Brief communication : maxillary lateral incisor morphology and uncommon trait expression : a case study from prehistoric Paa-ko , New Mexico

Prehistoric American Southwest exhibits a high frequency of dental morphological variability. This high variability may be the result of gene flow and subsequent genetic drift occurring in early periods (pre CE 900), though few studies report on dental variability in later periods. Morphological traits of the maxillary lateral incisors were analyzed from the Pueblo IV site of Paa-ko, New Mexico (CE 1300-1425) yielding high frequencies of four traits (shovel, double shovel, tuberculum dentale, interruption groove) and uncommon variants (barrel-shovel, triform, pegshaped). Lateral incisor morphology is underrepresented in the literature but could be useful in determining population migration and affinity.

Dental morphological traits, such as shoveling, tuberculum dentale, and interruption groove, reflect population affinity and movement (Scott and Turner, 1997).Within the New World, the greatest dental morphological variability is found in the prehistoric American Southwest (Scott et al., 1983;Bailey-Schmidt, 1995;Scott and Turner, 1997;McClelland, 2003;Kuba, 2006).This high variability may be a result of migration and genetic drift, as introduction of new genes and subsequent population isolation changes the expression of dental traits (LeBlanc et al., 2008).Mitochondrial DNA evidence from the prehistoric Southwest region suggests there was migration followed by population isolation in several areas, including the New Mexico Pueblo region (Malhi et al., 2003).Archaeological evidence indicates migration into the Southwest from surrounding areas prior to the Pueblo IV period (CE 900-1500; Wilcox and Haas, 1994;LeBlanc, 1999;Malhi et al., 2003;LeBlanc et al., 2008) was likely a response to resource unpredictability caused by warm droughts and high seasonal resource stress (Dean, 1996;Benson et al., 2007).Unfortunately, few bioarchaeological studies dealing with dental morphology in the later Pueblo IV period have been produced, limiting our ability to address this pre-contact period of population movement.This study interprets dental morphology of one tooth, the maxillary lateral incisor, from the site of Paa-ko, New Mexico in order to add to our understanding of Southwest dental morphological variability.
Expressions of discrete dental traits are used to determine biological distance, population movement, and evolutionary trends (Turner et al., 1991;Scott and Turner, 1997;Irish and Guatelli-Steinberg, 2003).Maxillary lateral incisors, having the highest amount of variation in trait expression of all tooth types, is a key tooth for understanding genetic relationships and population comparisons (Turner et al., 1991;Bailey-Schmidt, 1995;Scott and Turner, 1997).Scott and Turner (1997:32) describe categories of discrete trait variation in maxillary lateral incisors, including barrel-shovel, triform, and peg-shaped or conical incisors, which deviate from the more prevalent morphological forms of shovel and tuberculum dentale.
Studies on dental morphology throughout the prehistoric American Southwest reveal higher variability of maxillary lateral incisor trait expression than any other location worldwide, particularly for shovel, barrel shovel, and full expression of tuberculum dentale and its variants (Sofaer et al. 1972;LeBot and Salmon 1977;Turner and Swindler 1978;Kieser and Preston 1981;Bailey-Schmidt, 1995;Scott and Turner 1997;Burnett and Weets 2001;McClelland 2003;Bollini et al., 2008;LeBlanc et al., 2008).This study analyzes the frequency of maxillary lateral incisor traits from the prehistoric site of Paa-ko, New Mexico (CE 1300-1425), adding to the literature on Pueblo IV Southwest dental morphology.

MATERIALS AND METHODS
Skeletal remains analyzed for this study were recovered from Paa-ko, New Mexico between 1935 and 1937 as a joint venture of the Museum of New Mexico, the School of American Research, and the University of New Mexico, with funds allotted by the Works Progress Administration (Lambert, 1954).
Paa-ko is located between Albuquerque and Santa Fe, New Mexico (Lambert, 1954).Occupied from CE 1300-1425, Paa-ko was established through population migration during the Pueblo IV period (Lambert 1954).study uses a population count of 178 individuals, the majority of which are infants and children (9 years of age and younger).Taphonomic processes affecting the recovered dentition include severe postmortem damage and postmortem loss, both of which impacted observation and recording.For this study, only individuals with intact, fully erupted anterior maxillary dentition were used.Dental morphology was observed and recorded for 53 maxillary lateral incisors associated with 34 individuals; nine subadults (10-17 years), and 25 adults (18 + years).Males and females were not considered separately because of low correlation between sex bias and trait expression (Scott and Turner 1997;Kuba 2006).
Dental morphology of 37 discrete and continuous dental traits were examined and scored following the extensive sequencing of Turner et al's (1991) ASUDAS to ensure accurate recognition.For this study variations of shovel, double shovel, interruption grove, and tuberculum dentale were the focus.In cases where dental attrition or taphonomic processes inhibited scoring of morphological traits, traits were recorded as missing data.

RESULTS
Fifty-three maxillary incisors from 34 individuals were examined.Table 1 shows trait frequencies.
An outline of specific migration patterns is beyond the scope of this paper because its focus remains on one population; however, a discussion is necessary to determine the high variability and rare trait expression within the maxillary lateral incisor of Paa-ko.Increased trait prevalence is most likely the result of gene flow into the Southwest followed by period of population isolation (Malhi et al., 2003;LeBlanc et al., 2008).Migration into the region, including into the San Juan Basin (Benson et al., 2007) where Paa-ko is located, occurred prior to the Pueblo IV period.Lambert (1953) argues that Paa-ko was established by communities from the west.High frequencies of morphological variation and prevalence of uncommon traits may be reflective of the broad admixture resulting from eastern migration (Malhi et al., 2003).The Pueblo pattern of large site abandon-ment followed by isolated community aggregation (Fagan, 2000) likely resulted in short term genetic drift, and contributed to the higher prevalence of rare dental traits observed within Paa-ko.
Adaptive significance can also influence the expression of certain dental traits, as functional demands impact tooth morphology (Hunter and Jernval, 1995).Shoveling and tuberculum dentale may be adapted to strengthen teeth by adding to structural durability with extra enamel (Dahlberg, 1963;Bailey-Schmidt, 1995).However, this explanation is less likely to be the case because traits   associated with advantageous adaptive changes would be seen in high prevalence throughout the region, not localized to Paa-ko.Additionally, anterior teeth are not used in primary mastication.

CONCLUSION
Prehistoric Native American Southwest exhibits high dental variability in the lateral incisors (Sofaer et al., 1972;Bailey-Schmidt, 1995;Scott and Turner, 1997;McClelland, 2003), especially at Paako, New Mexico (CE 1300-1425).High prevalence of four discreet traits (shovel, double shovel, interruption groove, and tuberculum dentale) and their morphological variants (barrel-shovel, pegincisors, triform) were observed in Paa-ko, New Mexico.The high observed variability is likely the result of gene flow into the region followed by short term population isolation prior to the Pueblo IV period (CE 900-1500).Even though maxillary lateral incisor traits are good indicators of genetic relationships and population movement (Scott and Turner, 1997),they are underrepresented in the literature.This article provides an additional prehistoric Southwest population for future research on regional behaviors, migration, and population affinity.

TABLE 1 .
Frequency of Lateral Incisor Trait DISCUSSIONPaa-ko is a quintessential example of prehistoric Southwest dental morphological variation.