Journal of Paleontology, 91(5), 2017, p. 1069–1082 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.45
Increased xenarthran diversity of the Great American Biotic Interchange: a new genus and species of ground sloth (Mammalia, Xenarthra, Megalonychidae) from the Hemphillian (late Miocene) of Jalisco, Mexico
H. Gregory McDonald1 and Oscar Carranza-Castañeda2
1Bureau of Land Management, Utah State Office, 440 West 200 South, Salt Lake City, Utah USA 84101-1345 ⟨
hmcdonald@blm.gov⟩ 2Centro de Geociencias, Campus Juriquilla, Universidad Nacional Autónoma de México, C.P. 76230, Juriquilla, Querétaro, México ⟨
carranza@geociencias.unam.mx⟩
Abstract.—A new genus and species of megalonychid sloth, Zacatzontli tecolotlanensis n. gen. n. sp., is described from the late Hemphillian of Jalisco, Mexico. Comparison and analysis of the type specimen, a mandible, with other megalonychid sloths shows a closer relationship to South American taxa than those from North America or the Caribbean. This suggests that during the early stages of the Great American Biotic Interchange there were two separate dispersal events of megalonychid sloths—an earlier one represented by Pliometanastes and the later one by Zacatzontli n. gen. While the morphology of the spout of Zacatzontli more closely resembles that of Megalonyx, based on the current record, Zacatzontli does does not enter North America until after the evolution of Megalonyx from Pliometanastes. The role of the northern neotropics in South America as a staging area for South American taxa that entered North America is discussed.
Introduction
Ongoing research of the geology and paleontology of the late Tertiary basins in the central part of Mexico, between latitudes 23°N and 19°N, has been pivotal to expanding our knowledge of Hemphillian and Blancan North American Land Mammal age (NALMA) faunas found to the south of the classic localities of these ages in the North American Great Plains. The faunas recovered to date have a high mammalian diversity and represent the most important late Cenozoic faunal assemblages known south of the United States border. All of the sites that have been studied and collected consist of fluvial lacustrine deposits that form part of the Trans Mexican Volcanic Belt, and contain the earliest appearances of South American immigrants that dispersed into North America during the Hemphillian and Blancan (9.0–1.6Ma). The ages of these fossil records have been obtained by both
radiometric and paleomagnetic analyses. These have refined our knowledge of both the earliest existence of the Panamanian land bridge that was critical for the Great American Biotic Interchange (GABI) and the timing of the dispersal of taxa prior to its formation (Carranza and Miller, 1980;Miller and Carranza-Castañeda, 2001; Carranza-Castañeda and Miller, 2004; Flynn et al., 2005; Wood- burne, 2010; Carranza-Castañeda et al., 2013). During the early Blancan, the northward dispersal of South
American neotropical mammals into North America was more dynamic and involved more taxa than the earlier dispersal in the Hemphillian (~8 Ma). Evidence of this active interchange has been recorded at several localities in the San Miguel Allende Basin, in the state of Guanajuato. These important fossil sites encompass faunas from the latest Hemphillian, dated at 4.8Ma, as well as
various early Blancan localities, dated at 4.7Ma. We note that the Hemphillian-Blancan boundary as used here (4.8–4.7Myr; Flynn et al., 2005) is later than that of Lindsay et al. (2002) inwhich this boundary was defined as 4.9–5.0Ma based on stratigraphic sequences in Nevada. The Mexican faunas document the earliest records in North America of taxa originating in the neotropics of South America: Glyptotherium texanum Osborn, 1903 (Gillette et al., 2016), Paramylodon garbani (Montellano-Ballesteros and Carranza-Castañeda, 1981, 1986), and Neochoerus cordobae Carranza-Castañeda and Miller, 1988 (Miller and Carranza, 1998; Miller and Carranza-Castañeda, 1999; Carranza-Castañeda and Miller, 2004; Flynn et al., 2005; Carranza-Castañeda, 2016). All of these records strongly suggest that the second stage of the GABI occurred at least 1.5Myr earlier than had been previously accepted based on the presence of these taxa in faunas farther north in Arizona, Texas, and Florida. Results of additional studies in other basins in central
Mexico include localities in the State of Hidalgo, such as the Las Golondrinas locality, an important late Hemphillian fauna that includes Megalonyx, as well as the Zietla fauna from which several megalonychid molariforms were recovered (Carranza-Castañeda, 1991). Studies carried out in the state of Jalisco have revealed the
significance of the Tecolotlán Basin to our understanding the early stages of the GABI in Mexico. Here the stratigraphic sequence includes an important late Hemphillian fauna in the upper part of the unit, above which rest unconformably younger beds that have been assigned to the late Blancan–early Irving- tonian NALMA, and within this sequence, South American immigrants have been discovered (Kowallis et al., 2003, 2017; Carranza-Castañeda, 2016).
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