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Journal of Paleontology 91(5):1069–1082
as Hh3, so occurs later than the earliest well-dated record of Pliometanastes from the Siphon Canal locality in California at 8.19±.16Ma (Hirschfeld, 1981),which at themoment suggests a separate dispersal. The spout morphology of Zacatzontli n. gen. would appear to make it a better potential ancestor to Megalonyx than Pliometanastes, which has the more primitive elongated spout, but based on this single record it occurs after the earliest records of Megalonyx, which is also present in other similar age sites in Mexico. A second dispersal event of sloths in the later Hemphillian is also suggested by Thinobadistes, which also first occurs in the Hh3 (McDonald and Naples, 2008). In our analysis we recovered two major clades within the
megalonychids. The North American and Caribbean genera form one group, possibly reflecting their earlier dispersal and possibly a common shared ancestry, and the South American genera, possibly indicating subsequent evolution and diversifi- cation after the dispersal that founded the North American- Caribbean clade. The grouping of Zacatzontli n. gen. from the Hemphillian of Mexico with the South American clade rather than with the North American-Caribbean clade suggests that these two major groups of megalonychid sloths had already diverged at this time.
Paleobiogeography
Webb’s (2006) review of the GABI noted that the terrestrial fossil record for mammals in Central and North America unequivocally supports the presence of a significant water bar- rier between North and South America until ~4–3 Myr. Besides the mammals, evidence for this barrier is also provided by the birds, especially the ecological specialist tropical forest birds (Weir et al., 2009; Smith and Klicka, 2010). The age of Zacatzontli tecolotlanensis n. gen. n. sp. suggests that it, like the sloths that dispersed into North America earlier in the Hemphillian, most likely crossed a water barrier in order to enter North America. McDonald (2005) noted that of all the mammalian lineages
of South American origin that entered NorthAmerica during the Great American Biotic Interchange (GABI), the sloths were the most successful in terms of taxonomic diversity. The recognition of this new genus and species from the Hemphillian of Mexico increases our knowledge of that overall diversity during the earliest stages of the Great American Biotic Inter- change. The discovery of this new sloth also requires a reexamination of McDonald’s (2005) observation that at any one time there was only one representative of each of the major groups of sloths (megalonychid, nothrothere, megathere, and mylodont) present in North America at each stage of the GABI. This observation was biased by the very robust fossil record of xenarthrans from the temperate part of North America, primarily the United States and northern Mexico, which is in marked contrast to the smaller number of late Cenozoic localities and studies of the fauna from the tropical portion of southern Mexico and Central America. At least for the megalonychidae, we now have more than one representative of the family in the tropical portions of North America. In addition to Meizonyx salvadorensis Webb and Perrigo,
1985 from the middle Pleistocene of El Salvador (Webb and Perrigo, 1985), the recognition of a second genus of
megalonychid sloth, Zacatzontli n. gen., clearly demonstrates that the taxonomic diversity of fossil xenarthrans (and very likely other groups of South American origin as well) in the tropical portions of North and Central America is much greater than previously thought. This should not be unexpected given that the greatest taxonomic diversity of extant xenarthrans outside of South America today is in Central America and southern Mexico (i.e., the tropics), and includes all three genera of anteaters (Myrmecophaga tridactyla Linnaeus, 1758; Tamandua mexicana Saussure, 1860; and Cyclopes didactylus Linnaeus, 1758), two armadillos (Cabassous centralis Miller, 1899; and Dasypus novemcinctus Linnaeus, 1758; the only xenarthran taxon with a range that extends into the United States), and both genera of extant sloths, each represented by a single species (Bradypus variegatus Schinz, 1825 and Choloepus hoffmanni Peters, 1858), the “camp followers” of McDonald (2005). Webb (2006) noted that while geographically southern
Mexico and Central America are geologically tied to North America, having been widely and deeply separated from northern South America by the Bolivar Trough, today fauna and flora of this region are more similar toNeotropical South America—his Central American Paradox. As described byWebb (2006), Central America was conquered by the tropical fauna of South America, which is clearly indicated by the diversity of xenarthrans in this region. Because the majority of these taxa are restricted to tropical rainforest habitat, the northernmost limit for their distribution is at about the same latitude (±16°N) (Fig. 7).As discussed byMcDonald (2005), the variety of habitats in Central and North America acted as “nested sieves” that restricted the northern dispersal of xenarthrans, and it is clear that the majority of taxa dispersing out of South America were adapted to a tropical habitat and hence restricted to how far north their range could extend. While the sloths (Pliometanastes, Megalonyx, Thinobadistes, Glossotherium/Para- mylodon, Nothrotheriops,and Eremotherium) and the cingulates (Glyptotherium, Holmesina, Pachyarmatherium, Dasypus bellus Simpson, 1929, and to a lesser extent Pampatherium) extended their ranges into more northern temperate environments for geologically extended periods of time, it is becoming clear that there exists at least as great and possibly greater amount of currently undocumented xenarthran diversity in the tropical part of North America, which is only now being discovered. Fluctuating climatic conditions through the Pliocene and
Pleistocene did permit short-term northerly range expansions of some of the tropical taxa, such as the giant anteater, Myrmecophaga tridactyla, in the Middle Pleistocene (Irvingtonian) El Golfo fauna at 31.5°N (Shaw and McDonald, 1987), or even temperate taxa such as Megalonyx expanding its range as far north as the Yukon during the Sangamonian interglacial (McDonald et al., 2000).
Today, theNeotropics support an extremely large diversity of
living mammals with ~1500 recognized species currently known, representing ~30% of the total world mammal diversity. This biodiversity includes numerous endemic groups such as marsupials, xenarthrans (sloths, armadillos, and anteaters), caviomorph rodents, platyrrhine monkeys, and phyllostomid bats (Patterson and Costa, 2012). The existence of the variety of biomes present in the Neotropics (lowland rainforest, savannas, mountain forest, scrublands, and deserts) provides a partitioned
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