Almost nothing was known of the locomotor habits of notoungulates (or other endemic South American ungulates) prior to about 10 years ago, despite the presence of many excellent specimens. Since then, several studies have begun to explore the locomotor diversity in the group and the paleobiology of certain species/clades.
Shockey (1997) described notohippid postcrania from Salla, Bolivia, and suggested that some of these animals may have had significant forelimb mobility; the locomotor habits were not discussed in detail, however. Later,
Shockey (2001) suggested that a knee lock mechanism similar to those of horses might have been present in Toxodon (a toxodontid notoungulate), casting doubt on the supposed semi-aquatic habits of these Pleistocene mammals.
Elissamburu (2004) studied the late Cenozoic hegetotheriid notoungulate Paedotherium and inferred that it had both cursorial and fossorial (burrowing) characteristics, similar to some caviomorph rodents. Most recently,
Shockey et al. (2007) studied mesotheriid notoungulates and inferred highly fossorial habits for all members of the family for which adequate postcranial material is known. Other
recent observations on late Oligocene notoungulates from Salla (Shockey and Anaya,
in press) and Eocene notoungulates from Argentina (Shockey and Flynn,
2007) are forthcoming and should provide insights into the habits of more basal notoungulates.
The locomotor habits of most notoungulates remain uninvestigated, and much more information is needed to adequately characterize the roles of notoungulates in Cenozoic ecosystems. Studies of other interatheriids in particular would permit a more detailed analysis of the evolution of locomotor habits within this highly successful clade, perhaps providing insights into the group's diversity and distribution.Thanks to the studies noted above, a basic picture of notoungulate locomotor habits is emerging. The present study adds to this picture by inferring the habits of one of the most common interatheriid notoungulates, Protypotherium. Based on the analyses presented here, Protypotherium was most likely a generalized terrestrial mammal tending toward cursoriality. Its appendicular skeleton does not closely resemble that of any modern mammal, but is most similar to those of some medium-sized caviomorph rodents. Proximal and distal limb elements of Protypotherium mostly resemble those of cursorial mammals in qualitative characters, but intermediate elements are more similar to those of arboreal and semifossorial mammals. PCA and DFA indicate that appendicular bones statistically ally with those of some arboreal and semifossorial mammals. This may be a phylogenetic effect attributable to fossorial habits in ancestral interatheriids and/or notoungulates. In a phylogenetic context, the postcranial adaptations of Protypotherium are intermediate between those of more basal interatheriids and the middle Miocene Miocochilius; Interatherium appears to represent a divergent locomotor strategy within the family.