A revision of Titanosaurus Lydekker (dinosauria ‐ sauropoda), the first dinosaur genus with a ‘Gondwanan’ distribution

Abstract

Synopsis Titanosaurs represent approximately one‐third of sauropod diversity and were geographically widespread throughout the Cretaceous, especially on southern continents. Titanosaurs evolved numerous appendicular synapomorphies that account for their specialised ‘wide‐gauge’ limb posture, which can be recognised in their trackways. The macronarian origin of titanosaurs is only recently agreed upon and aspects of their inter‐relationships remain poorly understood. Titanosauria is named for the poorly known genus Titanosaurus, which was coined by Lydekker in 1877 on the basis of a partial femur and two incomplete caudal vertebrae. Fourteen species have since been referred to Titanosaurus, which distribute the genus across Argentina, Europe, Madagascar, India and Laos, and throughout 60 million years of the Cretaceous. Despite its centrality to titanosaur systematics and biogeography, the genus Titanosaurus has never been revised. A re‐evaluation of all Titanosaurus species recognises as diagnostic only five. The type species T. indicus is invalid because it is based on ‘obsolescent’ characters ‐ once diagnostic features that have gained a broadertaxonomic distribution over time. Consequently, the genus Titanosaurus and its co‐ordinated rank‐taxa (e.g. Titanosaurinae, Titanosauridae, Titanosauroidea) must be abandoned. The unranked taxon Titanosauria, however, remains valid. A new phylogenetic taxonomy is proposed for Titanosauria that utilises nodes that have been judged stable by the most recent cladistic analyses. The early appearance of titanosaur ichnofossils (Middle Jurassic) and body fossils (Late Jurassic) precludes a vicariant origin for the group, but such a pattern cannot yet be ruled out for lower‐level taxa within Titanosauria.