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Co-Occurrence of Mylodontid Sloths and Insights on Their Potential Distributions During the Late Pleistocene

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Co-Occurrence of Mylodontid Sloths and Insights on Their Potential Distributions During the Late Pleistocene Quaternary Research 85 (2016) 66–74 Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres Co-occurrence of mylodontid sloths and insights on their potential distributions during the late Pleistocene Luciano Varela ⁎,RichardA.Fariña Sección Paleontología, Facultad de Ciencias, Universidad de la República, Iguá 4225, 11400 Montevideo, Uruguay article info abstract Article history: Species distribution models (SDMs) for the last interglacial (LIG), the global last glacial maximum (LGM) and the Received 10 March 2015 Holocene climatic optimum (HCO) were generated for three extinct South American Pleistocene mylodontid Available online 20 January 2016 giant sloths, Glossotherium robustum, Lestodon armatus and Mylodon darwinii. They are recorded co-occurring in some localities including Arroyo del Vizcaíno site (AdV) in Uruguay. Co-occurrence records were studied Keywords: based on the overlap of their generated areas of potential distributions, and compared with the available Ground sloths Glossotherium biome reconstructions of South America during the LGM to analyze their distribution patterns, ecological require- Lestodon ments and possible interactions between them. Our results suggest that these sloths could have co-existed main- Mylodon ly in the Chaco-Paraná Basin and the plains in the Río de la Plata area. Areas of high suitability were observed for Xenarthra submerged parts of the continental shelf that were exposed during the LGM showing an overall increase in po- Species distribution models tential habitat compared to the LIG and HCO. This suggests that there was a drastic reduction in total available Ecological niche modeling areas of preferred habitat at the end of the Pleistocene. The co-occurrence of these sloths at the AdV site suggests Paleogeography the presence of vegetation indicative of mainly open, cold to temperate habitats but with mixed patches typical of Last glacial maximum humid climates. Quaternary © 2015 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction patterns of two gastropods. These methods have been increasingly used in paleobiogeography studies (see Varela et al., 2011; Franklin Species distribution models and the South American megafauna et al., 2015). The South American megafauna, an impressive assortment of giant Species distribution models (SDMs) represent an array of techniques mammals that went extinct at the end of the Pleistocene and one of used to model the potential distributions of species from their occur- the main sources for Darwin's ideas, has received considerable attention rence records and the environmental (mainly climatic) conditions in in the past decades in regard to their habits and paleoecological prefer- those locations (Guisan and Zimmermann, 2000; Guisan and Thuiller, ences (Fariña et al., 2013). Within the South American Pleistocene 2005). Among those techniques, maximum entropy approaches like megafauna the giant sloths are of particular interest due to their diver- the one implemented in MAXENT software package (Phillips and sity, very large size and lack of modern analogs, including studies of Dudik, 2008), have become widely used because of the ability to use their evolution (Gaudin, 2004), ecology (Fariña, 1996; Fariña and presence-only data, its robustness to limited amounts of samples and Blanco, 1996; Bargo et al., 2000, 2006a, 2006b; Vizcaíno et al., 2001; its higher predictive accuracy when compared to other methods Bargo and Vizcaíno, 2008) and extinction (Lessa and Fariña, 1996; (see Franklin, 2010). Species distribution models have been used re- Barnosky and Lindsey, 2010; Hubbe et al., 2013). Lately, some studies cently to study past and potential future distributions of extant species have focused on the biogeography of the megafauna. Gallo et al. (Torres et al., 2013), to better understand speciation processes (2013), for example, used the panbiogeographical method of track anal- (Raxworthy et al., 2008; Blair et al., 2013), the existence of ecologi- ysis and concluded that some current distributional patterns already cal processes like competitive exclusion or niche differentiation existed in the Pleistocene and that those patterns were highly influenced (Gutiérrez et al., 2014), and to evaluate the existence of climate refugia by the main plant communities. Lima-Ribeiro and Diniz-Filho (2012) (Waltari et al., 2007). In particular, Mumladze (2014) used species co- and Dantas et al. (2013) studied past potential distributions of Smilodon occurrence records and modeled potential co-occurrence areas to populator and Notiomastodon platensis, respectively, using