The paleoecology of Pleistocene giant megatheriid sloths: stable isotopes (δ13C, δ18O) of co-occurring Megatherium and Eremotherium from southern Brazil

The Pampa in subtropical Brazil (State of Rio Grande do Sul) is the only area of South America known so far where fossils of the Pleistocene giant megatheriid sloths Megatherium americanum, characteristic of subtropical-temperate areas, and Eremotherium laurillardi, widespread in the tropical zone, were discovered in the same deposits (Pessegueiro Creek and Chui Creek), but it is not clear whether this co-occurrence is a product of taphonomic mixture, or co-existence, which would imply niche partitioning. In order to understandtheir paleoecology and reconstruct the associated paleoenvironments, dentin samples of both megatheriids from the two sites were analyzed for their carbon (δ13C) and oxygen (δ18O) isotope ratios. The δ13C values of Megatherium indicate mixed diet of C3-C4 plants with higher content of the latter in Pessegueiro Creek, whereas the δ13C values of Eremotherium indicate C3-dominated diets, the more negative value in Pessegueiro Creek possibly related to the canopy effect. The δ18O of Eremotherium points to 18O-depleted water sources, possibly also influenced by the diet, whereas Megatherium ingested 18O-enriched water. The results show that co-existence of both megatheriids would have been ecologically possible, and that the Pampa was occupied by open grasslands/woodlands, with closed forests in Pessegueiro Creek. The δ13C of Eremotherium from intertropical Brazil indicate a more generalist habit than Megatherium from subtropical Brazil and Argentina, which probably facilitated its dispersion from the tropics up to subtropical areas along two different routes, one along the coastal plain, and the other along the Parana River Basin across central South America, following the southward expansion of riparian forests during warmer stages. The reduction of those forests during intervening cold stages possibly led to its disappearance in southern Brazil. Keywords: Quaternary, paleobiogeography, stable isotopes, Pampa, coastal plain, megafauna. RESUMO – O Pampa, no Brasil subtropical (Estado do Rio Grande do Sul), e a unica area da America do Sul conhecida, ate o momento, onde fosseis das preguicas megaterideas gigantes do Pleistoceno Megatherium americanum, caracteristica de areas temperadas subtropicais,e Eremotherium laurillardi, amplamente distribuida no zona tropical, foram descobertas nos mesmos sitios (Arroio Pessegueiro e Arroio Chui), mas nao esta claro se esta coocorrencia representa mistura tafonomica, ou coexistencia, o que implicaria particao de nicho. A fim deentender a paleoecologia e reconstruir os paleoambientes associados a esses taxons, amostras de dentina de ambos megaterideos dos dois sitios foram analisadas quanto as razoes isotopicas de carbono (δ13C) e oxigenio (δ18O). Os valores de δ13C de Megatherium indicam dieta mista de plantas C3-C4 com maior teor destas ultimas no Arroio Pessegueiro, enquanto os valores de δ13C de Eremotherium indicam dieta dominada por C3, sendo o valor mais negativo no Arroio Pessegueiro possivelmente relacionado ao efeito do dossel. O δ18O de Eremotherium indica fontes de agua empobrecidas em 18O, possivelmente tambem influenciadas pela dieta, enquanto Megatherium ingeria agua enriquecida em 18O. Os resultados mostram que a coexistencia de ambos os megaterideos teria sido ecologicamente possivel, e que o Pampa era ocupado por pastagens/matas abertas, com florestas fechadas no Arroio Pessegueiro. O δ13C de Eremotherium do Brasil intertropical indica um habitomais generalista do que Megatherium do Brasil subtropical e Argentina, o que pode ter facilitado sua dispersao desde os tropicos ate areas subtropicais ao longo de duas rotas diferentes, uma ao longo da planicie costeira e outra ao longo do Rio Parana na regiao central da America do Sul, seguindo a expansao para o sul das matas ciliares durante os estagios mais quentes. A reducao dessas florestas durante os estagios frios subsequentes possivelmente levou ao desaparecimento de Eremotherium no sul do Brasil. Palavras-chave: Quaternario, paleobiogeografia, isotopos estaveis, Pampa, planicie costeira, megafauna.