Palaeogeography, Palaeoclimatology, Palaeoecology
The first application of the mesowear method to endemic South American ungulates (Notoungulata)
Introduction
Notoungulates may be the most diverse mammalian clade devoid of living representatives. They ranged in size from less than 1 kg (e.g., the interatheriid Punapithecus; López and Bond, 1995, Hitz et al., 2006) to more than 1,000 kg (e.g., Toxodon; Fariña et al., 1998); their dentitions varied from brachydont and complete (e.g., Homalodotherium) to extremely hypsodont and highly reduced (e.g., Mesotherium); and their postcrania suggest diverse forms of terrestrial locomotion ranging from cursoriality (e.g., some interatheriids; Stirton, 1953, Cifelli, 1985, Croft and Anderson, 2008) to fossoriality (e.g., mesotheriids; Shockey et al., 2007). They occurred throughout South America from the Paleocene to the Pleistocene and some 150 genera in 13 families have been described (Cifelli, 1985, McKenna and Bell, 1997, Croft, 1999).
Within the great diversity of notoungulates, a common theme is evident: the group had a remarkable propensity to evolve hypsodont (high-crowned) dentitions. This trend occurred independently in at least four clades, resulting each time in species with predominantly or wholly hypselodont (i.e., ever-growing) dentitions (Simpson, 1980, Cifelli, 1985, Bond, 1986, Croft, 1999, Croft et al., 2003, Croft et al., In press). Only among rodents have hypselodont dentitions appeared independently as many times. (The generally accepted monophyly of xenarthrans – sloths, armadillos, glyptodonts – suggests hypselodonty was inherited from a common ancestor in this group; Engelmann, 1985, Rose and Emry, 1993, Delsuc et al., 2001, Gaudin, 2003.) Unlike in rodents, cheek tooth hypselodonty became a characteristic feature in notoungulates, present in all middle Miocene and younger species; it is a remarkable example of parallel evolution. Also noteworthy is the ‘precocious’ appearance of hypsodonty in notoungulates (and various other South American mammals) by the earliest Oligocene; similar changes did not characterize the mammalian herbivores of other continents until roughly 10–15 million yeas later (Patterson and Pascual, 1968, Simpson, 1980, Pascual and Ortiz Jaureguizar, 1990, Flynn and Wyss, 1998, Flynn et al., 2003, Bargo et al., 2006, Croft et al., In press). Clearly, the factors affecting the evolution of hypsodonty (and hypselodonty) in notoungulates must be clarified in order to understand: (1) the adaptive significance of hypsodonty among mammals in general; and (2) the early prevalence of this feature among South American mammals in particular.
The correlation between hypsodonty and diet/habitat has been discussed at length and has primarily focused on ungulates. Rather than attempting to summarize this extensive body of literature in a few paragraphs, we refer the reader to a handful of recent papers that provide useful summaries and discussions of key aspects of the hypsodonty issue in extant mammals: Janis and Fortelius (1988), an overview of hypsodonty and other wear-related dental adaptations in mammals; Williams and Kay (2001), a study that uses phylogenetically independent contrasts to assess hypsodonty in ungulates and rodents; Janis (2007), an overview of the dietary history of ungulates; Clauss et al. (2007), a review of adaptations in diverse groups of browsers and grazers; and Mendoza and Palmqvist (2008), an investigation of the correlation between hypsodonty and other variables in modern ungulates. For the purposes of the present study, we highlight two important aspects of this body of research.
First, investigations of hypsodonty in extant mammals have primarily focused on ungulates, particularly cetartiodactyls and perissodactyls. Only a single study (Williams and Kay, 2001) has examined the adaptive significance of hypsodonty in non-ungulates (rodents). Other than the notoungulate studies listed below, none has attempted to explore the correlation between diet and hypsodonty in extinct groups other than cetartiodactyls and perissodactyls using a method of dietary inference that is more or less independent of hypsodonty such as carbon isotopes, microwear, or mesowear (but see Vizcaíno et al., 2006, for a study of occlusal surface area in relation to diet in some extinct xenarthrans, an exclusively hypselodont group). The value of hypsodonty for inferring diet and/or habitat preference in other groups of mammals therefore remains relatively unexplored.
Second, no investigation of the adaptive role of hypsodonty has distinguished between hypsodont (high-crowned) and hypselodont (open-rooted) species. Such a delineation is unnecessary for extant ungulates (because all have rooted teeth), but should be taken in account for groups that include both very hypsodont and hypselodont species (e.g., rodents). Given a hypsodont and a hypselodont species with equal hypsodonty indices (i.e., relative crown height), the capacity of a hypselodont species to indefinitely offset tooth wear via additional growth makes a hypselodont species functionally different from a merely hypsodont one. Not taking such morphofunctional differences into account might obscure important differences in biology.
Despite the caveats noted above, it is apparent that hypsodonty is correlated both with grazing (i.e., eating grasses) and with open habitat feeding (i.e., feeding in areas of low vegetation) in extant mammals. Of these two factors, habitat appears to be more highly correlated, at least among ungulates (Mendoza and Palmqvist, 2008). Based on analogy with living ungulates, notoungulates generally have been interpreted as grazers and/or open habitat feeders (e.g., Patterson and Pascual, 1968, Simpson, 1980, Cifelli, 1985, Bond, 1986, Kay and Madden, 1997, Shockey, 1997, Flynn et al., 2003, Croft et al., 2004). Nevertheless, recent investigations of notoungulate diets based on carbon isotopes (MacFadden, 2005) and enamel microwear (Townsend and Croft, 2008a) have indicated that some hypselodont notoungulates may have been browsers (i.e., consumers of soft leaves, twigs, and/or fruit) and/or may have fed predominantly in closed habitats. It thus appears that hypselodonty (in notoungulates) may be less correlated with diet and habitat than hypsodonty is (in extant ungulates). This also might be true for other groups of mammals that have not been closely scrutinized.
