Paleogene Pseudoglyptodont Xenarthrans from Central Chile and Argentine Patagonia

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3536, 18 pp., 4 figures October 19, 2006 Paleogene Pseudoglyptodont Xenarthrans from Central Chile and Argentine Patagonia 1 2 3 MALCOLM C. MCKENNA, ANDRE´ R. WYSS, AND JOHN J. FLYNN ABSTRACT Herein we describe a new, large-bodied species of Pseudoglyptodon, a close sloth ally, from volcaniclastic deposits of the Abanico (5 Coya-Machalı´) Formation of the central Chilean Andean Main Range. This species, P. chilensis, is a rare element of the Tinguiririca Fauna, on which the recently formalized Tinguirirican South American Land Mammal ‘‘Age’’ is founded, being known from just two specimens. The holotype of P. chilensis, a partial skull and largely complete mandibles (preserving seemingly complete upper and lower dentitions), is by far the best-preserved specimen referable to Pseudoglyptodon known. As such, this material permits a more refined phylogenetic placement of this enigmatic xenarthran than has been possible previously, with Pseudoglyptodon representing the proximal outgroup to the clade including the most recent common ancestor of Choelepus and Bradypus, plus all its descendants (i.e., crown clade sloths). A fragmentary specimen from Argentina is removed from Glyptatelus and referred to Pseudoglyptodon. Although this specimen is distinct from P. chilensis and other previously recognized species of Pseudoglyptodon, it offers too meager a basis for formally establishing a new name. Finally, phylogenetic definitions of the names Phyllophaga and Tardigrada are proposed. Historically these terms have been used largely interchangeably, but here we advocate linking the latter to the crown clade. 1 Division of Paleontology, American Museum of Natural History ([email protected]). 2 Corresponding Author, Department of Earth Science, University of California, Santa Barbara, CA 93106 ([email protected]). 3 Division of Paleontology, American Museum of Natural History ([email protected]). Copyright E American Museum of Natural History 2006 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3536 INTRODUCTION topic dates to roughly 31.5 Ma (Wyss et al., 1993; Flynn et al., 2003)—early Oligocene South America continues to yield enlight- following the time scale of Swisher and ening paleontological surprises. Here we Prothero (1990). The skull preserves much of describe the first-known associated skull and the lower jaws and snout, but rearward from mandibles of Pseudoglyptodon Engelmann, the orbit the specimen is heavily damaged. 1987, from Eocene–Oligocene volcaniclastic Nonetheless, both petrosal bones are in sediments of the Abanico (5 Coya Machalı´) position (although ‘‘floating’’ in the hard Formation, Termas del Flaco, valley of the matrix), as are parts of the right zygomatic Tinguiririca River, central Chile. This is arch and mandibular condyle. All of the the first xenarthran from the Tinguiririca comparatively few teeth of the animal are Fauna (Wyss et al., 1994) to be described present, but the mandibles are clenched in detail. The two Chilean specimens are tightly to the skull. Separation of the mand- referred to a new, large-bodied species of ibles from the skull has not been attempted; Pseudoglyptodon, an aberrant early sloth instead, much of the dental pattern has relative, the type species of which, P. sallaen- been elucidated through computed tomo- sis, is based on a lower jaw from Branisa graphic (CT) scanning. The depositional Locality V-12, lower part of the Salla mechanism(s) accounting for the newly recog- Beds, Deseadan South American Land nized prevalence of mammal remains in Mammal ‘‘Age’’ (SALMA) of Salla, Bolivia post-Neocomian volcaniclastic strata of the (MacFadden et al., 1985). The name ‘‘Pseu- Andean Main Range remain(s) uncertain. doglyptodon’’ is intended to reflect the mor- Specimens described here may have been phology of the cheek teeth of these edentates, superficially resembling the cheek teeth of engulfed in a lahar or volcanic debris flow glyptodontids in their trilobate external and literally cooked to death, with the form, but lacking the central figure (an thinner parts of the skull and jaws reduced axial crest of osteodentine) typical of glypto- to cinders and only the more massive parts dontids. We also refer several teeth from remaining, more or less in their natural the Mustersan and Deseadan of Argentine positions. Postcranial elements were not re- Patagonia—previously interpreted as glyptate- covered, nor were any traces of osteoderms line glyptodontids—to Pseudoglyptodon. The that might have accompanied the skull. new Chilean species, P. chilensis, is similar Moreover, no obvious glyptodontid osteo- to P. sallaensis in many features but is derms are known from any of the localities