INTRODUCTIONFor most of the Tertiary, South America was geographically and biotically isolated from other major land masses – a period of "splendid isolation" for the mammals of that continent (Simpson 1980). A diversity of endemic clades flourished during this interval and representatives of many of these still characterize South American faunas today (e.g., armadillos, sloths, opossums, marmosets, capybaras, chinchillas; Patterson and Pascual 1968; Flynn and Wyss 1998). Some of these clades left no living representatives, most notably the various groups of hoofed plant eaters that likely filled niches presently occupied in South America by deer, camels, tapirs, peccaries, and other groups (Bond 1986; Bond et al. 1995; Croft 1999). Among these extinct ungulate clades, the notoungulates ('southern ungulates') were the most speciose and morphologically diverse; the group reached its zenith in the Oligo-Miocene and was still represented by several lineages in the Pleistocene (Cifelli 1985; Marshall and Cifelli 1990; Croft 1999). One of the most successful notoungulate clades in terms of temporal range, geographic range, and abundance was the Interatheriidae (suborder Typotheria). Although this group did not persist past the Pliocene, interatheriids are the longest ranging notoungulate family and are common constituents of most late Eocene to middle Miocene faunas (e.g., Simpson 1967; Tauber 1996; Reguero et al. 2003; Hitz et al. 2006; Croft 2007; see Croft et al. 2004 and Croft and Anaya 2006 for exceptions to this pattern). Like most typotheres, interatheriids were small to medium in size (body mass < 15 kg) and all Miocene species were characterized by ever-growing/rootless (hypselodont) molars. Like nearly all notoungulates, they lacked any form of cranial appendages. They have traditionally been regarded as cursorial (i.e., adapted for running and/or moving efficiently over long distances) and as grazers (i.e., consuming mostly grass and/or other low, open habitat vegetation) (Sinclair 1909; Scott 1932; Bond 1986; Tauber 1996). The idea that interatheriids were grazers has recently been called into question based on new data from enamel microwear (Townsend and Croft 2005, in press). The locomotor adaptations of the group have yet to be rigorously investigated; several studies have included descriptions of interatheriid postcranial elements (e.g., Sinclair 1909; Stirton 1953; Tauber 1996; Shockey and Anaya, in press; Hitz et al., in press), or included interatheriids in investigations of other notoungulates (Elissamburu 2004), but none of these has investigated potential locomotor habits relative to extant mammals.
This study focuses on the postcranial morphology of a common interatheriid, Protypotherium (Figure 1). The genus occurs in various Miocene faunas in Argentina (Bordas 1939;
Bondesio et al. 1980a,
b;
Barrio et al. 1989;
Kramarz et al. 2005), Chile (Flynn et al. 2002, in press;
Wyss et al. 2003), and possibly Venezuela (Linares 2004) but is best known from the coastal deposits of the Santa Cruz Formation of Patagonia, Argentina (approx. 50-51.5° S, Santa Cruz Province).
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