Lithostrotians

Lithostrotia is a clade of derived titanosaur sauropods that lived during the Early Cretaceous and Late Cretaceous. The group was defined by Upchurch ''et al.'' in 2004 as the most recent common ancestor of ''Malawisaurus'' and ''Saltasaurus'' and all the descendants of that ancestor. Lithostrotia is derived from the Ancient Greek , meaning "inlaid with stones", referring to the fact that many known lithostrotians are preserved with osteoderms. However, osteoderms are not a distinguishing feature of the group, as the two noted by Unchurch ''et al.'' include caudal vertebrae with strongly concave front faces (procoely), although the farthest vertebrae are not procoelous.

History of research

In 1895, Richard Lydekker named the family Titanosauridae to summarize sauropods with procoelous (concave on the front) caudal vertebrae. The name Titanosauridae has since been widely used, and was defined by Salgado and colleagues (1997), Gonzalaz-Riga (2003), and Salgado (2003) as a node-based taxon. According to a proposal by Wilson and Upchurch (2003) looks today much of the research on the use of that name from: Wilson and Upchurch published a revision of the genus ''Titanosaurus'' and declare the type species ''Titanosaurus indicus'' as invalid because it is based only on two vertebrae of the tail, showing no diagnostically usable features. Consistently these authors consider ranking groups that are based on ''Titanosaurus'' as the nominal taxon, Titanosauridae, Titanosaurinae and Titanosauroidea - also considered invalid. In 2004 Upchurch and colleagues presented the new group Lithostrotia to describe the same group as Titanosauridae, but instead it was not based upon a specific taxon. The name Lithostrotia is not currently recognized by all researchers.

Definition and synapomorphies

Upchurch and colleagues (2004) define the Lithostrotia as a node-based taxon that includes the last common ancestor of ''Malawisaurus'' and ''Saltasaurus'' and all descendants of that ancestor. According to this definition the Lithostrotia includes all forms that are more derived than ''Malawisaurus'' in phylogenies.

In addition to defining the group, Upchurch and colleagues gave two common derived features (synapomorphies), which serve to distinguish the group from non-members. The first is that all caudal vertebrae apart from the farthest distal were procoelous, meaning their front face was concave. Also, the front (proximal) caudal vertebrae were particularly strong procoelous. This first feature is also shared with Mamenchisauridae.

Unchurch ''et al.'' named Lithostrotia based on the presence of osteoderms in many members, but the eponymous osteoderms do not represent synapomorphy, as the evolutionary history of osteoderms is unknown within the titanosaurs. It may be this trait has developed multiple times independently within the titanosaurs and Lithostrotia, as osteoderms are known in many saltasaurids, '' Mendozasaurus'', ''Aeolosaurus'', ''Ampelosaurus'', and various other genera both within and outside Lithostrotia with different morphologies.

Systematics

Lithostrotia is a derived group of titanosaurs, excluding primitive forms such as ''Andesaurus'' and '' Phuwiangosaurus''. The possibly equivalent clade Titanosauridae was positioned in a phylogenetic analysis by Calvo ''et al.'' (2007), where it included all titanosaurs apart from ''Andesaurus'', though multiple primitive forms were not analyzed. Other phylogenies, by Unchurch ''et al.'' (2015), instead have found a few, non-lithostrotian titanosaurs, or nearly all non-brachiosaurid titanosauriformes within the group. Poropat ''et al.'' (2015) conducted a similar analysis to one of Unchurch ''et al.'' (2015). This analysis found that ''Andesaurus'', ''Argentinosaurus'' and ''Epachthosaurus'' were within Titanosauria but outside Lithostrotia, and the latter group included ''Malawisaurus'', ''Nemegtosaurus'', '' Diamantinasaurus'', '' Tapuiasaurus'' and ''Alamosaurus'' as basal lithostrotians outside Saltasauridae. Another phylogenetic analysis by Poropat and colleagues in 2016, partially reproduced below, found '' Diamantinasaurus'' as a non-lithostrotian titanosaur and the sister taxon of the contemporary ''Savannasaurus''.
The cladogram below follows Mocho ''et al.'' (2019) with the new subgroup called Lirainosaurinae.Mocho P, Páramo A, Escaso F, Marcos-Fernández F, Vidal D, Ortega F. 2019. Titanosaurs from Lo Hueco (Campanian-Maastrichtian) reveal new information about the evolutionary history of European titanosaurs, pp. 111. In: The Palaeontological Association (ed.), 63rd Annual Meeting, 15th–21st December 2019, University of Valencia, Spain, Programme Abstracts, AGM papers

Poropat ''et al.'' (2016)

Mocho ''et al.'' (2019)

While the clade Lithostrotia has been used in many different phylogenetic analyses as a description for a clade of titanosaurs, its use is not universal. The clade has been omitted from results as it has been considered a synonym of Titanosauria, or it has been used as either a very large clade encompassing almost all titanosaurs, or a smaller clade that excludes the taxa of Colossosauria. These incongruent results are because of the instability of the defining taxon ''Malawisaurus'', which may in fact represent a chimaeric assemblage of remains and requires re-evaluation to determine what Lithostrotia includes.

References

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