Morphological Chronoclines among Late Pleistocene Muskrats (Ondatra zibethicus: Muridae, Rodentia) from Northern Florida

Quaternary Research 58, 289–295 (2002) doi:10.1006/qres.2002.2385

Morphological Chronoclines among Late Pleistocene Muskrats (Ondatra zibethicus: Muridae, Rodentia) from Northern Florida Matthew C. Mihlbachler1 Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024

and C. Andrew Hemmings and S. David Webb Department of Vertebrate Paleontology, Florida Museum of Natural History, Gainesville, Florida 32611 Received March 14, 2002

tocene into the Holocene. Presently, muskrats range as far south as southern Louisiana, central Alabama, and northern Georgia (Willner et al., 1980); however, the late Pleistocene record of Ondatra is particularly abundant in Florida (Simpson, 1930; Lawrence, 1941; Martin and Webb, 1974; Gillette, 1976) and other regions where the species no longer occurs (Lewis and Johnson, 1997). Martin (1996) interpreted the Plio–Pleistocene fossil record of Ondatra as a single species with several successive “chronomorphs” that reveal “a pattern of modest change through much of late Pliocene and Pleistocene time, followed by a rapid increase during the last 0.6 million years and a period of dwarfing during the latest Pleistocene and Holocene” (p. 431). On a smaller spatial and temporal scale, Ondatra populations are known to respond phenotypically to short-term local environmental events (Martin, 1996). Body size and other biological variables such as population density, litter size, and mortality rates vary with temperature, seasonality, water-level fluctuation, rainfall variability, and food abundance (Boyce, 1978; Willner et al., 1980; Proulx and Buckland, 1986; Virgil and Messier, 1996). This response distinguishes populations of muskrats in space and time. Similar morphological patterns appear in the fossil record and are likewise assumed to be climate dependent. Semken (1966) found a chronocline of size increase in the lower first molar (m1) of muskrats through the Pleistocene, but Nelson and Semken (1970) concluded, from a limited amount of data, that the dimensions of the m1’s from interglacial periods appeared to be out of sequence. Based on their observation that the length/width (l/w) ratio in recent times is greater in northern populations, Nelson and Semken (1970) concluded that the m1 l/w ratio was a reliable paleotemperature indicator and that shifts in m1 proportions were evidence of chronoclinal translocations in response to climate change. Lewis and Johnson (1997) found a decrease in the m1 l/w ratio through the Pleistocene–Holocene boundary at Lubbock Lake, Texas, and interpreted the chronocline as a response to the transition from a cool damp climate to a hotter and drier climate.

The muskrat (Ondatra zibethicus) is presumed to have undergone a rapid phyletic size decrease near the end of the Pleistocene. Evolutionary changes in the size of middle to late Wisconsinan (ca. 32,000–12,300 14 C yr B.P.) muskrats from the Aucilla River, Jefferson County, Florida, were reconstructed by examining length and width of the lower first molar (m1). Body mass, estimated from m1 length, was relatively stable from 32,000 to 16,000 14 C yr B.P. and decreased only slightly by 12,300 14 C yr B.P. If the size trend found in the Aucilla River material is characteristic of the southeastern United States, a body size decrease after 12,300 14 C yr B.P. is needed to explain the smaller size of modern populations. It was previously thought that the length/width (l/w) ratio of the muskrat m1 was a paleoenvironmental indicator based on its presumed correlation with latitude in modern populations. We examined the length and width of modern muskrats from several geographic regions and found only a very weak trend in the size of the m1 between northern and southern populations; however, highly significant differences were found between regions of similar latitude. Our data indicate that chronoclines in the m1 of the Aucilla muskrat material and other such documented trends among fossil muskrats have paleoenvironmental significance, but it is not yet clear which environmental variables can best be predicted from them.  2002 University of Washington. Key Words: Ondatra; muskrat; rodent; evolution; late Pleistocene; Wisconsinan; Florida; Aucilla River; chronocline. C

INTRODUCTION

At the end of the Pleistocene, many mammal species underwent punctuated shifts in body size, possibly in response to climatic changes (Marshall and Corruccini, 1978; Martin, 1996). The muskrat (Ondatra zibethicus) underwent both geographic range shifts and body mass changes from the late Pleis1

To whom correspondence should be addressed. E-mail: mihlbach@amnh. org. Fax: 212 769 5842. 289

0033-5894/02 $35.00 C 2002 by the University of Washington. Copyright  All rights of reproduction in any form reserved.

