A titanosaur specimen (MZSP-PV 807) from the Early Cretaceous (late Barremian-early Aptian) Quiricó Formation in Minas Gerais, southeastern Brazil was described as the holotype of the new genus and species Tapuiasaurus macedoi by Zaher et al. (2011). In the original description (focused solely on the skull), Zaher et al. recovered this taxon as a member of Nemegtosauridae sensu Wilson (2002), but the cladistic placement of Tapuiasaurus as sister to Nemegtosaurus rather than Rapetosaurus created a 55 million year ghost lineage, and Poropat et al. (2015) recovered Rapetosaurus as a sister taxon of Isisaurus within Saltasauridae and Nemegtosaurus as more basal among advanced lithostrotians, indicating that cranial characters cited as diagnostic for Nemegtosauridae by Wilson (2002) occur in more than one lithostrotian clade. Wilson et al. (2016) provided a more detailed description of the skull of the Tapuiasaurus macedoi, and utilizing new morphological data, they re-analyzed the cladistic position of Tapuiasaurus and found it to be extremely labile, further confirming that Tapuiasaurus was not as closely related to Nemegtosaurus as previously thought. Meanwhile in their description of the lithostrotian Sarmientosaurus from Argentina, Martinez et al. (2016) also recovered Tapuiasaurus as distantly related to Nemegtosaurus. In an unpublished thesis describing of the postcranial skeleton of Tapuiasaurus, Navarro (2019) agrees with Wilson et al. (2016) that Tapuiasaurus is distantly related to Nemegtosaurus, but finds Tapuiasaurus to fall in a more basal position within Lithostrotia along with Isisaurus and Rapetosaurus. Given that the inter-relationships among derived titanosaurs are still a work in progress despite recent advances, I will allow the main findings of the phylogenetic analysis of Tapuiasaurus by Navarro (2019) and their implications for lithostrotian macroevolution do the talking, especially in light of the recent paper by Poropat et al. (2021) recovering Sarmientosaurus outside Lithostrotia as the sister taxon of Diamantinasaurus.
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| Skull of the holotype of Tapuiasaurus macedoi (MZSP-PV 807) (after Zaher et al. 2011) |
In the cladistic analysis in part 4 of his thesis (titled "Results"), Navarro (2019) recovers Tapuiasaurus as the sister taxon of the East Asian taxon Yongjinglong and an unnamed lithostrotian titanosaur from the Adamantina Formation of the Prata municipality in southeastern Brazil, with all three taxa forming a clade sister to Isisaurus and Rapetosaurus. He erects the nomen ex dissertationae "Tapuiasaurinae" to include Tapuiasaurus, Yongjinglong, the Prata titanosaur, and probably Gobititan, and the clade formed by "Tapuiasaurinae", Isisaurus, and Rapetosaurus falls as phylogenetically intermediate between the most basal lithostrotians (Malawisaurus and Rukwatitan) from East Africa and the most derived lithostrotians (Eutitanosauria sensu Navarro 2019). The recovery of Isisaurus and Rapetosaurus as outside Saltasauridae (contra Poropat et al. 2015) adds a new twist in understanding lithostrotian macroevolution, because their placement as sister to "Tapuiasaurinae" would imply that more than one clade of lithostrotian titanosaurs acquired Nemegtosaurus/Rapetosaurus-like cranial features by the Aptian-Albian interval, since Yongjinglong hails from the Aptian-Albian upper part of the Hekou Group of northern China, and hence temporally intermediate between Tapuiasaurus and Rapetosaurus. Yongjinglong itself was originally described as an opisthocoelicaudiine saltasaurid by Li et al. (2014) but was recovered outside Titanosauria as a euhelopodid by Mannion et al. (2019). Although the vast majority of colossosaurian titanosaurs are based on specimens that include no skull material, a recent paper by Averianov et al. (2021) recovering the Early Cretaceous (Hauterivian) genus Tengrisaurus as a relative of Colossosauria suggests that both saltasaurids and non-saltasaurids diverged from the most basal lithostrotians and the Isisaurus+Rapetosaurus+"Tapuiasaurinae" clade by the Hauterivian, in which case complete skull material for many colossosaurian taxa is required to determine how the cranial anatomy of colossosaurians compares to that of saltasaurids, Rapetosaurus and Tapuiasaurus.
With respect to the systematic paleontology section of Chapter 1 in Navarro's (2019) thesis, the genera Adamantisaurus, Arrudatitan, and Trigonosaurus are grouped together in a distinct clade that is sister to Rinconsauria, dubbed "Trigonosaurinae". However, Aeolosaurus and Gondwanatitan are not included by Navarro in his cladistic analysis even though he offers a stem-based definition for Aeolosaurini (under which only Aeolosaurus and Gondwanatitan are included), and cladistic analyses by França et al. (2016), Hechenleitner et al. (2020), Silva et al. (2021), and Soto et al. (2022) recover these genera as closely related to the taxa included by Navarro (2019) in Rinconsauria and "Trigonosaurinae", with the latter two papers grouping Trigonosaurus with Bravasaurus, Gondwanatitan, and Uberabatitan rather than Aeolosaurus and Arrudatitan. Since the thesis by Navarro (2019) is unpublished, and the holotype of Aeolosaurus rionegrinus is less complete than that of Gondwanatitan faustoi, the proposed clade "Trigonosaurinae" is best treated with a high level of caution, considering that the material preserved in the holotype of Adamantisaurus (MUGEO 1282) is far less substantial than that of Arrudatitan and Trigonosaurus.
In his discussion of the comparisons of the postcranial skeleton of Tapuiasaurus with other titanosaurs, Navarro notes that the manus of the Tapuiasaurus macedoi holotype differs from that described for the Australian titanosaur Diamantinasaurus in having manual phalanges in the first metacarpal (unlike the 2-2-2-2-2 phalangeal formula described for Diamantinasaurus), while noting that Opisthocoelicauda has a manus with reduced ossified phalanges (see Poropat et al. 2015). Although he notes that the vast majority of titanosaur taxa do not preserve manual material, he interprets the phalangeal formula of Tapuiasaurus as reflective of a gradual reduction of the manus as a derived condition for titanosaurs more derived than Andesaurus and Diamantinasaurus. The holotype of Savannasaurus elliotorum, which hails from the same geologic unit and location as Diamantinasaurus, also has the phalangeal formula of Diamantinasaurus (Poropat et al. 2016). Although Diamantinasaurus and Savannasaurus are recovered in a distinct titanosaur clade (Diamatinasauria) outside Lithostrotia as more derived than Andesaurus rather than sister taxa of Andesaurus as in Navarro (2019), and no phalangeal remains are known for Andesaurus or Ninjatitan, the phylogenetic placement of Tapuiasaurus by Navarro (2019) is apparently consistent with Navarro's conclusion that the most primitive titanosaurs retained a 2-2-2-2-2 phalangeal formula. Likewise, Navarro (2019) notes that the pes of Tapuiasaurus differs from derived lithostrotians like Alamosaurus, Mendozasaurus, and Opisthocoelicauda in having 10 phalanges (as does Gobititan), concluding that Tapuiasaurus exhibits a combination of a gradually reduced manus and a pes with 10 phalanges (reduced in more derived lithostrotians).
References:
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