about twice the size of the latter. A second in the Abanico Formation at Termas del specimen probably referable to P. chilensis Flaco, even though such durable elements is known from Termas del Flaco, but it would be expected to have withstood de- yields limited useful information. A third position. Well-preserved dasypodid osteo- specimen, referable on present evidence to derms occur in moderate abundance in P. chilensis, was described by Florentino strata near Termas del Flaco, but these Ameghino (1897) from the couches a` are unlikely to pertain to Pseudoglyptodon. Pyrotherium (Deseadan in current terminolo- This absence of glyptodontid osteoderms gy) of Patagonia, being placed in the poorly might be argued to reflect the general known early glyptodont species Glyptatelus scarcity of this taxon in these deposits (with tatusinus. only two specimens recovered) rather than The Chilean specimens described here are the taxon’s actual lack of osteoderms. We derived from concretionary nodules harvested would point out, however, that dasypodids, in place from volcaniclastic sediments of the which are known from equally few Abanico (5 Coya Machalı´) Formation of the dental remains in these strata, are nonetheless central Andean Main Range. The Tinguiririca fairly abundantly represented by osteo- Fauna forms the basis of the recently formal- derms—sometimes as large, articulated por- ized Tinguirirican SALMA (Flynn et al., tions of the carapace. In short, if P. chilensis 2003); the age of the fossiliferous strata in possessed obviously glyptodontid dermal ar- this area is constrained by 40Ar/39Ar radioiso- mor, it seems highly unlikely that these would 2006 MCKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS 3 have gone undetected, given the extensive SYSTEMATICS collecting efforts undertaken in the area to date. XENARTHRA The dental pattern exhibited by PHYLLOPHAGA OWEN, 1842, Pseudoglyptodon chilensis sheds light on a va- AS MODIFIED BELOW riety of issues concerning xenarthran fossils and phylogeny. It is at once apparent that Pseudoglyptodon Engelmann, 1987: 217 the new Chilean animal is closely similar in most respects but size to Engelmann’s TAXONOMIC NOTE: Confusingly, different Pseudoglyptodon sallaensis from the De- taxonomic names are currently used to refer to seadan assemblage of Salla, Bolivia, and the same minimally inclusive clade encom- passing the xenarthran mammals com- to teeth once referred to two species of monly known as sloths: Tardigrada and the early glyptodont Glyptatelus Ameghino, Phyllophaga. Here we propose phylogenetic 1897, from the Mustersan and Deseadan of definitions (sensu de Queiroz and Gauthier, Argentina. Various features of the skull 1990) to remedy this ambiguity, tying each and mandible of Pseudoglyptodon are name to a different clade. We define Phyl- clearly slothlike, however, as Engelmann lophaga (a name coined by Owen, 1842, but (1987) first appreciated. The newly revealed generally disused until resurrected by occlusion of the caniniform teeth, wherein McKenna and Bell, 1997) as all xenarthrans the lower caniniform tooth occludes almost more closely related to Bradypus or Choloepus directly opposite the upper caniniform tooth, than to myrmecophagids or dasypodids. presages the ‘‘reversed occlusion’’ seen in Consistent with familiar, present-day usage, numerous sloths including Choloepus,and we tie the name Tardigrada to the crown the small number of cheek teeth recalls sloths clade. Thus, Tardigrada is defined as the most as well. recent common ancestor of Bradypus and The new material from Chile clarifies Choloepus plus all of its descendants. The somewhat the problem of glyptateline rela- distinction between these names is especially tionships by reinforcing the disassociation relevant to the current study because—as of the type osteoderms from the teeth dubi- detailed below—new specimens from Chile ously referred to this group by Ameghino argue that Pseudoglyptodon is not a member and accepted by various later commentators of Tardigrada (the crown clade), but (e.g., Hoffstetter, 1958: 573; Scillato-Yane´, rather that it represents its nearest known 1977: 250). We believe that the teeth described outgroup (and hence is a member of by Ameghino as pertaining to two species Phyllophaga). of Glyptatelus instead should be referred to TYPE SPECIES OF PSEUDOGLYPTODON: P. sal- Pseudoglyptodon, an aberrant animal with laensis Engelmann, 1987: 217. Holotype of P. tardigrade affinities now known from more sallaensis, PU 20552, collected from Branisa’s informative material than was available to locality V-12, lower Salla beds, Salla, Bolivia. Engelmann in 1987. Pseudoglyptodon may OTHER MATERIAL: Other instances of orig- have possessed osteoderms, of course, as inally

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