metric variables is exemplified by the data for metaconid crown

3.3. Metric wear data related to tooth eruption The sequence of tooth eruption is related to ontogenetic age of bighorn (Table 2). Several of the MCC specimens were made up of mandibles with multiple teeth, and a couple of them had molars that were in early stages of eruption, that is, the occlusal surface of one or more molars was not yet fully erupted, meaning that the occlusal surface was not yet in the occlusal plane. I used the tooth eruption data in 45

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Table 4 Bighorn m3 metaconid crown heights (mm) ordered from greatest (youngest) to least (oldest). Gap column calculated based on row in which value occurs minus next value. See Fig. 6. Specimen

Metaconid ht

Gap

89 47 108 1 64 107 91 105 19 3 22 37 35 24 38 29 17 82 63 36 39 7 93 111 65 6 20 103 13 4 14 43 71 9 62 2 80 5

53.10 52.12 51.80 51.32 51.24 51.20 50.50 50.10 49.06 48.8 48.68 48.6 48.00 47.38 46.52 45.0 42.7 41.50 39.74 37.3 37.00 36.7 36.50 36.20 35.62 34.8 34.76 34.04 33.6 33.10 32.7 32.4 32.18 28.2 26.30 20.66 19.70 18.00

0.98a 0.32 0.48 0.08 0.04 0.70 0.40 1.04a 0.26 0.12 0.08 0.60 0.62 0.86 1.52b 2.30b 1.20b 1.76b 2.44b 0.30 0.30 0.20 0.30 0.58 0.82 0.04 0.72 0.44 0.50 0.40 0.30 0.22 3.98b 1.90b 5.64b 0.96a 1.70a

Table 5 Metric data (mm) for two incompletely erupted m3s. Estimated age

Metaconid ht

Entoconid ht

Lobe 1 width

Lobe 2 width

24–26 months 30–32 months

50.01 51.32

46.70 47.28

6.48 7.54

5.00 5.06

in the other the m3 has not quite fully emerged and the cusps are not yet completely in the occlusal plane (estimated age 30–32 months). Metric data for each align with these estimated ages (Table 5). Crown heights are greater in the younger specimen, contrary to expectations, whereas lobe widths are greater in the older specimen, as expected. However, differences in all dimensions are so small that little confidence can be placed in suggesting, on the basis of the measurements, that crown heights do not reflect age whereas lobe widths do. 4. Discussion Knowing the ontogenetic age of individual bighorn sheep in a zooarchaeological collection would reveal such things as season of hunting, herd demography (mature rams only, ewes and immature lambs, or all ages and both sexes), and possibly selective procurement and processing. Previous research by a zooarchaeologist determined the ontogenetic ages at which fusion of particular long bone epiphyses occurs in bighorn (Walker, 1987). Ontogenetic age determination of bighorn is clearly important to wildlife scientists (e.g., Geist, 1966; Turner, 1977). And yet, so far as I can determine, neither group of researchers has progressed very far in their efforts to refine the tooth eruption schedule or explored the utility of tooth wear patterns among bighorn sheep despite proven success with North American bison in such endeavors. The Moses Coulee Cave bighorn sheep dentitions provide necessary insights to eruption and wear patterns among this species. Ordinal-scale wear stages documented for domestic sheep mandibular molars work well as recording devices for documenting bighorn sheep mandibular molars. Metaconid crown height, entoconid crown height, lobe 1 width, and lobe 2 width all correlate with wear stages. All of these variables are known to covary with ontogenetic age in other ungulate species. Insufficient mandibles with partially erupted dentitions exist in the Moses Coulee Cave assemblage to determine correspondence between the known modern mandibular tooth eruption sequence and wear stages, crown heights, and lobe widths. Nevertheless, statistically significant relationships between variables assumed to reflect ontogenetic age indicate detailed study of a large sample of modern bighorn sheep mandibles of known ontogenetic age would provide paleozoologists and neozoologists with a valuable analytical tool. And, given the results reported here, it would seem worthwhile to examine a large collection of bighorn mandibular dentitions collected from individuals of known ontogenetic age to facilitate development of that analytical tool.

Note: average gap = 0.95 mm. a Gap. b Major gap.

Acknowledgments Thanks to Jerry R. Galm and Stan Gough of Archaeological and Historical Services, Eastern Washington University, for providing me with the Moses Coulee Cave faunal collection. Michael C. Wilson provided encouragement. Thanks to Jonathan Driver and Bryan Hockett for their suggestions.

Fig. 6. Metaconid crown heights of bighorn m3s ordered from greatest (youngest) to least (oldest). See text for discussion of how gaps were identified, and Table 4 for data.

Table 2 to assign an ontogenetic age to the mandibular dentitions with not yet fully erupted molars, and then aligned those ontogenetic ages with the metric data for the included teeth. Unfortunately, only two mandibular dentitions had molars or premolars not yet erupted or still erupting and also retained an m3. Both represent Payne's (1973) wear stage 2, or Grant's (1982) stage “b” (Fig. 2). In one the m3 is just emerging through the mandible (estimated age 24–26 months) whereas

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