The next winner, coming in at #8, in our PLOS Paleontology Top 10 Open Access Fossil Vertebrates contest, is Sarmientosaurus musacchioi, which was published in April of this year in PLOS ONE. Sarmientosaurus is no humble creature; rather it belongs to one of the most massive groups of organisms to ever inhabit this planet: the titanosaurs. But beyond its grandeur of simply being an impressive titan, the discovery of Sarmientosaurus presented a prime opportunity for the researchers involved in the study, an opportunity that just doesn’t happen often enough for titanosaur researchers. The type material of Sarmientosaurus includes a complete skull.
“Titanosaurs included the biggest land animals ever, so we want to know as much about them as we can,” said Matt Lamanna, a co-author in this study. “But to truly understand a creature, you need to have its head. And because titanosaur skulls are super-rare, lots of important aspects of how these dinosaurs lived and behaved have really been anybody’s guess.”
Titanosaur skulls are indeed rare. Of the 60 or so described titanosaurs, only four (four!) include somewhat-to-nearly complete skulls. That includes this newcomer Sarmientosaurus, from the Upper Cretaceous Bajo Barreal Formation of Patagonia, Argentina, as well as Nemegtosaurus (from the Upper Cretaceous Nemegt Formation of Mongolia), Rapetosaurus (from the Upper Cretaceious Maevarano Formation of Madagascar), and Tapuiasaurus (from the Lower Cretaceous Quiricó Formation of Brazil). Of course, many of the other known titanosaur taxa have fragmentary material from the skull and jaws, but relative to the amount of postcranial material preserved in the fossil record, skulls are considerably rare, and a lot of that is due to the fragility of the skull, compared to other skeletal material, such as the massive vertebrae or legs, for example. The skull was also not crushed or distorted due to taphonomic processes, and its preservation allowed the team to examine it using a variety of methods. The skull went to Larry Witmer’s lab at Ohio University. Witmer (one of the co-authors of the study) used CT scanning methods, retrieving a host of information.
“The Sarmientosaurus skull is beautifully-preserved, which meant that we could tease out a ton of information.” said Witmer. “It was really exciting for us to work through the CT scan data because it gave us a glimpse into the biology and lifestyle of this animal like we rarely get with dinosaurs.”
As can be seen in the 3-D videos above, the team was able to reconstruct organs based on the shape of the cranial cavities for the brain, inner ear, and eyeballs. Typical of most sauropods, the brain is incredibly small relative to the size of its body. But, as it turns out, Sarmientosaurus probably had fantastic vision and hearing compared to some of its relatives. The inner ear morphology and orientation suggests that Sarmientosaurus was well-attuned to low frequencies, and also potentially held its head down low, facing downwards while feeding on low vegetation.
The study also tested the evolutionary relationships of Sarmientosaurus to other titanosaurs and sauropods, and recovered it as a basal lithostroation titanosaur, lower in the phylogenetic tree than Nemegtosaurus, Rapetosaurus, and Tapuiasaurus, and the most plesiomorphic taxon of the group to have a complete skull preserved. However, the study suggests that Sarmientosaurus represents a “ghost lineage”, co-occuring with more derived titanosaurs such as Tapuiasaurus (from the Aptian of Brazil), yet retaining many of the more plesiomorphic characters of the clade. The features that Sarmientosaurus retains offers support and insight into the relationship of titanosaurs to brachiosaurs, suggesting that titanosaurians are closely related to members of Brachiosauridae and other titanosauriforms. This hypothesis of relationships is not novel, but this study is one of the first to base that hypothesis on solid cranial characters, rather than solely or predominantly on postcranial elements.
Martínez RDF, Lamanna MC, Novas FE, Ridgely RC, Casal GA, Martínez JE, et al. (2016) A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria. PLoS ONE 11(4): e0151661. doi:10.1371/journal.pone.0151661
Additional coverage of Sarmientosaurus:
This post includes edited press materials provided by the Carnegie Museum of Natural History.