Paleobiology, 29(4), 2003, pp. 506±519 A paleoecological paradox: the habitat and dietary preferences of the extinct tethythere Desmostylus, inferred from stable isotope analysis Mark T. Clementz, Kathryn A. Hoppe, and Paul L. Koch Abstract.ÐThe Desmostylia, an extinct order of mammals related to sirenians and proboscideans, are known from the late Oligocene to late Miocene of the North Paci®c. Though often categorized as marine mammals on the basis of fossil occurrences in nearshore deposits, reconstructions of desmostylian habitat and dietary preferences have been somewhat speculative because morpho- logical and sedimentological information is limited. We analyzed the carbon, oxygen, and stron- tium isotope compositions of enamel from Desmostylus and co-occurring terrestrial and marine taxa from middle Miocene sites in California to address the debate surrounding desmostylian ecol- ogy. The d13C value of tooth enamel can be used as a proxy for diet. Desmostylus had much higher d13C values than coeval terrestrial or marine mammals, suggesting a unique diet that most likely consisted of aquatic vegetation. Modern aquatic mammals tend to exhibit lower variability in d18O values than terrestrial mammals. Both fossil marine mammals and Desmostylus exhibited low d18O variability, suggesting that Desmostylus spent a large amount of time in water. Finally, the Sr isotope composition of marine organisms re¯ects that of the ocean and is relatively invariant when com- pared with values for animals from land. Sr isotope values for Desmostylus were similar to those for terrestrial, rather than marine, mammals, suggesting Desmostylus was spending time in estu- arine or freshwater environments. Together, isotopic data suggest that Desmostylus was an aquatic herbivore that spent a considerable portion of its life foraging in estuarine and freshwater ecosys- tems. Mark T. Clementz. Department of Earth Sciences, University of California, Santa Cruz, California 95064. E-mail: [email protected] Kathryn A. Hoppe. Department of Environmental Science, Policy & Management, University of California, Berkeley, California 94720. E-mail: [email protected] Paul L. Koch. Department of Earth Sciences, University of California, Santa Cruz, Califorina 95064. E-mail: [email protected] Accepted: 5 March 2003 Introduction vores in coastal ecosystems has no modern an- Mammalian herbivores are a minor com- alog, so the dynamics of ancient coastal eco- ponent of the total fauna in modern marine systems may have been very different from and coastal ecosystems, limited to just four si- those today. renian species. Nonetheless, these herbivores The feeding ecology and habitat preferences are thought to play critical roles in structuring of desmostylians are not well understood, but the species richness and productivity of these before we can begin to explore what part, if ecosystems (Bowen 1997; Peterken and Con- any, desmostylians played in past coastal eco- acher 1997). These effects may have been even systems, we need such basic autecological greater in the past, when the diversity of data. Here, we reconstruct the ecology of one large-bodied, mammalian herbivores was genus, Desmostylus, through stable isotope higher (Domning and Furusawa 1992; Aran- analysis of tooth enamel. Using carbon iso- da-Manteca et al. 1994; Domning 2001). Other topes, we assessed feeding preferences, ex- groups of mammals may have exploited the ploring levels of dependence on terrestrial abundant vegetation in shallow coastal waters versus aquatic food sources. To assess the ad- that were widespread from the Eocene aptation of Desmostylus to aquatic habitats, we through the Miocene, such as the Desmostylia, used a method that relies on contrasts in ox- an extinct group of hippo-sized mammals re- ygen isotope variability between terrestrial lated to sirenians. The coexistence of several and aquatic species. Finally, we explored dif- species of large-bodied mammalian herbi- ferences in habitation of marine, estuarine, q 2003 The Paleontological Society. All rights reserved. 