The Titanosaurus (in?)validity project Part 4 - Bonatitan

Today's taxon to be compared with Titanosaurus in our continuing series is Bonatitan reigi, described by Martinelli and Forasiepi (2004) as based on two partially preserved individuals, but redescribed by Salgado et al. (2015) as five fragmentary individuals, of which a braincase among the material labeled MACN-PV RN 821 is now the holotype. Other elements under that number and MACN-PV RN 1061 are paratypes, including a mid caudal neural arch probably coming from a smaller individual than the holotype. It derives from the Allen Formation of Argentina.

Titanosaurus indicus lectotype GSI 2191 in ventral (top), anterior (left), left lateral (center) (all after Lydekker, 1879) and left lateral (right) views (after Mohabey et al., 2013; scale = 50 mm).

This case is a bit different from the previous three because we have no preserved mid caudal centra for Bonatitan, and the only somewhat comparable caudal neural arch is from a more proximal position than Titanosaurus. As established before, Titanosaurus shows a more distal-esque morphology than saltasaurines in lacking transverse processes and being narrower compared to its length. But I'm still not sure if this makes Titanosaurus the paravian of titanosaurs or saltasaurines the caenagnathoids of titanosaurs (or both?). Regardless, Bonatitan does have well-developed transverse processes based on the lateral articulations on the neural arch, and the broader morphology expected of a saltasaurine, but its features (tall neural peduncle, prezygapophyses with dorsal projection distally) suggest it's around position 4-5 compared to 6-11 in Titanosaurus. And if there were six vertebrae between these, maybe they could transition from a Bonatitan type to a Titanosaurus type. That being said, the width difference still seems implausible to me and the posterior position of the neural peduncle compared to the anterior centrum edge looks to be less than half as much as Titanosaurus, which has also been a consistent trend in saltasaurines.

Bonatitan reigi paratype MACN-PV RN 1061 in anterior (left) and right lateral (reversed; right) views (after Salgado et al., 2015).

Neuquensaurus' caudal sequence has shown that other neural arch characters like the neural peduncle width and posterior shape between the neural arch and centrum are highly variable within an individual, so we can't use those to distinguish Bonatitan and Titanosaurus. For completeness' sake, there is a ~second caudal vertebra preserved with a deep ventral keel over at least the posterior half of the ventral midline (considered diagnostic by both sets of authors), but no taxon so far has described or illustrated if the first two caudals differ from more distal elements in ventral topography (they often do in theropods).

Is Bonatitan Titanosaurus? Probably not, but we're hindered because due to position and incompleteness they aren't that comparable. In particular, the centra seem to be the most positionally consistent in titanosaurs so far within this portion of the tail, and we just don't have one for Bonatitan. There is a diagnostic character listed by both Martinelli and Forasiepi and Salgado et al. involving the vertebra which could be checked in Titanosaurus- "dorsal to middle caudal vertebrae with deep oval to circular pits on both sides of the prespinal lamina", but the published figures and photo of Titanosaurus don't expose the area well enough. This is also a difficulty in comparing Titanosaurus' neural arches- the lack of a dorsal or posterior view, since the short acute triangle with possible prespinal fossa in anterior view in Lydekker's drawing isn't that useful.

Next time- Ibirania.

References- Lydekker, 1879. Indian pre-Tertiary Vertebrata. Part 3. Fossil Reptilia and Batrachia. Palaeontologica Indica (series 4). 1, 20-33.

Martinelli and Forasiepi, 2004. Late Cretaceous vertebrates from Bajo de Santa Rosa (Allen Formation), Río Negro province, Argentina, with the description of a new sauropod dinosaur (Titanosauridae). Revista del Museo Argentino de Ciencias Naturales 6(2), 257-305.

Mohabey, Sen and Wilson, 2013. India’s first dinosaur, rediscovered. Current Science. 104(1), 34-37.

Salgado, Gallina and Paulina Carabajal, 2015 (online 2014). Redescription of Bonatitan reigi (Sauropoda: Titanosauria), from the Campanian-Maastrichtian of the Río Negro Province (Argentina). Historical Biology. 27(5), 525-548.