The Late Jurassic-Early Cretaceous worldwide record of basal Titanosauriforms and the origin of Titanosaurians (Sauropoda): New evidence from the Aptian (Lower Cretaceous) of Chubut Province, Argentina

SEBASTIÁN APESTEGUÍA; OLGA GIMÉNEZ

Bozeman, Montana

Congreso; 59 Annual Meeting of vert Pal; 2001

Soc of Vert Pal

After the Bathonian coincident with the opening of the Uralian Sea (165 mya) and before the split of Neopangea into Laurasia and Gondwana by the opening of the Tethys sea (141 mya), basal Titanosauriforms spread through Neopangea. During Late Jurassic and Early Cretaceous times the group was relatively abundant and their remains are found in distant relicts of Neopangea, including North America, the UK, France, Istria, South America, Australia, Africa, and Thailand. Until now, their incomplete remains and their characterization via plesiomorphic features, without a detailed phylogenetic study, gave them a ‘brachiosaur’ status and made it imposible to evaluate the phylogenetic positions of their most succesful component: The Titanosauria. In Cenomanian times the group vanished from most of the world, remaining clearly present only in Gondwana.  From there, they would spread in Late Campanian times as shown by the highly derived phylogenetic status of the extra-gondwanan taxa Alamosaurus and Opisthocoelicaudia. From the Gorro Frigio Formation, Chubut Province, an ‘E.Feruglio’ museum team discovered the remains of a large titanosauriform with strongly opisthocoelous dorsal centra bearing hyposphene-hypanthrum articulations, acuminate pleurocoels, centro-parapophyseal laminae, short, unforked and flattened neural spines and a postspinal laminae that suggests the presence of the unpreserved prespinal laminae. A ‘step-shaped’ articulation in the scapulo-coracoid resembles those of Brachiosaurus brancai and BYU 9462.  The scapula also bears curved posteroventral and half straight antero-dorsal borders and a flat transverse section.  The coracoid is square. Upchurch’s Titanosauridae or Sanz’s ‘P’ node are not defined by dorsal vertebrae features, but Salgado et al.’s Titanosauridae are diagnosed by the absence of hyposphene-hypanthrum, present in the new taxa. Although preliminary, the present evidence shows a non-Titanosauridae titanosaur condition, closer to Andesaurus than to Chubutisaurus.  This new taxon probably confirms the first Aptian argentinian titanosaur. However, this doesn’t necessarily imply an argentinian origin for Titanosauria because most of those worldwide recorded basal titanosauriforms are poorly known and are not scored yet in a cladistic framework.