Bonitasaura salgadoi: The Missing Link (Or one of them, anyway).
Size: The one juvenile specimen that was found was 23 feet (7 meters) long.
Time Period: The Santonian Stage of the Late Cretaceous Period.
Locale: The Bajo de la Carpa Formation of Argentina.
Name: The generic name means “La Bonita lizard,” in reference to “La Bonita,” the quarry where it was found. The specific name honors Leonardo Salgado, an Argentinian paleontologist of great renown.
This is another sauropod skull post. Like my posts about Abydosaurus and Tapuiasaurus, it’s about the animal’s skull and what it means. Also like these posts, it’s about a macronarian, that famous lineage leading to dinosaurs like Saltasaurus and Giraffatitan.
This animal’s remains are from a partial sub-adult skeleton found in sandstone. Remarkably, this animal is preserved with a skull that resembled that of the diplodocids. This skull’s lower jaw had a sharp ridge set behind a reduced set of teeth. The ridge may have supported a sheath not unlike a beak, which probably had a counterpart in the upper jaw. The sheath would work in tandem with the peg-like front teeth in order to rake vegetation into the mouth and then crop it. Interestingly, Bonitasaura was the only sauropod known to have a beak.
The neck of Bonitasaura was short, and its back vertebrae had projections for muscle attachment. This indicates that the neck was often used intensively during feeding.
Bonitasaura’s skull resembles that of some diplodocid sauropods, animals that titanosaurs were often confused with in the past. Some titanosaurians exhibited long, low skulls without the nasal arches of other macronarians like Brachiosaurus. The lower jaws of these titanosaurians were squared off with comb-like teeth. The titanosaurians resembled diplodocids in their limb proportions and whip-like tails. Bonitasaura permits the recognition of square jaws in a titanosaurian. It also had a peculiar skull shape that increases the known diversity of titanosaurian skulls. The known skulls belong to Rapetosaurus, Nemegtosaurus, and Tapuiasaurus. Unlike the skull of Rapetosaurus, the skull of Bonitasaura is short from front to back and rather tall, resembling the skulls of more basal macronarians, such as Camarasaurus and Malawisaurus. The skull of Rapetosaurus is long and low.
Despite its resemblance to diplodocids, Bonitasaura was a titanosaurian. It was related to mid-sized to large titanosauroids that lived during the Turonian and Campanian stages of South America, and not (as was previously believed) to the nemegtosaurids of the Northern Hemisphere.
The strong resemblance that Bonitasaura bore to the diplodocids shows that titanosaurians rushed in to fill niches left by their extinction. Titanosaurs and diplodocoids may have even competed for food and other resources before the extinction of the latter. Though I said in my post with Tapuiasaurus that the resemblance between Tapuiasaurus and Rapetosaurus indicates that they didn’t need to evolve, and thus didn’t compete with diplodocids as much. I believe that this might have been misconstrued, as I was referring to the one group. Along with the diplodocid niche, titanosaurs took over the niches of all other sauropods (and in the south, all large herbivores).
The titanosaurs are a group whose skulls are poorly known. Still, another skull being found is surprising. Bonitasaura is bizarre, because it doesn’t resemble many of the other titanosaurs whose skulls are known. It had a strange beaked skull which would have sheared and raked plants like their bygone cousins, the diplodocids. These dinosaurs were long-gone by the time the titanosaurs had achieved complete dominance, but their looks and feeding habits were reflected in animals like Bonitasaura, descendant of the animals that ousted them from their feeding niches.