Morphology of the forelimb of Psilopterus bachmanni (Aves, Cariamiformes) (Early Miocene of Patagonia)

DEGRANGE, FEDERICO; NORIEGA, JORGE I.; VIZCAÍNO, SERGIO F.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Universidad Nacional de San Juan

Psilopterines are the smallest and most gracile predatory birds belonging to the large terrestrial family Phorusrhacidae, which were abundant during the Tertiary of South America. Two taxa are recognized from the Santacrucian (Early-Middle Miocene) beds of the Argentinean Patagonia: Psilopterus lemoinei (Moreno y Mercerat, 1891) and Psilopterus bachmanni (Moreno y Mercerat, 1891) both described in a very ambiguous manner by Sinclair y Farr (1932). The first species is very well-known by several individuals, including skulls, fore and hind limb bones and has been recently redescribed by Degrange and Tambussi (in press). The smallest one, P. bachmanni, had its fore limb skeleton practically unknown until the recent collection made by a joint Museo de La Plata-Duke University expedition. The fossils herein described come from the locality Puesto Estancia La Costa (= Corriguen Aike), situated at the coast of Santa Cruz Province, between Coyle and Gallegos rivers, and were recovered from the middle levels of Estancia La Costa Member (level 5.3), Santa Cruz Formation. The specimen (MPM-PV4243) is housed at the Museo Regional Provincial Padre Manuel Jesús Molina and includes associated bones of both forelimbs (proximal and distal ends of left humerus, both fragmentary zeugopods, and a nearly complete left manus) and a fragmentary furcula. Differences in size and several others related to the hind limbs with P. lemoinei (Degrange and Tambussi, in press) allow us to refer this new specimen to P. bachmanni. The furcula has a prominent “V” shape when viewed frontally, with very robust corpus claviculae and lacks apophysis furculae. The humerus shows no significant differences with P. lemoinei (see Degrange and Tambussi in press). In the ulna, the olecranon is robust but poorly salient. The manus of this species was previously unknown and is here described for the first time. The os carpi ulnare et radiale are both very stout bones. The carpometacarpus has processi extensorius et alularis very marked and stout. The os metacarpale majus is very robust and depressed. The phalanx digiti minoris is flattened. The phalanges digiti majoris are both very robust. It has been proposed that some species belonging to Psilopterus were able to fly (Tonni and Tambussi, 1988; Degrange and Tambussi, unpublished data) in a clumpsy manner. While this work is not intended to certify this condition, the information provided here represents an effort to bring some new data useful in future systematical, morpho-functional and evolutionary studies.