SDMs and study the importance of interspecific competition in the distribution showed the reduction of suitable areas at the beginning of the Holocene. In particular, Lima-Ribeiro et al. (2012) analyzed the potential distribu- tions of Eremotherium laurillardi and Megatherium americanum during ⁎ Corresponding author. the Late Pleistocene and showed a drastic reduction in potentially suit- E-mail address: [email protected] (L. Varela). able areas for both species at the end of the Pleistocene. http://dx.doi.org/10.1016/j.yqres.2015.11.009 0033-5894/© 2015 University of Washington. Published by Elsevier Inc. All rights reserved. L. Varela, R.A. Fariña / Quaternary Research 85 (2016) 66–74 67 We apply Paleo-Species Distribution Models (PSDMs) to predict the co-occurrence of three species of mylodontid ground sloths: Lestodon armatus, Mylodon darwinii and Glossotherium robustum. L. armatus and G. robustum are rather similar morphologically, with bulk feeding habits having been proposed (Bargo and Vizcaíno, 2008) and differing primar- ily in their body sizes at ~4000 kg for L. armatus (Fariña et al., 1998; Bargo et al., 2000), and between 750 and 1500 kg for the gracile and ro- bust, possibly sexually dimorphic, forms of G. robustum, respectively (Bargo et al., 2000; Christiansen and Fariña, 2003). In contrast, M. darwinii was inferred to have been a selective feeder (Bargo and Vizcaíno, 2008) and had a body mass between 1000 and 2000 kg (Christiansen and Fariña, 2003). The Arroyo Del Vizcaíno site The Arroyo del Vizcaíno site (Fariña et al., 2014a), near the town of Sauce, Canelones, Uruguay (34°37′ S, 56°02′ W, 25 m above mean sea level [asl]) has yielded over one thousand remains of at least 27 individ- uals mostly belonging to megafaunal species. About 95% of those re- mains have been assigned to the giant sloth L. armatus,butM. darwinii and G. robustum have been found, as well as glyptodonts (Glyptodon clavipes, Panochthus tuberculatus and Doedicurus clavicaudatus)and other Pleistocene species including Toxodon platensis, the fossil horses Equus (Amerhippus) neogeus and Hippidion principale, a deer, a probosci- dean and the saber-toothed felid S. populator. This site, which has been dated to ~30 ka on the basis of radiocarbon ages obtained from samples of bone and wood (Fariña and Castilla, 2007; Fariña et al., 2014a), is important as it may provide evidence for early human presence in South America (Fariña et al., 2014a; Fariña, 2015). The climatic conditions and sea level were similar to those experienced during the global last glacial maximum (LGM) several millennium later at ~26.5 to 19.0 ka (Clark et al., 2009). Figure 1. Records of Glossotherium robustum, Lestodon armatus and Mylodon darwinii used in this study. Material and methods period, precipitation of driest period, precipitation seasonality, precipi- Species occurrences and paleoclimate data tation of wettest quarter, precipitation of driest quarter, precipitation of warmest quarter and precipitation of coldest quarter. Data on L. armatus, G. robustum or M. darwinii occurrences were ob- tained from a review of the literature and the Paleobiology Database Paleo-Species Distribution Models (http://paleobiodb.org). Sixty-one localities containing these Pleisto- cene taxa occur in South America (Fig. 1). Species occurrence at these lo- The potential distribution of the three studied sloths was recon- calities and their assigned ages are listed in Table 1. Although systematic structed by ecological niche modeling using the software MAXENT v. revisions might be needed to determine the validity of some species 3.3.3k (Phillips and Dudik, 2008). MAXENT utilizes a maximum entropy within these genera, we considered Mylodon and Lestodon as monospe- approach to species distribution modeling with presence-only data cific genera according to Esteban (1996) and Czerwonogora and Fariña (Phillips et al., 2006). Paleo-Species Distribution Models (PSDMs) (2013), respectively, and G. robustum as the only valid species for the were constructed for the LGM from occurrence records between 13 Pleistocene according to McAfee (2009). Therefore, all the recorded and 40 ka (Table 1), assuming that the species were also present during occurrences of the genera for the Pleistocene were considered as the LGM. The obtained model was then projected to predict the species belonging to the mentioned species. potential distribution during the HCO and LIG (according to GCMs). A General Circulation Model (GCM) reconstructing the paleoclimate As high correlation is expected between many variables, an initial for the last interglacial (LIG) at ~130 to 115 ka (Dahl-Jensen et al., run in MAXENT and a jackknife test were performed to identify vari- 2013), the LGM, and Holocene climatic optimum (HCO) at ~8 to ables with minimal or no contribution to the models. Those variables 5.5
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