Late middle to late Pleistocene paleoecology and paleoenvironments in the coastal plain of Rio Grande do Sul State, Southern Brazil, from stable isotopes in fossils of Toxodon and Stegomastodon

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Abstract

Stable isotopes (δ13C, δ18O) in teeth of extinct mammals have been used as indicators of diets and climate conditions, which in turn are used for paleoenvironmental reconstructions. The first analysis of stable isotopes in Pleistocene mammals from Southern Brazil is reported here. The analyzed taxa were the notoungulate Toxodon and the proboscidean Stegomastodon from Late Middle to Late Pleistocene fossiliferous beds exposed along Chuí Creek, in the coastal plain of Rio Grande do Sul State. The δ13C isotopes indicate that Toxodon was a mixed-feeder that fed mostly on C4 plants, while Stegomastodon was a browser to mixed-feeder with preference for C3 plants, with less C4 plants in the diet compared to modern African elephants. Comparison with carbon isotopic values in fossils of these taxa from other Late Quaternary localities in South America shows an increased proportion of C3 plants in the diets of both Toxodon and Stegomastodon in higher latitudes. The values of δ18O in both taxa seem to reflect the isotopic composition of the ingested water rather than the isotopic content in the plants of which they fed on, and the observed variations are likely to mirror variations of the patterns of precipitation. Paleoclimatic inferences and possible causes for the (pseudo)extinction of these taxa in southern Brazil are also discussed.

Highlights

► Stable isotopes in mammalian enamel are tools for paleoecological reconstructions. ► Teeth of fossil herbivores from Southern Brazil were subject to isotopic analyses. ► The two analyzed taxa had browsing to mixed diets, feeding on C3 and C4 plants. ► The results suggest that a grassland environment existed in the area. ► Changes in vegetation may have played a role in the extinction of these mammals.

Introduction

The Coastal Plain of Rio Grande do Sul State (CPRS), in southern Brazil, contains fossiliferous assemblages of Pleistocene age. The most important and studied is the one found in Chuí Creek (Fig. 1). The fossil assemblage is found in a sedimentary layer exposed along the banks of the creek, and includes mostly large-bodied extinct mammals correlated to the Lujanian Stage/Age from the Pampean region of Argentina (e.g. Paula Couto and Cunha, 1965, Soliani, 1973, Lopes et al., 2009). This assemblage is a mixture of Pampean taxa (e.g. Doedicurus sp., Lestodon armatus and Mylodon darwinii) and taxa with Brazilian affinities (Eremotherium laurillardi, Protocyon troglodytes, Holmesina paulacoutoi). Besides these, there are taxa indicative of humid (Myocastor coypus, Holochilus brasiliensis) and dry/semiarid environments (Dolichotinae sp.; Microcavia sp., Catagonus stenocephalus) (Oliveira et al., 2005, Ubilla et al., 2008, Kerber et al., 2011, Lopes, in press). The taphonomy of the fossils indicates that the assemblage is parautochthonous, composed of remains reworked and mixed by fluvial dynamics (Lopes et al., 2009). ESR ages obtained from fossil teeth show that the fossil assemblage from the Chuí Creek encompasses a time interval between Late Middle (~ 226 ka BP) and Late (~ 34 ka BP) Pleistocene at least (Lopes et al., 2010).

The mixing of organisms related to different environments, the large time interval represented by this assemblage, and the reworking by fluvial dynamics, indicate that paleoenvironmental reconstructions based solely on the faunal composition of the fossil assemblage can be difficult (Lopes, in press). The climate of the South America during the Middle to Late Pleistocene was highly variable, driven by Milankovitch cycles and millennial-scale oscillations (Clapperton, 1993, Salgado-Laboriau et al., 1998, Rabassa et al., 2005, Whitney et al., 2011). Such variation is likely to have influenced the range of habitats and the distribution (and possible the extinctions) of large terrestrial mammals, but paleoenvironmental data from southern Brazil are so far based on pollen records (Behling and Lichte, 1997, Behling, 2002) and speleothems (Cruz et al., 2005, Cruz et al., 2006, Wang et al., 2007). The results of the first analyses of stable isotopes in fossil mammals from Chuí Creek, as an attempt to understand their paleoecology and make inferences about the climate and environments in southern Brazil during the Late Middle to Late Pleistocene are presented here.

LGP: Laboratory of Geology and Paleontology, Universidade Federal do Rio Grande, Rio Grande do Sul, Brazil.

Section snippets

Geological setting

Chuí Creek is a small fluvial system located in the coastal area of southernmost Brazil, near the town of Santa Vitória do Palmar (Fig. 1). It flows over a plain located some 11 m above sea level, between two Pleistocene coastal barriers (Fig. 2), each one formed by a marine highstand during interglacial epochs (Villwock and Tomazelli, 1995). The creek was originally a shallow stream, but in the early 1960s it was further excavated for agricultural purposes, exposing a layer containing fossils

Methods

In recent years, stable isotopes in organic remains have been used as tools for paleoecological and paleoenvironmental reconstructions in continental environments of the Paleogene and Neogene (Koch et al., 1995, Macfadden and Cerling, 1996, MacFadden et al., 1996, Koch, 1998, Kohn and Cerling, 2002, Palmqvist et al., 2003). Among terrestrial mammals, the main object of the studies has been tooth enamel, that has a mineral phase composed of hydroxylapatite (Ca10[PO4, CO3]6 [OH, CO3]2) and

Carbon isotopes

The results of the 13C and 18O analyses of both taxa, with the ages of the dated specimens (Lopes et al., 2010; unpublished data) are listed in Table 1. Isotopic data from African elephants (Loxodonta africana) from Bocherens et al. (1996) are also presented for comparison.

Nine samples of Toxodon exhibit δ13C values between − 4.0‰ and − 5.5‰, except for one sample (LGP-E0011) that exhibits value of − 8.8‰; the standard score (z) shows that it is an outlier (Hair et al., 2005). The mean value of δ13

Carbon isotopes

The δ13C values obtained for both taxa indicate a browsing to mixed-feeding habit based mostly on C3 plants for Stegomastodon, and indicate a mixed diet for Toxodon, composed of C3 and C4 plants (Fig. 5). The values obtained for Stegomastodon are very close to those found in modern African elephants, but the higher coefficient of variation seen in the latter indicates a wider range of dietary preferences that include large amounts of C4 plants, related to the seasonal feeding pattern. Most of

Conclusions

  • The δ13C values presented here indicate that Stegomastodon was a browser to mixed feeder, with a high content of C3 plants in its diet, while Toxodon was a mixed feeder that fed mostly on C4 plants, probably grasses, and had a very specific dietary composition regarding the proportion o C3/C4 plants in Southern Brazil.

  • The δ13C values in Stegomastodon are similar to those found in modern L. africana, but the lower coefficient of variation found in δ13C values of Stegomastodon indicate that it

Acknowledgments

The authors are grateful to Dr. Juliano Marangoni (IMEF-FURG) for helping with the statistical analysis and for two anonymous referees for corrections and suggestions. This research was funded by CNPq (Doctorship grant for the senior author and research grant 307940/2010-1 for A.M. Ribeiro).

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