The Rise of Non-Titanosaur Macronarians in South America

Abstract

Although the origin of Neosauropoda probably dates back to the Early?Middle Jurassic, it is not until the Late Jurassic that Macronaria become well represented in the fossil record. Unlike the great diversity of South American titanosaurs, basal macronarians are relatively scarce in the fossil record; even so, they provide valuable information for better understanding the first steps at the origin of this clade. The only non-titanosauriform macronarian sauropod from South Americais Tehuelchesaurus from the Oxfordian-Tithonian of Argentina, while all other basal macronarians found up to date are titanosauriforms (either Brachiosauridae or Somphospondyli). Brachiosaurids were abundant in the Jurassic, but they apparently became extinct at the Jurassic/Cretaceous boundary all over theworld except in North America. Isolated elements from the Late Jurassic of Argentina and Padillasaurus from the Early Cretaceous of Colombia were suggested as brachiosaurids, but these assignments are questionable. Until now, no clear somphospondylans have been recorded in Jurassic levels. In South America, basal, non-titanosaur somphospond ylansare represented by three taxa registered in Argentina: Chubutisaurus and Ligabuesaurus from the Early Cretaceous (Aptian?Albian) and Malarguesaurus from the Late Cretaceous (Turonian?Coniacian). Here, we provide a complete revision on the fossil record of non-titanosaur macronarians from South America and the current phylogenetic status of them.