Our study uses mesowear analysis to test whether functionally hypselodont notoungulates (i.e., notoungulates in which tooth roots formed only in very old individuals, if at all) were grazing and/or feeding predominantly in open environments. Mesowear analysis is a method that gauges the relative roles of attrition (tooth on tooth wear) and abrasion (food on tooth wear) in shaping tooth cusp morphology over an animal's lifetime (Fortelius and Solounias, 2000). Broadly speaking, abrasion dominates in mammals feeding on grasses and other low vegetation, resulting in low, rounded or flat tooth cusps; attrition dominates in mammals feeding on softer leaves and twigs, resulting in high, sharp cusps. Either process can dominate in frugivores, with low, blunt cusps forming when many hard seeds are consumed. The term mesowear was coined to indicate that it reflects the diet of an animal over a period of time greater than that of enamel microwear (formed over days to weeks) but less than what could be termed ‘macrowear,’ wear dictated by population- or species-specific craniodental morphology (developed over many generations) (Fortelius and Solounias, 2000). It primarily has been used to infer diet in extinct species based on correlations between tooth cusp shape and diet in extant mammals. Mesowear has been examined in three major clades of extant ungulates (cetartiodactyls, perissodactyls, hyracoids) and has been used to infer diet in a variety of extinct groups (primarily cetartiodactyls) including antilocaprids (pronghorns), bovids (antelopes, cattle, etc.), cervids (deer), equids (horses), giraffids (giraffes), merycoidodontids (oreodonts), palaeomerycids (extinct horned ruminants), moschids (musk deer), and tragulids (chevrotains and mouse deer) (e.g., Kaiser et al., 2000, Franz-Odendaal and Kaiser, 2003, Kaiser and Fortelius, 2003, Kaiser and Solounias, 2003, Franz-Odendaal and Solounias, 2004, Semprebon et al., 2004, Mihlbachler and Solounias, 2006, Rivals and Semprebon, 2006, Kaiser and Rössner, 2007, Merceron et al., 2007, Rivals et al., 2007a, Rivals et al., 2007b, Schubert, 2007, Semprebon and Rivals, 2007, Rivals et al., 2008).
Mesowear analysis has not been applied previously to endemic South American ungulates such as notoungulates. As detailed below, we chose to study notoungulates from Salla, Bolivia, owing to the ready availability of large samples of well-preserved notoungulate specimens from this locality.
Section snippets
Salla Fauna
The Salla Fauna derives from rich late Oligocene strata exposed as badlands near the town of Sahalla (also known as Salla) in northwest Bolivia, approx. 100 km southeast of La Paz (17° 8–14' S, 67° 37–40' W; Fig. 1). The locality lies within the Eastern Cordillera of the Andes, and the badlands are exposed at elevations of ca. 3500–4000 m. The formation has yet to receive an official geologic designation, and has been referred to simply as the Salla Beds (MacFadden et al., 1985). The 540 m
Study taxa
Mesowear analysis requires a sample of at least 10 individuals, preferably more (Fortelius and Solounias, 2000). We therefore restricted our analysis to the three most common ungulates from Salla: a mesotheriid notoungulate (Trachytherus), an ‘archaeohyracid’ notoungulate (Archaeohyrax), and an interatheriid notoungulate (Federicoanaya) (Fig. 2). Each species is represented in our dataset by 16–21 specimens (see below).
The Salla mesotheriid was recently recognized as a new species of
Discriminant Function Analyses (DFA)
The DFA of the expanded modern ungulate dataset resulted in two canonical discriminant functions, the first of which was significant (p > 0.001, Wilks' lambda). Percentage of individuals with blunt cusps had the greatest influence on this function (loading = − 0.791); percentage of individuals with high occlusal relief had an effect nearly as great, but positive (loading = 0.753). Percentage of individuals with sharp cusps was correlated with occlusal relief but with a lower magnitude (loading = 0.594).
Inferred diets of Salla notoungulates
Taken together, these analyses suggest: (1) all three notoungulates included some grass in their diet and/or fed at least part of the time in open habitats; (2) these notoungulates were partitioning available dietary resources by foraging in different areas and/or varying the composition of their diets; and (3) Trachytherus had the most abrasive diet of the three notoungulates, and Federicoanaya had the least abrasive.
Specifically, Trachytherus appears to have been a committed grazer or to have
Conclusions
South American notoungulates represent an exceptional radiation of small to large-bodied herbivores that characterized South American faunas for more than 50 million years. The parallel acquisition of hypselodonty in several notoungulate lineages makes this clade an excellent model for testing hypotheses of the relationship between tooth crown height and diet/habitat in extant and extinct mammals. Such testing requires well-supported dietary interpretations for extinct taxa based on several
Acknowledgments
We thank F. Rivals and G. Semprebon for providing us with unpublished mesowear data and for helpful comments; B. Shockey for providing the photograph of Trachytherus in Fig. 2; B. MacFadden and R. Hulbert for access to fossil specimens at UF; and T. Matson for access to the modern ungulate specimens at the Cleveland Museum of Natural History. We appreciate the efforts of anonymous reviewers, whose suggestions improved this manuscript. This research was supported by the Case Western Reserve
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