Andean lineage of Pleistocene Megatherium: geographical implication

FRANÇOIS, PUJOS; RODOLFO, SALAS; JEAN, MATTOS

Norman, Oklaoma, Estados Unidos

Congreso; 62nd Annual Meeting of the Society of Vertebrate Paleontology; 2002

The Society of Vertebrate Paleontology

    The subfamily Megatheriinae was extremely diverse from the late Miocene (Montehermosan) to late Pleistocene (Lujanian) in South America. Megatherium americanum is known from meridional areas of South America while Eremotherium laurillardi is distributed throughout the tropical and subtropical areas of North and South America. The large M. americanum is the only well known species of its genus.    Several Pleistocene Megatherium species have been discovered in the Andes: M. medinae (Chile), M. sundti (Bolivia), M. tarijense (Bolivia and Peru), and M. elenense (Ecuador and Peru). Two new species of Megatherium (M. sp. nov. 1 and M. sp. nov. 2) from Peru are being currently described. All the Andean taxa share the following characters: (1) reduction of total body size (from 20 to 60%); (2) raised, prominent, and posteriorly extended occipital condyles; (3) reduced deltopectoral crest of the humerus; (4) short rostrum; (5) relatively flat femur; and (6) enlargement of the surface of the ectal facet of the astragalus.    Megatheriinae are usually considered bipedal and/or quadrupedal. In these Andean megatheriines the morphology of the humerus and the position of the occipital condyles suggest a predominantly quadrupedal locomotion, which may be related to the Andean relief. Metacarpals IV and V are sub-equal in size and longer than the others, bear dorsodistal tuberosities, and are tightly articulating. This peculiar morphology reflects lateral contact of the hand with the ground during locomotion. While walking, digits four and five contact the ground at an angle between 30 to 45&deg. Furthermore several carpal/metacarpal facets (Mc III/Mc IV, Mc IV/Mc V, and magnum/Mc III) are parallel. This particular arrangement produces a highly stable structure.    These Megatheriinae may have evolved endemically in the Andean region. Preliminary phylogenetic analysis confirms the existence of a distinct Andean lineage of Megatherium that can be clearly separated from the clade containing [M. altiplanicum + M. americanum].