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TABLE 1 Descriptive Statistics of Lower First Molar Length (L) and Width (W) Data (in mm), Mean L/W Ratios, Estimated Mass, and Approximate Ages of the Samples in Radiocarbon Years Locality

ICH

LS

MC

SH

HMR

1A

BHC

PL

LRR

BP

3B

LR

A3

Ref. age (14 C 103 yr B.P.) N L Mean L SD L Min L Max L CV W Mean W SD W Min W Max W CV L/W Mass (kg)

2 Wis 30 7.63 0.59 5.96 8.62 7.73 3.16 0.21 2.63 3.54 6.62 2.41 1.05

1, 3 31–32 50 8.19 0.36 7.44 9.14 4.38 3.36 0.15 2.94 3.64 4.44 2.44 1.35

1 24 31 (32) 8.31 0.41 7.29 9.18 4.95 3.30 0.14 3.01 3.60 4.25 2.50 1.42

1, 4, 5 12.3–12.1 46 (61) 8.08 0.42 7.03 9.00 5.20 3.15 0.17 2.79 3.45 5.42 2.57 1.29

— Wis 16 (18) 8.61 0.32 8.06 9.22 3.75 3.43 0.22 3.07 3.92 6.31 2.52 1.61

— Wis 5 8.54 0.35 8.26 9.14 4.05 3.36 0.11 3.27 3.54 3.18 2.54 1.57

6 20.5 5 8.23 0.23 7.89 8.50 2.85 3.21 0.08 3.08 3.29 2.51 2.57 1.37

5, 6, 7, 8 12.5–12.3 4 7.74 0.60 6.92 8.24 7.81 3.18 0.27 2.85 3.50 8.35 2.43 1.10

9 Wis 3 8.23 0.45 7.90 8.74 5.51 3.38 0.15 3.25 3.55 4.60 2.44 1.37

6 16 3 8.21 0.45 7.69 8.5 5.50 3.33 0.19 3.11 3.47 5.74 2.47 1.36

6 15 2 7.78 — — — — 2.94 — — — — 2.66 1.12

— Wis 1 7.68 — — — — 2.83 — — — — 2.71 1.07

— Wis 1 7.95 — — — — 3.39 — — — — 2.35 1.21

ICH = Ichetucknee River; LS = Latvis-Simpson (8JE1500); MC = Matten-Childers (8JE 604); SH = Sloth Hole (8JE 121); HMR = material collected along the Half Mile Rise section of the Aucilla River and probably from multiple areas; 1A = unnamed site; BHC = Black Hole Cave; PL = Page Ladson (8JE 591); LRR = Little River Rapids (8JE603); BP = Bent Palm (8JE608); 3B = unnamed locality; LR = Ladson Rise (8JE602A3); A3 (area 3); N = number of observations; Min = minimum; Max = maximum; CV = coefficient of variation; mass = mass estimated from equation in Martin (1996). When the number of length and width measurements was unequal, the number of width measurements is given in parentheses. References: 1 = table 1, this paper; 2 = Martin and Webb, 1974; 3 = Mihlbachler, 1998; 4 = Hemmings, 1998; 5 = Webb et al., 1998; 6 = Dunbar et al., 1990; 7 = Newsom et al., 1993; 8 = Webb et al., 1991; 9 = Muniz, 1998.

