Evolution and phylogeny of titanosaur sauropods

The fragmentary preservation of most titanosaurs, and the use of different criteria produce controversial aspects in the cladistic analyses. For example, Titanosauria, as a taxon that includes different titanosaur groups was proposed by Bonaparte and Coria (1993), on the paper where described the giant Argentinosaurus, maybe the most large sauropod known. Salgado et al., (1997a) defined Titanosauria as “the most recent common ancestor of Andesaurus delgadoi and Titanosauridae, and all of its descendants”. Later, this node-based clade, was re-defined using different criteria.

First, Sereno (1998) proposed to characterize Titanosauria through a stem-based definition as “all somphospondyls closer to Saltasaurus than to Euhelopus”. Second, Salgado (2003a) followed the definition of Sereno. Third, Wilson and Upchurch (2003) adopted the node-based definition originally proposed by Salgado et al. (1997a). This changing panorama has important consequences for the study of basal titanosaurs.



For instance, if we follow the original definition, Phuwiangosaurus, Ligabuesaurus, Malarguesaurus and Chubutisaurus place outside Titanosauria. In a second alternative, following the definition of Sereno (1998), some of them could be considered as titanosaurs, such as emphasize González Riga et al. (2009a). In this context, a carefully revision of the phylogenetic definitions of the clades and named used during the last decade is necessary.

The phylogenetic study of titanosaur sauropods is in its first stages. For example, the taxonomic description of the most South American titanosaur taxa do not include a cladistic analysis or, at least, a preliminary phylogenetic study. For example, from 27 titanosaur genus known in South America, only 6 (i.e. Mendozasaurus, Rinconsaurus, Ligabuesaurus, Muyelensaurus, Futalongkosaurus and Malarguesaurus) have cladistic analyses in their description (González Riga, 2003; Calvo and González Riga 2003; Bonaparte et al., 2006; Calvo et al., 2007b; Calvo et al., 2007a; González Riga et al., 2009a).

This indicates that the most papers comprise topics of diagnosis, description and comparison, without further considerations.
Most of the cladistic studies of sauropods include few titanosaur taxa. However, in the last year, the analyses of Salgado et al. (1997a), Sanz et al., (1999), Curry Rogers and Foster (2001), Smith et al., (2001); Wilson (2002), González Riga (2003), Calvo and González Riga (2003), Upchurch et al., (2004), Curry Rogers (2005), Bonaparte et al., (2006), Calvo et al. (2007a; 2007b), and González Riga et al. (2009a) increased the number of titanosaur genus analyzed and show a progresive inclusion of characters usefull for titanosaurs.

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