Systematics
The fossil record of titanosaurs, as plains-dwelling herbivores, has been disappointingly fragmentary for such a widespread and successful group (they represent roughly a third of the total sauropod diversity known to date). Only recently have skulls or relatively complete skeletons (see Rapetosaurus) of any of the roughly 50 species of titanosaur been discovered. Many species are poorly known. Much material may be reclassified and some genera renamed as understanding of the clade grows.
The family Titanosauridae was named after and anchored on the poorly known genus Titanosaurus, which was coined by Richard 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 Period. Despite its centrality to titanosaur systematics and biogeography, a re-evaluation of all Titanosaurus species recognises only five as diagnostic. The type species T. indicus is invalid, because it is based on 'obsolescent' characters - once diagnostic features that have gained a broader taxonomic distribution over time. Consequently, use of the genus Titanosaurus has largely been abandoned. The most well known Titanosaurus specimens have since been re-assigned to other genera, including Isisaurus.
Some paleontologists (such as Sereno, 2005 ) have contended that Titanosaurus is too poorly known to use as a basis for classification, family names for which it is the type genus (e.g. Titanosaurinae, Titanosauridae, Titanosauroidea) should not have other genera referred to them. Weishampel et al., in the second edition of The Dinosauria, also did not use the family Titanosauridae, and instead used several smaller titanosaur families such as Saltosauridae and Nemegtosauridae.
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