These local observations of microevolutionary change suggest that muskrats are good paleoclimate indicators; however, more examples from other regions are needed to ascertain the reliability of the patterns thus far investigated. This paper examines chronoclinal changes in the dimensions of the m1 of late Pleistocene muskrats from several localities along a 5-mile stretch of the Aucilla River (Jefferson County, Florida) ranging in age from 32,000 14 C yr B.P. (mid-Wisconsinan) to 12,300 14 C yr B.P. (late Wisconsinan). An assemblage from the Ichetucknee River (Columbia County, Florida) is also examined. Additionally, the paleoecological significance of size change in the m1 is statistically evaluated with data on modern muskrats from various latitudes. Florida Muskrat Record The Aucilla River contains a rich record of late Pleistocene biotic and cultural remains. There are now 199 Aucilla muskrat m1 s in the collection at the Florida Museum of Natural History (Gainesville, Florida). There is only a single Holocene record of muskrats in Florida dated to 3,150 14 C yr B.P. (Bullen, 1958); however, it was found in association with cultural material and could have been transported from elsewhere. Therefore, the Aucilla River localities contain the youngest radiometrically dated records of muskrat populations in Florida. The Aucilla River is generally 1–3 m deep, and occasionally runs underground, forming a series of landlocked channels. The channel is perforated with sinkholes that were once backfilled with sediments during lower water levels during the

Wisconsinan. The recent winnowing of fine sediments has resulted in the accumulation of vertically deflated surface assemblages of fossils and artifacts lying in the bottoms of 8- to 10-m-deep depressions (Webb, 1976; Dunbar et al., 1990). Current conditions of the Aucilla River suggest that horizontal transport was minimal. (1) The Aucilla River has a shallow gradient and a low flow velocity that stops or reverses daily with the tides. (2) There is virtually no sediment load in the river. (3) The limestone bottom has jagged features that prevent horizontal movement. (4) Corresponding parts of broken bones, mastodon tusks, and ivory tool fragments have been found side by side, sometimes with fragments in the surface assemblage and other fragments within or protruding from uneroded strata. Muskrat assemblages from seven Aucilla River localities are associated with intact bone-bearing carbon-dated strata (Table 1). Previously unpublished carbon dates that are relevant are listed in Table 2. MATERIALS AND METHODS

We measured anteroposterior length and bucco–lingual width of the first lower m1 with digital calipers accurate to the hundredth of a millimeter (Table 1). Modern m1 s from 12 populations from Alabama, Indiana, Kansas, Louisiana, Maryland, Minnesota, Nebraska, New Hampshire, New York, and Pennsylvania were measured (Table 3). All modern specimens are housed in the American Museum of Natural History, New York, and the National Museum of Natural History, Washington, DC. We avoided measuring the occlusal surface because the

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LATE PLEISTOCENE MUSKRATS FROM FLORIDA

TABLE 2 Previously Unpublished Carbon Dates Associated with Pleistocene Vertebrate Fauna from the Aucilla River Given in Standard Radiocarbon Years Locality

Provenence

Age (14 C yr B.P.)

Laboratory no.

Method

Material

SH MC MC LS LS LS

Unit 210, level 14 1990 test trench—reddish brown clay/silt Level 1, trench 2 Unit 16.5 dung layer Unit 17, dung layer Unit 17, level 4, dung layer

12,180 ± 60 24,010 ± 920 23,670 ± 210 31,610 ± 240 31,140 ± 220 31,900 ± 400

Beta-119350 Beta-95136 Beta-126827 Beta-85549 Beta-119344 Beta-124753

AMS RM AMS AMS AMS AMS

mastodon bone wood turtle bone Cucurbita seed mastodon bone Cucurbita seed

LS = Latvis-Simpson; MC = Matten Childers; SH = Sloth Hole; Beta = beta analytic; AMS = accelerated mass spectrometry; RM = standard radiometric.

extent of dental wear influences the occlusal area of the tooth (Viriot et al., 1993). The base of the crown was avoided because it is often located below the surface of the bone in many adults (Galbreath, 1954; Viriot et al., 1993). We were able to maximize the sample size and minimize age-related error by measuring a point about 3–4 mm above the base of the crown. The data were subjected to statistical analyses to examine (1) the differences in the absolute dimensions (size) of the m1 (analysis of variance [ANOVA]) and (2) the differences in the relative dimensions (shape) of the m1 (l/w ratios, regression, and (analysis of covariance [ANCOVA]). RESULTS

