Glossotherium chapadmalense (Kraglievich, 1925) Robertson, 1976

Mcafee, Robert K., 2009, Reassessment of the cranial characters of Glossotherium and Paramylodon (Mammalia: Xenarthra: Mylodontidae), Zoological Journal of the Linnean Society 155 (4), pp. 885-903 : 896-898

GLOSSOTHERIUM CHAPADMALENSE

The South American Pliocene sloth Glossotherium chapadmalense is a curious taxon with diverse taxonomic possibilities relative to the Pleistocene forms of Paramylodon and Glossotherium . In addition to being an early species of Glossotherium , G. chapadmalense has been put forth as a potential sister taxon to Paramylodon because of the similarities with the late Blancan specimens of North America. Those shared characters have also raised the question of whether G. chapadmalense belongs as a species of Glossotherium or if it represents another genus (i.e. Eumylodon Ameghino, 1904 ). The type specimen of G. chapadmalense (MACN 8675) is known from a slightly damaged skull and mandible ( Figs 8 , 9 ), and was studied secondarily to determine its taxonomic relationship to G. robustum and P. harlani . As a result of the damage in the posterior skull, only a subset of cranial measurements were obtainable, and those measures were supplemented with data taken from Kraglievich’s (1925) study when the specimen was more complete. The data obtained are given in Table 4 with comparisons to the measurement averages for Glossotherium and Paramylodon .

Glossotherium chapadmalense is approximately similar in size to G. robustum but is smaller, which is expected as a number of sloth taxa increased in size toward the Pleistocene ( McDonald, 1977, 1997; Hirschfeld, 1985; Saint-André & De Iuliis, 2001; De Iuliis, 2006; Pujos, 2006). The posterior length of the palate also falls within the range expected for Glossotherium , but could be a reflection of the overall smaller size of the animal. Size alone is a poor character for inclusion as a species of Glossotherium , requiring further evaluation of the ratios and qualitative characters compared to those already established as generically significant.

Among the ratios related to skull width, there is a lack of clarity with overlapping ranges, with the values for G. chapadmalense either falling within the overlap or very near to the upper or lower boundary of the other genus. Placement within either genus would be questionable given the ambiguous nature of the variance in those ratio ranges. Glossotherium chapadmalense does not show the greater anterior Min, the minimum value; Max, the maximum value; X, the mean value; s, one standard deviation from the mean. width that defines G. robustum , and its related ratios are more equivalent to those of P. harlani with its narrower muzzle, as are the ratios for the posterior regions of the skull. The remaining ratios place G. chapadmalense within the ranges of Glossotherium , especially for palatal lengths ratio and the ratio for palate length versus anterior width ( Table 4). Of the upper dentition left intact, M4 resembles that of Glossotherium in the posterior lobe being quite narrow and not close to being equal in width to that of the anterior lobe ( Fig. 8B ). As a result of the damage, it was not possible to assess more of the qualitative skull characters, such as the inflation of the pterygoid sinus, the width of the parasagittal crest, or the development of the temporal fossa.

The lack of quantitative variables for the mandible in the Pleistocene forms puts the onus of identification on the qualitative characters. Of those characters put forth, G. chapadmalense resembles G. robustum in the concave border of the predental spout that provides the anterior end with a lateral flare ( Fig. 9 ). With damage to the posterior ends of the mandible, it is not possible to determine the shape of the ascending coronoid process to see if it resembles the curved shape of Glossotherium or the straight shape of Paramylodon .

Figures

Figure 8. Skull of Glossotherium chapadmalense, type specimen (MACN 8675) in lateral (A) and ventral (B) views.

Figure 9. Mandible of Glossotherium chapadmalense, type specimen (MACN 8675) in occlusal view.