McDonald and Carranza-Castañeda—New Miocene ground sloth from Mexico
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until 4–5Myr when FAD starts to increase, peaking during the Pleistocene. They concluded that the study of paleodiversity patterns and paleobiogeography in the Americas is currently biased by the samples available (Carrillo et al., 2015). Extrapolating from the possibility that there may exist a
diverse number of currently unknown extinct sloth and other xenarthran taxa in the tropical portions of South America brings us full circle to the issue that many of the North American sloth taxa, such as Pliometanastes, Zacatzontli n. gen., and Thinobadistes, are not present in South America from similar- aged faunas, or at least are not known from the well-documented similar-aged faunas of Argentina and the more temperate parts of South America. Past studies have focused on the dispersal of taxa between
Figure 7. Comparison of maximum latitude of extant and fossil North American xenarthrans: (1) extant taxa, (2) extinct taxa. The striped bar for Myrmecophaga is the Irvingtonian range extension, the two bars for Tamandua show the differences on the Pacific and Gulf coasts, the two bars for Dasypus show the differences on the Pacific and Gulf coasts, and the striped bar for Megalonyx shows its Sangamonian range extension.
environment that enhances the overall species richness (Tews et al., 2004) along with the latitudinal gradient diversity (Jablonski et al., 2013). Croft (2007) used the Simpson Index of faunal similarity to
compare the middle Miocene Quebrada Honda Fauna from mid- latitudes in Bolivia with the slightly older high-latitude fauna of Collón-Curá Argentina and noted that it was more similar to the older fauna than to the contemporaneous low-latitude fauna of La Venta in Colombia. He suggested isolating mechanisms existed between the low and middle latitudes during the early and/or middle Miocene. Carrillo et al. (2015), using dissim- ilarity analysis, showed that distinct tropical and temperate faunal assemblages existed in South America during the middle Miocene (Colloncuran–Laventan [≈ Langhian–Serravallian]) and late Miocene (Huayquerian–Montehermosan [≈ Tortonian– Piacenzian]). Their analysis of first appearances of taxa in the faunas examined showed that across time and latitude the faunistic movements related to GABI began during the late Miocene (~10 Ma) with the oldest records found at higher latitudes. The number of FAD remained relatively low
North and South America in both directions following the formation of the land bridge by ~3 Ma, as correlated with the expansion of savannas and grasslands in the Neotropics during glacial periods (Webb, 1991, 2006; Leigh et al., 2014). It appears that North American taxa seem to have been more successful utilizing more temperate biomes, whereas, in contrast, the South American taxa dominated in the tropics (Webb, 1991, 2006; Leigh et al., 2014). Therefore, attempting to derive the North American sloth taxa from taxa from the temperate part of South America should be reconsidered. Rather, it may be that at least some of the North American sloth taxa are instead derived from an ancestral stock that lived in the northern, and thus tropical, part of South America, which is an area that is greatly underrepresented by faunas in general as well as those that date to the beginning of the GABI. Gaudin and Croft (2015) attributed the rarity of early xenarthran remains not only to the general scarcity of fossil localities from tropical latitudes in South America, but also to low population densities associated with myrmecophagy and the lack of durable, enamel- covered teeth, which have been the basis for many of the described mammalian taxa from this region. The sediments in
many of the known sites in tropical South America are marine- fluvial in origin, and therefore represent significantly different environments than inland sites in the temperate latitudes (per- sonal communication, D. Croft, 2017). The large number of early records of sloths in Hemphillian faunas in North America is an artifact of more fieldwork in this region, which is a mirror image of what is seen in temperate South America. With an increase in the recovery of sites in both tropical CentralAmerica (Laurito and Valerio, 2012) and South America (Rincon et al., 2016), it may be just a matter of time before Zacatzontli n. gen. and some of the other “endemic” North American sloth taxa are found in the tropical part of South America and a fuller under- standing of xenarthran diversity and the participation of this group during the early stages of the GABI is possible.
Conclusions
The discovery of a new genus and species of megalonychid sloth, Zacatzontli tecolotlanensis, from the late Hemphillian of Jalisco, Mexico increases the diversity of taxa of South American origin that participated in the GABI. Zacatzontli n. gen. in Mexico and another megalonychid sloth, Meizonyx,inEl Salvador clearly show there is a greater diversity of taxa of South American origin in the northern Neotropics than had been
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