ANOVA The modern muskrats were divided into a southern (LA and AL) and a northern group (KS, IN, MN, NE, NH, NY, PA, MD). ANOVA was marginally significant for length ( p = 0.08) and width ( p = 0.04), indicating a small latitudinal difference between northern and southern populations. We found that if we

divided the populations into three major geographical regions (northeast: MD, NH, NY, PA; southeast: AL, LA; Midwest– Great Plains: IN, KS, MN, NE) highly significant differences among the lengths and widths of the groups were found ( p < 0.0001 for length and width). The northeast group had the largest average lengths and widths (7.84 × 3.30 mm). Despite its similarity in latitude with the northeast group, the Midwest–Great Plains group had the smallest average dimensions (7.46 × 3.11 mm). Finally, the southeast group was intermediate in size (7.52 × 3.14 mm). We conclude that there is a highly significant regional effect in the m1 dimensions that is not related to latitude. Among the fossil samples, the Ichetucknee River muskrats have a mean m1 length (7.63 mm) and width (3.16 mm) that was within the range of modern populations and similar to those from Alabama and Louisiana. The majority of the Aucilla samples, however, have mean values that are larger than those of modern populations. In fact, Aucilla River muskrat teeth were significantly longer than modern muskrats of the largest of the three modern geographic groups, the northeast region ( p < 0.0001); however, the width differences were insignificant ( p = 0.30). ANOVA was continued with the Aucilla River assemblages

TABLE 3 Descriptive Statistics of Lower First Molar (ml) of Modern Muskrat Populations Region

ALb

ALm

LAb

LAm

KS

IN

MN

NE

NH

NY

PA

MD

N L Mean L SD L Min L Max L CV W Mean W SD W Min W Max W CV L/W

12 7.58 0.29 7.25 8.20 3.86 3.15 0.12 2.91 3.38 3.93 2.41

11 7.67 0.26 7.32 8.09 3.38 3.18 0.10 3.05 3.38 3.04 2.41

18 7.39 0.31 6.96 7.98 4.20 3.12 0.12 2.82 3.35 3.84 2.37

26 7.53 0.32 7.04 8.19 4.27 3.13 0.12 2.87 3.33 3.95 2.41

10 7.54 0.28 7.20 7.97 3.70 3.04 0.18 2.79 3.39 5.86 2.48

30 7.64 0.27 7.00 8.14 3.59 3.11 0.20 2.46 3.47 6.39 2.47

29 7.44 0.22 7.01 7.89 2.93 3.17 0.11 2.96 3.50 3.53 2.34

54 7.35 0.23 6.75 7.83 3.13 3.09 0.13 2.81 3.37 4.29 2.38

9 7.83 0.23 7.48 8.14 3.52 3.30 0.16 3.04 3.57 4.87 2.37

20 7.75 0.25 7.09 8.12 3.27 3.30 0.09 3.16 3.47 2.69 2.35

17 8.02 0.35 7.39 8.6 4.34 3.26 0.12 3.05 3.53 3.66 2.46

37 7.79 0.30 7.34 8.54 3.82 3.31 0.15 2.96 3.67 4.50 2.36

ALb = Bayou la Batre, Alabama; ALm = Mobile Bay Delta, Alabama; LAb = Belair, Louisiana; LAm = Morgan City, Louisiana; KS = Greenwood Co., Kansas; IN = Culver, Indiana; MN = Fort Snelling, Minnesota; NE = Brown Co., Nebraska; NH = Grafton Co., New Hampshire; NY = Greene Co., New York; PA = Pennsylvania; MD = Laurel, Maryland. Other abbreviations are as in Table 1.

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FIG. 1. Mean lengths and widths of modern and fossil muskrat populations superimposed over RMA regressions of modern and fossil muskrat m1’s. Solid circles represent fossil samples and open squares represent modern populations. Standard error bars are also shown for the fossil samples. Abbreviations are as in Tables 1 and 3. RMA = reduced major axis.

with large sample sizes (LS, MC, and SH), revealing significant differences in molar dimensions between these groups (length: p = 0.05; width: p < 0.0001).

