Hypsodonty in Pleistocene ground sloths was reported by Bargo et al. (2006), but no data was provided for megalonychids. Herein, hypsodonty indices (HI) are presented for 22 Megalonyx specimens (mean = 1.06, SD = 0.10), and statistical analysis suggests that there were no significant changes in HI during the ontogeny or phylogeny of the genus during the Pleistocene.

Introduction

In their recent paper Bargo et al. (2006) calculated hypsodonty indices (HI) for eleven taxa of Pleistocene ground sloths (Xenarthra: Tardigrada). Hypsodonty, or tooth crown height, has been explored for a number of mammal groups (see Bargo et al. 2006 for a list of references), but sloth teeth, like all xenarthran teeth, have no enamel, and hypsodonty is likely affected by a number of factors. Bargo et al. (2006) reviewed several such factors, including diet, habit, and habitat preference. However, while the taxa analyzed included mylodontids and megatheriids from North America and South America, Bargo et al. (2006) did not include megalonychids, citing inaccessibility to material.

Within the Xenarthra, there are several families of ground sloths, including Mylodontidae, Megatheriidae, and Megalonychidae (McKenna and Bell 1997). Within the Megalonychidae, the genus Megalonyx comprises a chronospecies from the late Hemphillian through late Rancholabrean North American Land Mammal Ages (NALMA) of the Pliocene and Pleistocene Epochs (McDonald 1977). During the Pleistocene the chronospecies was represented by Megalonyx leptostomus Cope, 1893 during the late Blancan, Megalonyx wheatleyi Cope, 1871 during the Irvingtonian, and Megalonyx jeffersonii (Desmarest, 1822) during the Rancholabrean (Bell et al. 2004). An overall increase in size and a few qualitative character changes (see McDonald 1977) appear to account for the differences among the three species of Pleistocene Megalonyx known from over 152 sites in North America (McDonald 2003) from Florida (Hulbert 2001) to Alaska (Stock 1942).

It is the purpose of this paper to present hypsodonty indices for three species of Megalonyx (Xenarthra: Megalonychidae) to compliment the work of Bargo et al. (2006) and add to the known database of hypsodonty indices for Pleistocene ground sloths. Additionally, HI values in Megalonyx are compared across space and time to assess any changes in hypsodonty.

Materials and methods

Mandibular elements (see Table 1) were measured with Mitutoyo MyCAL 0–8”/0200 mm digital calipers to the nearest 0.1 mm. Hypsodonty index (HI) was calculated as in Bargo et al. (2006) using depth of the mandible (below the third molariform tooth) divided by the length of the molariform tooth row (Fig. 1) and multiplied by 100. In specimens for which both sides of the mandible are intact, HI was calculated for each side, and HI for the individual was then determined by adding HI of both sides and dividing by two.

To determine if hypsodonty values changed during the ontogeny or phylogeny of Megalonyx, mean HI was compared for three subsets of data as follows:

(1) Ontogeny: HI for juveniles from the Blancan of Florida was compared to HI for adults from the Blancan of Florida. Juveniles and adults were compared from the same time and region to minimize the effects of temporal and geographic variation, respectively. Results were statistically analyzed using a Kruskal-Wallis one-way analysis of variance.

(2) Temporal: HI for specimens from the Blancan, Irvingtonian, and Rancholabrean were all compared using a Kruskal-Wallis one-way analysis of variance.

(3) Geographic: HI for specimens from the Irvingtonian NALMA: the early Irvingtonian Port Kennedy Cave, Montgomery County, Pennsylvania; the late-middle Irvingtonian McLeod Mine, Levy County, Florida, and the late Irvingtonian Camelot local fauna, Dorchester County, South Carolina were all compared using a Kruskal-Wallis one-way analysis of variance.

Results

Table 1 provides the list of Megalonyx specimens studied, measurements and HI obtained. Table 2 provides means and standard deviations for three sets of Megalonyx samples. HI values ranged from 0.95 to 1.17, mean = 1.05, SD = 0.06, n = 22. The mean HI value of Megalonyx is closest to that of Megatherium americanum (mean HI = 1.02) reported by Bargo et al. (2006). Statistical analyses revealed no differences in HI for Megalonyx across ontogeny, time, or space (see Table 2):

(1) Ontogeny: juveniles from the Blancan of Florida: mean HI = 1.06, SD = 0.10, n = 3; adults from the Blancan of Florida: mean HI = 1.10, SD = 0.04, n = 2; results of comparison of both samples using Kruskal-Wallis test: H = 0.3333; df = 1; p = 0.5637.

(2) Temporal: specimens from Blancan: mean HI = 1.08, SD = 0.07, n = 5; specimens from Irvingtonian: mean HI = 1.05, SD = 0.06, n = 11; specimens from Rancholabrean: mean HI = 1.00, SD = 0.05, n = 2; results of comparison of all three samples using Kruskal-Wallis test: H = 3.5528; df = 2; p = 0.1692.

(3) Geographic: specimens from Port Kennedy: mean HI = 1.06, SD = 0.10, n = 3; specimens from Camelot: mean HI = 1.04, SD = 0.05, n = 4; specimens from McLeod: mean HI = 1.05, SD = 0.05, n = 4; results of comparison of all three samples using Kruskal-Wallis test: H = 0.0530; df = 2; p = 0.9738.

Fig. 1.

Measurements used to calculate hypsodonty index (HI) in Megalonyx. HI is calculated as the depth of the mandible (DM), usually measured below the third molariform tooth (see B), divided by the length of the tooth row (LTR; see A), and multiplied by 100. Specimen displayed is AMNH F:AM 99187—mandible Megalonyx wheatleyi from the middle Irvingtonian McLeod Mine, Florida, in occlusal (A) and lateral (B) views.

