On the systematics on Hapalops Ameghino, 1887 (Xenarthra: Megalonychidae)

GERARDO, DE IULIIS; FRANÇOIS, PUJOS

Ottawa, Ontario, Canadá

Congreso; 66th Annual Meeting of the Society of Vertebrate Paleontology; 2006

The Society of Vertebrate Paleontology

    Hapalops is the most abundantly recovered fossil sloth from the early – middle Miocene Santacrucian of Patagonia. Its remains have been known for more than a century, but all important systematic work on this genus predates 1910. Until recently xenarthran paleontologists had long clung to the idea that the main sloth clades (Mylodontidae, Megalonychidae, Nothrotheriidae, and Megatheriidae) developed from Miocene forms, and that Hapalops occupied a phylogenetically central position among later sloths. Although Santacrucian sloths could provide a wealth of information on sloth evolution and diversity, their basic taxonomy, particularly of Hapalops, remains poorly understood, and this has hindered higher level systematic studies. Some 26 Hapalops species are still formally recognized, even though most are based on partial or fragmentary specimens from a limited geographic region of Patagonia. A revision is clearly required, particularly in view of the wide range of intraspecific variation established recently for other sloth species by several authors. Two main reasons for the lack of progress is that available remains, including several collections in major North and South American institutions, have not been studied as a whole, and that strict stratigraphic information is not available for the older collections. Recent field work in the coastal exposures of the Santa Cruz Formation between Ríos Coyle and Gallegos led by S.F. Vizcaíno and M.S. Bargo (Museo de la Plata) and R. Kay (Duke University), has yielded several reasonably complete new specimens, including skulls and mandibles with associated skeletons, of Hapalops. A preliminary morphological and metric analysis based on skull and dental of these remains and those of the classical Santacrucian fauna recovered by the Princeton Expeditions at the end of the 19th century suggest the existence of four main size and morphological types. Further analyses will include Hapalops remains in collections in Buenos Aires, Chicago, and New York.  Understanding the systematics of Hapalops, the oldest “osteologically” well-known sloth, is fundamental to further phylogenetic analyses of fossil and extant sloths.