from southern and northern regions fall on the opposite sides of a regression line, we did not find this pattern to be clear in our modern data (Fig. 1). Likewise, the l/w ratios of the modern populations showed no visible trend with latitude (Fig. 2). Although the ANOVA results found significant differences in absolute dimensions, the l/w ratios did not detect a significant difference in the proportions of specimens from the three regions: northeast, Midwest–Great Plains, and southeast (ANOVA: p = 0.291). In the fossil muskrats, there is a gradual increase in average l/w proportions from LS–BHC (32,000–20,500 14 C yr B.P) followed by a decrease at BP and PL (16,000–12,500 14 C yr B.P) and another increase at SH (12,300 14 C yr B.P) (see Table 1 for an explanation of sample abbreviations). These shifts are roughly consistent with the timing of the last glacial maximum between 22,000 and 14,000 14 C yr B.P. (Crowley and North, 1991), but they are counterintuitive to Nelson and Semken’s (1970) conclusion that glacial age muskrats should have higher l/w ratios rather than lower l/w ratios. The major shifts are entirely dependent on rather limited samples sizes at BHC, BP, and PL (N = 5, 3, and 4, respectively) and are, therefore, suspect. The simplest interpretation, based on those localities with better sample sizes (LS, MC, and SH), is a moderate increase in the l/w proportions from 32,000 to 12,300 14 C yr B.P. Finally, the Ichetucknee specimens had a low l/w ratio, the smallest absolute proportions, and were similar in size and proportion to that of modern southern muskrats.

Regression and L/W Ratios

ANCOVA

The reduced major axis slope (RMA) is calculated as the standard least squares regression slope divided by the square root of the R 2 value (correlation coefficient) and makes no assumptions about dependent and independent variables (LaBarbera, 1989). Regressions of the modern and fossil samples were both significant ( p < 0.0001) and produced similar slopes (modern = 0.48; fossil = 0.41), although the R 2 values were low (Fig. 1). In contrast to Nelson and Semken’s (1970) conclusion that muskrats

ANCOVA tests the significance of the difference in the vertical position of the regression lines of multiple groups regressed on the same variables (Packard and Boardman, 1988). It assumes that the slopes are equal. We treated width as the dependent variable and regressed it against the independent variable, length. Therefore, the null hypothesis we were testing was that there were no significant differences in length-corrected tooth width between the groups.

FIG. 2. Mean length/width ratios and standard error bars for lower first molar (ml) of fossil and modern muskrat samples. Modern samples are arranged from left to right in correspondence to latitude from north to south. Fossil samples are from left to right in chronological order from oldest to youngest. The single exception is the Ichetucknee sample of which the age is not constrained by carbon dates. Abbreviations are as in Tables 1 and 3.

LATE PLEISTOCENE MUSKRATS FROM FLORIDA

Analysis of the modern southern and northern groups indicated no significant difference (R 2 = 0.31; p = 0.184). In contrast, the grouping of the samples into three geographic regions (northeast, southeast, Midwest–Great Plains) again was highly significant (R 2 = 0.38; p < 0.0001). An analysis including the groups Ichetucknee, Aucilla, and modern muskrats did not show significant differences in length-corrected molar width (R 2 = 0.29; p = 0.2268). This result is inconsistent with the RMA regression (Fig. 1) and the l/w ratios (Fig. 2) which suggest that Aucilla m1 s are generally narrower than modern m1 s. The discrepancy can be attributed to the low correlation of length and width within and among populations, because the difference in the regression slopes of RMA and least squared regressions are greater at lower R 2 values (LaBarbera, 1989). A final ANCOVA was performed on the three Aucilla localities with adequate sample sizes. This test demonstrated that there were significant shifts through time in the length-corrected widths of the m1 (R 2 = 0.28; p < 0.0001) among Aucilla River muskrats. This result was consistent with those of the other methods that suggest a gradual narrowing of the m1 from LS (32,000 14 C yr B.P.) to MC (24,000 14 C yr B.P.) to SH (12,300 14 C yr B.P.). DISCUSSION

We were concerned that the data were distorted by thin deposits of geothite that builds up with variable thickness on fossil material in some parts of the Aucilla River (Dunbar et al.,1990). The coefficients of variation of the largest Aucilla samples ranged from 4.38 to 5.20 mm for length and from 4.25 to 5.42 mm for width. These values do not indicate that geothite deposits introduce additional variability because they are within the range of coefficients of variation in the modern samples and fall within the expected range of dental variation for mammalian species (Simpson et al., 1960). Nor was the thickness of the deposits sufficient to alter the absolute measurements. The Ichetucknee sample, which lacks geothite deposits, exhibited a degree of variability greater than do modern populations (7.63 mm for length and 6.62 mm for width). This finding is not surprising considering that the Ichetucknee River material was not collected from a spatially confined sinkhole assemblage and may be time-averaged.

