The largest dinosaurs of all time had a bad habit of losing their heads. When a titanosaur died, its small skull often wound up far from its massive body, making it hard for paleontologists to track down an animal’s noggin millions of years later.
But now, a complete skull from a newly named species of titanosaur is offering paleontologists a detailed look at how these giants sensed the world around them.
Sarmientosaurus musacchioi was an herbivorous dinosaur that lived 95 million years ago in what is now Patagonia in Argentina. Paleontologist Rubén Martínez at the National University of Patagonia San Juan Bosco found several neck vertebrae and an ossified tendon, as well as the beautifully preserved skull. The remains are described today in PLOS ONE.
“More than 60 legit titanosaur species have been named to date,” but Sarmientosaurus is only the fourth to have an entire skull, says study co-author Matthew Lamanna, a paleontologist at the Carnegie Museum of Natural History in Pennsylvania.
What’s more, “Sarmientosaurus has probably the most complete and best preserved skull of any sauropod from South America to date,” says paleontologist Mathew Wedel of Western University of Health Sciences in California.
Anatomical investigations and CT scans of the fossil cranium hint at how Sarmientosaurus fits on the titanosaur family tree. The shape of the skull and teeth indicate that Sarmientosaurus was an old-fashioned titanosaur, with a more primitive look than its relatives living at the same time.
Until now, Wedel says, there was an anatomical gap between the earliest titanosaurs and the older sauropods that they evolved from, like Brachiosaurus.
“To me, Sarmientosaurus is cool because it bridges that gap,” Wedel says. “You can take one look at this thing and say, ‘Yeah, cool, we’ve been waiting for someone like you.’”
An animation shows a 3-D model of the Sarmientosaurus skull and its brain cavity. (Video by WitmerLab, Ohio University)
The skull also revealed that Sarmientosaurus had senses that set it apart from its titanic kin. “The eye sockets of Sarmientosaurus are proportionally humongous,” Lamanna says, which may mean this dinosaur had better vision than some of its relatives.
The dinosaur’s inner ear was suited to picking up low-frequency sounds, the study shows, indicating that these dinosaurs probably communicated with low rumbles, says Lamanna, whose work has been funded in part by the National Geographic Society Committee for Research and Exploration.
Curiously, the new study also suggests that this mighty dinosaur lumbered through life looking like an oversize Eeyore. Based on the inner ear and preserved neck tendon, Sarmientosaurus probably kept its snout pointed down toward the ground at the end of a low-slung neck.
The study authors suggest that this posture may mean Sarmientosaurus was a grazer browsing on low-lying vegetation. But while Wedel says that the animal may well have had a downward-facing snout, he’s not yet convinced about its feeding habits.
“The inner ear usually tells us something about the alert posture of an animal, not its feeding posture,” Wedel says. The narrow snout of Sarmientosaurus might mean this dinosaur was better suited to browsing at different levels than pulling up plants from the forest floor.
Still, Wedel says the authors are to be commended for their thorough detail in describing the new skull. Lamanna adds that Sarmientosaurus helps expand our understanding of the titanosaurs as a motley crew.
“Even for many hard-core dino fans, sauropods are kind of ‘seen one, seen them all,’” Lamanna says. But the titanosaurs alone included an array of distinct dinosaurs, some with broad teeth and some with narrow biters, others with short necks and some with long. Sarmientosaurus is only the newest face in that grand picture of dino diversity.