Table 1.

Hypsodonty indices of Megalonyx. HI = mandibular height/tooth row length times; 100. All measurements are in millimeters (mm). See note in Discussion regarding Megalonyx wheatleyi/jeffersonii. Abbreviations: BLA, Blancan; DM, depth of the mandible; HI, Hypsodonty Index; IRV, Irvingtonian; LTR, length of the molariform tooth row; NALMA, North American Land Mammal Age; RLB, Rancholabrean.

Table 2.

Mean values of Hypsodonty index (HI) for Megalonyx from various Pleistocene sites. Data subsets represent ontogenetic, temporal, and geographic variation. Abbreviations: n, sample size; SD, standard deviation.

Discussion

Specimens from the late Irvingtonian Camelot local fauna from Dorchester County, South Carolina deserve a special note here. In this paper I refer to the sample as Megalonyx wheatleyi/jejfersonii because specimens show characters of both M. wheatleyi and M. jeffersonii as described by McDonald (1977). The Camelot population was apparently in a state of transition within the chronospecies. The detailed morphometries of this sample is beyond the scope of this paper, but a detailed description of the Megalonyx elements from Camelot forms the basis of the author's Ph.D. dissertation and will, therefore, be presented in another paper.

Fig. 2.

Drawing of Megalonyx skull, based on Leidy (1855: pls. 1 and 3) showing upper dentition in left lateral (A) and occlusal (B) views. Positions of caniniforms and molariforms are indicated. Not to scale.

Mean HI for Megalonyx was 1.05, close to HI (1.02) for Megatherium americanum reported by Bargo et al. (2006). These represent the highest mean HI values for all of the ground sloths studied. However, as Bargo et al. (2006) indicated, determining the reasons for increased hypsodonty in these taxa is problematic as there are likely several contributing factors. In fact, the comparability of HI in different ground sloths is questionable because dental formulas vary among taxa. For example, LTR is different for Megatherium (with four lower molariforms) and Megalonyx (with three lower molariforms and a longer diastema); furthermore, tooth morphology differs among various ground sloth taxa (Gerardo De Iuliis, personal communication 2008). See also Fig. 2 for an illustration of the skull and upper tooth morphology of Megalonyx. Accordingly, similar HI values for Megatherium and Megalonyx do not necessarily indicate similar diets. Still, Megatherium was not a grazer, but “… better suited for consuming a variety of turgid or moderate to soft tough food items.” (Bargo et al. 2006:57). Furthermore, Megalonyx is generally considered to be a browser (Kurtén and Anderson 1980; Schubert et al. 2004). A recent isotope study by Kohn et al. (2005) supported a forest-dwelling, browsing niche for Megalonyx.

The analysis of HI over space and time revealed no significant change in hypsodonty in Megalonyx. Admittedly, sample sizes were small for some subsets of data (e.g., n = 2 for Rancholabrean specimens and for adults from the Blancan of Florida). However, sample sizes reported in Bargo et al. (2006) were also relatively small (five samples where n = 5). Still, the prevailing trend appears to be that there was no change in hypsodonty during the ontogeny or phylogeny of Megalonyx.

Fig. 3.

Comparison of molariforms of two taxa of Megalonyx from South Carolina. A. SCSM 2006.1.101, Megalonyx leptostomus from the late Blancan Walrus Ditch Local Fauna near Summerville, in Dorchester County, South Carolina, in occlusal (A₁) and lateral (A₂) views. B. SCSM 2004.1.26.2, Megalonyx wheatleyi/jeffersonii from the late Irvingtonian Camelot Local Fauna near Harleyville, in Dorchester County, South Carolina, in occlusal (B₁) and lateral (B₂) views. While M. jeffersonii is larger than both M. leptostomus and M. wheatleyi, hypsodonty indices were statistically the same in all species.

Mean HI values for juveniles (1.06) and adults (1.10) of Megalonyx from the Blancan of Florida were not significantly different. The morphology of juvenile Megalonyx molariforms is different than that of adults in that juvenile molariforms are broader at the base than the occlusal surface. However, HI was measured using the length of the molariform tooth row, which is correctly assessed at the level of the alveoli (i.e., the base of the tooth). It appears that whatever the factors contributing to hypsodonty, there was little or no change during the ontogeny of Megalonyx.

Mean HI values for Megalonyx from the Blancan (1.08), Irvingtonian (1.05), and Rancholabrean (1.00) were also not significantly different. As stated before, the primary change in the chronospecies over time was an increase in size (McDonald 1977). Apparently, changes in the mandible and teeth remained proportional (see Fig. 3). The fact that mean HI values remained statistically the same from the Blancan through the Rancholabrean can be added to other morphological similarities in the chronospecies of Megalonyx that suggest a revision of taxonomy may be in order. The number of species and rates of evolution are currently under study for this genus and will be reported in a future paper.

Finally, mean HI values for Megalonyx from the different Irvingtonian sites were not statistically different (1.06 for Port Kennedy, PA; 1.04 for Camelot, SC; 1.05 for McLeod, FL). It appears, then, that geographic variation was also not a factor affecting hypsodonty. Megalonyx had the largest geographic distribution of any North American ground sloth, ranging from Mexico to Alaska and coast to coast (McDonald 1977; McDonald et. al 2000; Woodburne 2004). More data is needed to assess HI for other specimens at the periphery of the range to see if differences do indeed exist. Hopefully, further research will shed light on the reasons for apparent fixed degree of hypsodonty in Megalonyx, and if similar patterns exist in other ground sloths.