293

for the Aucilla samples range from 1.61 to 1.07 kg. The Aucilla material is consistent with Martin’s (1993, 1996) claim that Wisconsinan muskrats from the Southeast were larger than in modern times, although these estimates fall within the known body mass range of modern muskrats (0.68–1.82 kg) (Nowak, 1999). Martin (1996) suggested that the size decrease that resulted in smaller modern muskrats occurred sometime after 16,000 14 C yr B.P. Our data indicate that body mass was relatively constant from 32,000 to 16,000 14 C yr B.P. The mass estimate for Page–Ladson muskrats suggests that an anomalous punctuated size decrease followed by a rapid increase occurred sometime between 16,000 and 12,300 14 C yr B.P. The sample size of Page–Ladson m1 specimens (N = 4) does not allow us to rule out a sampling error as the cause for the anomaly. The Sloth Hole sample is only a few hundred years younger, has more specimens (N = 46), and indicates that only a minor body size decrease had occurred by about 12,300 14 C yr B.P. A further size decrease must have occurred after this time to achieve the smaller body mass found in modern populations from the southeast. It is not known if Florida populations were extirpated before or after undergoing subsequent size shifts. The Ichetucknee specimens are smaller than those from the Aucilla River and it is probable that they represent a different time period. Their smaller size suggests that they may be younger than Sloth Hole, perhaps between 11,000 and 9,000 14 C yr B.P. If this age estimate is correct, the average mass estimate of the Ichetucknee muskrats, superimposed on Figure 3, would fit on the segment younger than the Aucilla deposits and older than the modern specimens. A confirmation of this age for the Ichetucknee material would serve to narrow the timing of the size decrease to very near the end of the Pleistocene or the earliest Holocene. We cannot, however, discount the possibility that the Ichetucknee specimens are older than the oldest Aucilla River material or that they are a time-averaged mixture of specimens.

Body Mass Trends For muskrats, m1 length is a good body mass indicator (Martin, 1996). The mean body mass estimates of each fossil sample (Table 1) was calculated with the exponential equation M = 0.71L 3.59 , derived from arvicoline rodents (Martin, 1993), where M represents mass in grams and L is the length of the m1. Martin (1993) estimated the mass of Ichetucknee River muskrats as 1.08 kg. The average mass estimate for Ichetucknee specimens reported here is similar (1.05 kg), and like that of modern populations from the Southeast. Mean mass estimates

FIG. 3. Plot of average body mass estimates against time of Aucilla River muskrat assemblages and modern populations from Alabama and Louisiana. The large decrease in mass at 12,300 14 C yr B.P. (PL) is possibly a sampling error (N = 4). Abbreviations are as in Tables 1 and 3.

