McDonald and Carranza-Castañeda—New Miocene ground sloth from Mexico
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Figure 6. Strict consensus of 4 trees (0 taxa excluded), “Implicit enumeration” based on TNT (Tree New Technologies), Goloboff et al., 2008; RI = Retention Index, defined to be 0 for parsimony uninformative characters; CI = Consistency Index, CI = 1 if there is no homoplasy. Characters used for analysis from Scott, 1903–1904; Kraglievich, 1923; Simpson, 1932; Matthew and Paula Couto, 1959; Hooijer, 1962; Paula Couto, 1967; McDonald, 1977; Mayo, 1980; Naples, 1982, 1987; Patterson et al., 1992; Gaudin, 1995; White and MacPhee, 2001; Carlini and Scillato-Yané, 2004; Cartelle et al., 2008; De Iuliis et al., 2009, 2016.
known from mandibles, although often incomplete, so while the first two genera are included in our analysis of the taxonomic
relationships of Zacatzontli n. gen., many characters could not be scored, which further reduces the strength of any interpreta- tions of its relationship to these taxa. Since Meizonyx from the middle Pleistocene of El Salvador is known from the mandible, it has been included in this analysis. Despite the many megalonychid genera currently described,
many are based on fragmentary remains and could not be included in this or other analyses (e.g., Gaudin, 2004;McDonald et al., 2013). Consequently, our analysis here is only intended to provide a first approximation of the possible relationship of Zacatzontli n. gen. to the other megalonychids with which it can be compared (Fig. 6). A fuller analysis will require the recovery of better-preserved and more complete material, particularly the cranium. Given that the analysis is limited to the mandible and is therefore restricted in the number of characters available, we consider the resulting tree to provide a tentative representation of the phylogenetic relationship of Zacatzontli n. gen. and the
megalonychid taxa examined, and it should only be considered a first approximation that will require additional testing as more material becomes available. Although this analysis does recover a North American-
Caribbean clade and a SouthAmerican clade (Fig. 6), some of the resulting relationships are quite different from those recovered by Gaudin (2004) andMcDonald et al. (2013), such as the recovery
of Pliometanastes as the sister taxon to Parocnus,andMegistonyx as the sister group to the other South American megalonychids and not to Ahytherium. The position of the oldest megalonychid, Deseadognathus, higher on the tree probably reflects the incom- pleteness of Deseadognathus, and the unresolved trichotomy of Proschismotherium, Mesopotamocnus,and Australonyx probably reflects the small number of characters in the data set. Despite these limitations, there is a general agreement between our and previous analyses on the broader relationships within the Mega- lonychidae. The results, therefore, do have heuristic value since we have been able to include genera that have not been included in previous analyses. Given the limited number of taxa represented by mandibles complete enough to obtain a reasonable number of characters, our goal was not to produce a comprehensive taxo- nomic revision of the family Megalonychidae, but rather to explore at a more basic level the possible relationships of the North and Central American megalonychids to each other and contemporaneous South American genera. Of special interest of course is where Zacatzontli n. gen. fits in these relationships in order to set the stage for future research.Of interest here iswhether there was a single lineage of megalonychid sloths that dispersed into Central and North America and subsequently radiated or whether the diversity of megalonychids in Central and North America is the result of multiple distinct lineages of mega- lonychids dispersing north at different stages of the GABI. Zacatzontli n. gen. is currently known froma single locality dated
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