Unusual caudal series of Titanosauridae of the Late Cretaceous in the Rio Colorado Formation, border between the Neuquen and Mendoza provinces, Argentina.

GONZÁLEZ RIGA, BERNARDO J.; CALVO, JORGE O.

Buenos Aires

Simposio; 7º International Symposium on Mesozoic Terrestrial Ecosystems; 1999

Commitee, International Symposium on Mesozoic Terrestrial Ecosystems

The outcrops widely exposed on both margins of the Colorado River, in the border of Neuquén and Mendoza provinces, mainly belong to the Río Colorado Formation (Coniacian-Santonian). The fossils have been found in overflow and floodplain deposits. The Arroyo Seco locality (Mendoza) has yielded 70 bones belonging to two Titanosauridae sauropods. One of them has typical procoelous anterior caudals, and distal and middle caudals slightly procoelous. At Cañadón Arroyo Seco, near Rincón de los Sauces city (Neuquén), more than 150 bones have been recovered. They belong to three Titanosauridae sauropods and to one not still determinate theropods. A preliminary study of the Titanosauridae materials indicates that they belong to a new species. They have preserved cranial, material, cervicals, dorsals, sacrals and procoelous caudals, hand and forelimbs, scapular and pelvic girdle and osteoderms. One of these specimens is very unusual: three articulated middle caudals are procoelous, amphicoelous and bocinvex; and two posterior caudals are opisthocoelous and biconvex. The body caudal series are completely different from the other  two sauropod specimens. Nevertheless, the neural arches are too similar. These features impede to separate them as two different species. The Loma del Lindero locality, close to Rincón de los Sauces city, has yielded 200 disarticulated bones belonging to two Titanosauridae specimens and one Abelisauridae theropod. Most of caudal vertebrae recovered are typically procoelous, but al least one is an opisthocoelous middle posterior caudal. It was assumed that Titanosauridae always have strongly procoelous middle caudals with occasional amphicoelous ones. However, these new findings show that al least some Titanosauridae have slightly procoelous, opisthocoelous and / or amphicoelous middle caudals. This evidences suggest that caudals may have multiple variable morphology along its tail. It id important to interpreted at the family level isolated middle caudals in sites where we could expect to find typical Titanosauridae.