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Significance of Molar Proportions Following Nelson and Semken’s (1970) conclusion that temporal and latitudinal chronoclines in the m1 proportions were a response to environmental variation, others (Lewis and Johnson, 1997, 2002; Johnson et al., 2000) have subsequently used the l/w ratio as a tool for diagnosing muskrat subspecies and paleoenvironmental variables, even in circumstances where sample sizes are extremely limited (N = 1–4). In our data there was extensive overlap in the dimensions and l/w ratios among individuals of different regions, indicating that single specimens and small samples cannot appropriately distinguish one population or one muskrat subspecies (sensu Lewis and Johnson, 2002) from another. Additionally, the l/w ratio is here regarded as an inferior method because it failed to find highly significant geographic trends in the modern data that were located by other statistical methods. Nonetheless, the finding of highly significant regional effects, although they are not strongly related to latitude, suggests that the dimensions of muskrat dentition contain paleoenvironmental information. Boyce (1978) found that total annual precipitation and the variation of monthly precipitation was correlated with body size in muskrats. Both are factors that influence food availability. These findings suggest that chronoclinal patterns in the sizes of Quaternary muskrats might be strongly related to changes in rainfall patterns, seasonality, or habitat quality. However, these climate variables need to be correlated with morphological variables that are commonly available to paleontologists (e.g., tooth dimensions) before paleoenvironmental inferences can be made with muskrat fossils. That objective is beyond the scope of this analysis but a larger data set on molar dimensions of modern populations with associated climate data could address this question. We must still conclude, however, that if a correlation is found, the high degree of overlap in the molar dimensions of muskrats from different regions will limit paleoenvironmental inferences to large sample sizes. In light of these findings, it is currently difficult to interpret shifts in the proportions of the Aucilla muskrats as adaptive responses to climate change. Nonetheless, chronoclinal shifts in the m1 proportions were identified by all of the statistical methods. The simplest interpretation of the data based on the largest samples (LS, MC, and SH) is that there was a gradual increase in relative m1 width through the Wisconsinan. Pollen records indicate that the climate before 30,000 14 C yr B.P. in Florida was similar to that of today but drier. The period of 30,000–14,000 14 C yr B.P. was dry and not much colder than today, although seasonal variation may have increased. The time between 14,000 and 12,000 14 C yr B.P. was characterized by mesic conditions, a warm-summer climate, and colder winters with a mean January temperature below −5◦ C (Watts and Hansen, 1988; Watts et al., 1992). The trend in the Aucilla River muskrat dentition is consistent with an interpretation of changing molar dimensions as an adaptive response to increasing seasonality and colder local conditions toward the end of the Pleistocene. Without a strong correlation of modern muskrat molar dimensions with

known environmental variable(s), however, the association of the chronocline with more specific aspects of climate change remains circumstantial. CONCLUSION

Shifts were found in the size and shape of north Florida muskrats through the Wisconsinan. Body mass estimates generated from m1 length indicate that size appears to have been relatively stable from 32,000 14 C yr B.P. to 16,000 14 C yr B.P. Size, according to samples with sufficient numbers of observations, seems to have decreased slightly by 12,300 14 C yr B.P. If further size decrease occurred synchronously throughout the southwest, our findings indicate that it must have occurred sometime later than 12,300 14 C yr B.P. It is not clear whether Florida muskrat populations experienced further size decrease at the end of the Pleistocene or if the local populations were extirpated before this occurred. In addition, shifts in the proportions of the Aucilla muskrat m1 s did occur through the Wisconsinan; however, sample size was a factor in limiting our interpretation to a simplistic increase in length/width proportions through time. Our attempt to correlate this variable with latitude in modern populations yielded very weak results. Statistical techniques such as ANOVA and ANCOVA on large samples were more effective than length/ width ratios in locating significant regional effects in the data. The most significant differences were found among regions of similar latitude. We are thus forced to conclude that the dimensions of muskrat teeth are not simple temperature indicators as suggested by Nelson and Semken (1970), but may relate to a mosaic of environmental variables such as temperature, seasonality, food abundance, food quality, rainfall variability, and other factors that determine habitat optimality for muskrats. We hesitate to ascribe paleoenviromental significant to the shifts in m1 proportions of Aucilla river muskrats but it is likely that novel interpretations of paleoenvironment made from fossil muskrats will be possible with further studies on the correlation of dental dimensions in modern muskrats and modern climate variables. ACKNOWLEDGMENTS We are especially grateful to Bill Gifford, Joe Latvis, Jack Simpson, and the many volunteers who helped out in the field. Linda Gordon (U.S. National Museum), Ryan Means (Coastal Plains Institute), and Robert Randall (American Museum of Natural History) loaned specimens or allowed access to collections. Jim Dunbar, Mark Muniz, and Dennis Ruez, Jr. provided information and comments at various stages in the progression of the manuscript. This research was funded by the Florida Department of State’s Bureau of Historic Preservation and the Society of Vertebrate Paleontologys Patterson Award, awarded to the senior author.

REFERENCES Boyce, M. S. (1978). Climatic variability and body size variation in the muskrats (Ondatra zibethicus) of North America. Oecologia 36, 1–19.

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