A Complete Skull of an Early Cretaceous Sauropod and the Evolution of Advanced Titanosaurians

Hussam Zaher,Ricardo Pires-Domingues,Nelson Jorge Da Silva,Peter Larson,Rubén Juarez-Valieri,Claudio Riccomini,Alberto B Carvalho,Diego Pol,Diógenes De Almeida Campos,Paulo M Nascimento,Paul Sereno

doi:10.1371/journal.pone.0016663

Abstract

Advanced titanosaurian sauropods, such as nemegtosaurids and saltasaurids, were diverse and one of the most important groups of herbivores in the terrestrial biotas of the Late Cretaceous. However, little is known about their rise and diversification prior to the Late Cretaceous. Furthermore, the evolution of their highly-modified skull anatomy has been largely hindered by the scarcity of well-preserved cranial remains. A new sauropod dinosaur from the Early Cretaceous of Brazil represents the earliest advanced titanosaurian known to date, demonstrating that the initial diversification of advanced titanosaurians was well under way at least 30 million years before their known radiation in the latest Cretaceous. The new taxon also preserves the most complete skull among titanosaurians, further revealing that their low and elongated diplodocid-like skull morphology appeared much earlier than previously thought.

Highlights

Titanosaurians are known as the most diverse group of sauropod dinosaurs, including one-third of all known genera of sauropods [1,2]
The phylogenetic analysis shows that Tapuiasaurus is deeply nested within an advanced titanosaurian clade formed by nemegtosaurids and saltasaurids, as the sister taxon of Rapetosaurus (Figure 7; Figure S4)
The advanced titanosaurian clade is supported by the presence of postcranial features such as a proximodistally elongated coracoid, procoelous middle and posterior caudal vertebrae, and distal condyles expanded on the anterior surface of the humerus [15], only the first one being preserved in Tapuiasaurus

Summary

Introduction

Titanosaurians are known as the most diverse group of sauropod dinosaurs, including one-third of all known genera of sauropods [1,2] Their characteristic wide-gauge trackways have already been recorded in the Middle Jurassic [3,4], titanosaurians are mostly known from Late Cretaceous postcranial remains [2]. Their abundance in Upper Cretaceous sediments has been regarded as a result of the successful radiation of a clade referred as the ‘‘advanced titanosaurians’’. The absence of well-preserved titanosaurian cranial remains from the Early Cretaceous presents a major hurdle to understanding the fossil record of this group. The present discovery represents the first described titanosaurian skull for the South American continent

Methods

Results

Conclusion