The record of Opisthodactylus Ameghino 1891(Aves, Rheiformes) from the late Miocene of northwestern Argentina

NORIEGA, J.I.; VEZZOSI, R. I.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Universidad Nacional de San Juan

The monotypic genus Opisthodactylus was initially placed in its own family, Opisthodactylidae (Ameghino, 1895), but later transferred to Rheidae due to its strong resemblance with living rheas (Patterson and Kraglievich, 1960; Tonni and Tambussi, 1986). The type species, Opisthodactylus patagonicus Ameghino 1891, is restricted to the Santacrucian (Early-Middle Miocene) beds of Patagonia. This poorly described taxon was based on very fragmentary materials which are deposited at the British Museum of Natural History, including three incomplete distal ends of tarsometatarsi and a distal tibiotarsus among other bones. In this contribution we report a well preserved specimen belonging to Opisthodactylus sp. which comes from the ?Araucanense medio? levels at the Santa María valley (= Yocavil) in Tucumán Province (sensu the original label of its collector, A. Castellanos). Although the available information about the stratigraphic provenance does not follow the modern rock nomenclature, the bearing fossil horizon can be correlated with the Andalhuala Formation (Castellanos, 1946; Marshall and Patterson, 1981; Bossi and Muruaga, 2009). The referred materials (MUFYCA-647) are housed at the Museo Florentino y Carlos Ameghino (Instituto de Fisiografía y Geología, Rosario) and include associated elements of both hind limbs (right femur, both tibiotarsi and tarsometatarsi, and six pedal phalanges). The new specimens are assigned to Opisthodactylus sp. by sharing quite similar dimensions and the following characters with the type materials: Distal end of tarsometatarsus broadened, with marked spread of trochleae II and IV; articular facets of trochleae III and IV transversally widened and markedly grooved; shaft depressed dorso-ventrally above the level of trochleae; prominence for attachment of internal ligament continuous with that for transverse ligament on distal tibiotarsus; tendinal groove deep and ending proximally. More detailed studies will probably support the erection of a new species. The finding will contribute to emend the generic diagnosis and to bring additional phylogenetic information. In addition, this record extends the biochron of Opisthodactylus from Santacrucian to Huayquerian age (Late Miocene). Likewise, the presence of a probable younger new species of Opisthodactylus in Tucumán Province has a paleobiogeographic significance that must be evaluated to understand the natural history of neogene rheids living at Northwestern and Patagonian regions of Argentina.