Lucas et al., eds., 2008, Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44. 331 VARIATION AND SEXUAL SIZE DIMORPHISM IN PLEISTOCENE GROUND SLOTHS (XENARTHRA)

DONALD R. PROTHERO AND KRISTINA R. RAYMOND

Department of Geology, Occidental College, Los Angeles, CA 90041

Abstract—According to the paleontological literature, ground sloths (especially megatheres) show unusually high variability compared to other mammals. We evaluated this hypothesis by measuring all the common limb elements of the mylodont Paramylodon (=Glossotherium) harlani from the late Pleistocene Rancho La Brea tar pits in Los Angeles, and the nothrothere Nothrotheriops shastensis from Rancho La Brea and also from late Pleistocene San Josecito Cave, Nuevo Leon, Mexico. We find no evidence of unusually large variability (as measured by coefficient of variation and other statistics) in any postcranial element of either of these taxa. This is consistent with the fact that some megatheres (such as Eremotherium laurillardi) show clear evidence of sexual dimorphism in size and shape, but P. harlani skulls exhibit shape, but not size, dimorphism. We conclude that sexual size dimorphism is not as widespread in sloths as once assumed.

INTRODUCTION METHODS In recent years, the paleontological literature about ground sloths With a digital calipers and tape measure, we measured all the has concluded that they are highly variable, and that species should not unbroken adult humeri, radii, ulnae, femora, tibiae, astragali and patellae be distinguished based on small differences in size (Cartelle and of Rancho La Brea P. harlani and N. shastensis in the collections of the Bohorquez, 1982; Cartelle and De Iuliis, 1995). Cartelle and De Iuliis Page Museum, and all the suitable bones of N. shastensis in the LACM (1995) measured large samples of the megathere Eremotherium laurillardi collections from San Josecito Cave (Arroyo-Cabrales and Polaco, 2003). from both North and South America. They reported (p. 833) that one of The samples from Rancho La Brea range from 10,000-40,000 years in their samples “demonstrates variation of nearly 35% in linear measure- age (Marcus and Berger, 1984), and those from San Josecito Cave range ments among both adult and juvenile members of a single population.” between 10,500 and 44,000 years old (Arroyo-Cabrales and Polaco, They show (fig. 3) this extreme variability in several different samples of 2003). The landmarks for each measurement are indicated in Table 1, or astragali of those sloths. Although they do not report the actual statistics in figure 2 of Cartelle and DeIuliis (1995). Data were processed with in this paper, we analyzed the unpublished data (Fig. 1A) provided to us Excel spreadsheets, and some of the statistics were calculated using by G. de Iuliis (pers. commun., 2006). The coefficient of variation, or CV Systat software. Results are given in Table 1 and Figure 1, and discussed (defined as 100 x standard deviation/mean) of the astragali from their below. Toca das Oncas sample, Jacobina, Bahia, Brazil, is 12.96 for the height and 11.02 for the width. This is considerably larger than the CV = 10 DISCUSSION threshold indicated by most research for the variability within a single As can be seen from Table 1, virtually all the CVs from all of the species (Kurtén, 1953; Simpson et al., 1960; Yablokov, 1974). Cartelle variables for P. harlani are less than 10, and most are in the range of 3-6 and De Iuliis (1995) suggest that this high astragalar variability is due to typical of a single species (Kurtén, 1953; Simpson et al., 1960; Yablokov, the fact that Eremotherium laurillardi shows extreme sexual size dimor- 1974). The sole exception is the midshaft depth (measured in the ante- phism in all other aspects of the skeleton. From such data, Cartelle and rior-posterior plane) of radii of P. harlani, which had a CV of 13.45. Bohorquez (1982) and Cartelle and De Iuliis (1995) concluded that other However, this feature of the bone might be expected to be highly vari- “species” of Eremotherium defined solely on size are all junior syn- able, since there is an irregular crest running along the radius that makes onyms of Eremotherium laurillardi. Our plot (Fig. 1A) does seem to the cross-section highly variable, and affects this measurement. McDonald show a very wide range of sizes for this (admittedly small) sample, and (pers. commun., 2007) suggests that the variability of this crest may be there does seem to be some clustering into larger and smaller morphs. related to the size of the muscles that attach to it, which are presumably How widespread is sexual size dimorphism in ground sloths? larger in older individuals. McDonald (2006) documented shape dimorphism in the skulls of the In particular, the large sample (n = 88) of astragali of P. harlani in mylodont Paramylodon harlani from Rancho La Brea tar pits (Stock, the Page Museum collections only show a CV of 7.13 for the proximal- 1925), Los Angeles, California. [McDonald (1995) showed that distal axis, and 6.73 for the transverse width. This contrasts with the Paramylodon is the proper generic name for this taxon, and that the CVs of 12-13 measured by Cartelle and De Iuliis (1995) for astragali of E. commonly used name “Glossotherium” is only applicable to South laurillardi. The lack of a clearly bimodal clustering is also apparent in a American material]. McDonald (2006) described two distinct morpholo- graph of these data (Fig. 1B). Clearly, the lack of size dimorphism in the gies in the skull, a “robust” and a “gracile” form, but did not assign skulls of P. harlani is consistent with all the observations of variability in genders to these two morphs, since there are no clear-cut criteria for nearly every postcranial element of this species. deciding which type of sloth skull is male and which one is female. The normal methods of calculating the significance of this differ- Although the skulls showed clear shape dimorphism, there was no sepa- ence, using analysis of variance, does not apply here, because the bones ration based on size (McDonald, 2006, fig. 4). Both robust and gracile of P. harlani are less than half the size of comparable bones of E. laurillardi. skull morphs overlapped completely in size distributions. McDonald Conventional significance testing using ANOVA requires that the samples (pers. commun., 2006) provided us with his unpublished data, and we be from the same or similar populations, and would be distorted by the calculated the CVs for all his measurements. Significantly, we found no differences in scaling between the two species. Thus, we used Levene’s CVs greater than 9 in his data (and all the rest of the CVs were in the (1960) method, as modified by Sokal and Braumann (1980) and Van normal range of 3-6). Valen (2005), for calculation of the significance of the difference between 332 TABLE 1. Measurements of sloth postcranials (in mm).