0094-8373/00/2904-0005/$1.00 HABITAT AND DIETARY PREFERENCES OF THE EXTINCT TETHYTHERE DESMOSTYLUS 507 FIGURE 1. Proposed cladogram for the mirorder Tethytheria modi®ed after Inuzuka et al. 1995. and terrestrial systems by identifying differ- eria (Domning et al. 1986; Novacek and Wyss ences in strontium isotope composition 1987) (Fig. 1). The Desmostylia is composed of among taxa. two families, the Paleoparadoxidae, which possessed low-crowned or bunodont molars, Background and the Desmostylidae, which had high- The Desmostylia. Desmostylians are large, crowned or hypsodont molars (Inuzuka et al. hippo-sized mammals known to have inhab- 1995; Inuzuka 2000). Both families coexisted ited the Paci®c coast of North America and along the northern Paci®c coast of Asia and Asia from the late Oligocene (;28Ma)tolate North America until the late middle Miocene. middle Miocene (;10 Ma) (Barnes et al. 1985; Desmostylian ecology is currently a mys- Inuzuka et al. 1995; Inuzuka 2000). Several tery. Though most desmostylian fossils have characters of the skull suggest that desmos- been found in nearshore marine deposits, tylians share a common ancestry with sireni- there are occasional reports of desmostylians ans and proboscideans, which are classi®ed associated with terrestrial mammals at sites with desmostylians in the mirorder Tethyth- where the depositional environment is un- clear. Desmostylian morphology has also gen- erated con¯icting ecological interpretations. Early reconstructions portrayed desmostyli- ans with a pinniped-like posture (Fig. 2A), suggesting they were fully aquatic swimmers (Repenning 1965), but recent studies compar- ing desmostylians with extant aquatic and ter- restrial mammals have challenged this idea. Cole and Domning (1998) argued that des- mostylian postcranial elements were most FIGURE 2. Three different skeletal reconstructions of similar to those of modern terrestrial ungu- desmostylians based on morphological data and pro- lates or extinct ground sloths, and they sug- posed lifestyle. A, Cole and Domning (1998) proposed gested a mode of locomotion and a lifestyle that Paleoparadoxia exhibited a posture and joint me- chanics similar to those of extinct ground sloths and the similar to those of the semiaquatic hippopot- extant hippopotamus. B, Repenning's (1965) reconstruc- amus. A more radical interpretation was sug- tion of Desmostylus, with a posture similar to that of modern otariid pinnipeds, was based on interpreting gested by Inuzuka (1984). Unlike other mam- the fore- and hindlimbs as modi®ed into ¯ippers. C, mals, which hold their limbs directly under Inuzuka (1984) proposed a completely new style of pos- the body, Inuzaka's reconstructed desmosty- ture for Desmostylus, termed herpetiform, which was thought to enhance stability in high-wave-energy envi- lians had a ``herpetiform'' or sprawling pos- ronments. ture similar to lizards, which would have pro- 508 MARK T. CLEMENTZ ET AL. vided a high degree of stability against lateral ences in photosynthetic physiology, sources of forces, such as those produced by waves in ®xed carbon, and environmental conditions. nearshore ecosystems. It is unclear which (if In terrestrial ecosystems, differences are re- any) interpretation is correct. lated to photosynthetic pathway (O'Leary Most interpretations imply some use of 1988), resulting in relatively high d13Cvalues aquatic habitats by desmostylians, but the ex- (213 6 2½) for plants using C4 photosynthe- tent to which they foraged within these eco- sis (e.g., warm-climate grasses), low d13C val- systems is uncertain. Cranial and dental mor- ues (227 6 3½) for C3 photosynthesizers phology, as well as their phylogenetic posi- (e.g., trees, most shrubs, herbs, and cool-cli- tion, suggests that desmostylians were prin- mate grasses), and d13C values that can be any- cipally herbivores, though at least one study where between these extremes for plants us- has proposed that they consumed mollusks ing CAM photosynthesis (e.g., most succu- (McLeod and Barnes 1984). Their northern lents). In aquatic systems, environmental con- geographical distribution and the deposition- ditions (dissolved [CO2], mixing of the water al settings of their occurrences led Domning column, nutrient supply, etc.) have a stronger et al. (1986) to conclude that desmostylians in¯uence on primary-producer d13C values, primarily ate marine vegetation (e.g., macroal- creating differences in mean d13C values for gae, seagrass) in cold, marginal marine envi- kelp (217 6 4½), seagrass (210 6 3½), near- ronments. However, if desmostylians were ca- shore and offshore marine phytoplankton pable of terrestrial locomotion (Cole and (220 6 2½, 223 6 2½, respectively), and Domning 1998), they could have come ashore freshwater vegetation (226 6 7½) (Osmond to feed on terrestrial plants, ®lling an ecolog- et al. 1981; Boon and Bunn 1994; Hemminga ical niche similar to that of the hippopotamus. and Mateo 1996; Rau et al. 2001; Ravens et al. Thus, even the dietary preferences of this ex- 2002). tinct group of mammals are unresolved. Many studies have exploited these differ- Carbon Isotopes and Foraging Preferences. ences in primary-producer d13C values to trace Analysis of carbon isotopes in biogenic sub- the foraging habits of terrestrial and marine strates (e.g., bone or enamel carbonate, colla- consumers (Cerling and Harris 1999; Cle- gen) has proven useful for paleodietary recon- mentz and Koch 2001) (see Table 1). Within