FIGURE 1. Plot of two size dimensions in mm of astragali of (A) Eremotherium laurillardi (based on data provided by G. de Iuliis) and of (B) Paramylodon harlani. Even though they are very different in overall size, and the sample sizes are different, it is clear that the P. harlani shows one continuous cluster with no apparent dimorphim, while that of E. laurillardi is widely scattered with possibly two definable clusters. two different populations of astragali of these two species. We found that the differences in variance between the E. laurillardi astragali (both dimensions) and the P. harlani astragali were significant at the 0.05 con- fidence level ( v1-v2 = 2.15 for anterior-posterior length, 3.10 for width; d.f. = 19). This is not surprising, given the huge difference in CVs. Although the total samples of the nothrothere N. shastensis are much smaller, the same trends are observed. The sample of skulls of this species is too small to determine if there are two different sexual dimorphs, but the measurements of the postcrania reveal no significant size dimor- phism (Table 1). Nearly all the CVs were in the normal 3-8 range, and only a few were greater than 10. Unusually variable dimensions included the distal end of the ulna (both dorso-ventral and lateral dimensions) and ulna, next to the radial articulation) give rise to high variability in shape as the midshaft dorso-ventral thickness of the tibia. In both of these cases, captured in our measurements. Significantly, the lowest CVs in both P. these are oddly-shaped parts of the bone, where variable ridges of bone harlani and N. shastensis consistently occur with the total length of the for muscular attachments (in the tibia) or odd articulations (at the distal bone, which is the most sensitive measure of total size, and might also be 333 expected to be highly variable given the difficulty of locating consistent extreme sexual size dimorphism, no significant size variability is ob- landmarks on the articular ends. Thus, our measurements capture some served in the large sample of the nothrothere Nothrotheriops shastensis shape variability within both species, but it is clear that the overall size and the mylodont Paramylodon (= Glossotherium) harlani from Rancho variability is much less, in contrast to the conditions observed in La Brea and from San Josecito Cave. In addition, the large sample of Eremotherium laurillardi. astragali of P. harlani from different pits at Ranch La Brea shows no To calculate the significance of this difference, we again used the significant change in size from 40,000 to 10,000 years ago. Only two Levene (1960) method, as modified by Sokal and Braumann (1980) and species were available for study in this research, so studies of large Van Valen (2005). As expected, the variances of the astragali (both di- samples of sloth bones should be conducted on any other species with mensions) of E. laurillardi were significantly different from those of N. sufficient material to see what trends are more common within the shastensis ( v1-v2 = 2.11 for anterior-posterior length, 2.48 for width; p = Xenarthra. 0.05; d.f. = 5). We also calculated the significance of the differences of variances of astragali (both dimensions) of N. shastensis vs. P. harlani. ACKNOWLEDGMENTS Given their low CVs, it would be expected that these populations would We thank C. Shaw, J. Harris, and S. Cox for access to the Page not be significantly different, and this prediction holds true ( v1-v2 = Museum collections, and X. Wang, L. Barnes, and S. Mcleod for access 0.053 for anterior-posterior length, 0.049 for width; p = 0.05, d.f. = 5). to the LACM collections. We thank G. McDonald and G. De Iuliis for their help and advice on this project, and for allowing us to use their CONCLUSIONS unpublished data. We thank G. McDonald, S. G. Lucas, and G. De Iuliis Contrary to the widely reported observation (based on the for helpful comments on the manuscript. We dedicate this paper to the megatheres Eremotherium and Megatherium) that ground sloths showed late Howard Levene and Les Marcus, who taught Prothero much about biostatistics during his time at Columbia University.

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