By Ángela Posada-Swafford
Illustrated by Henry González
Plenty of dinosaur fossils can be found throughout the world: femurs, hips, vertebrae, ribs, fingers, and even the occasional tooth. But dinosaur skulls are another matter altogether: they are so fragile that most don’t survive the ravages of time and erosion. So, for as powerful as these creatures were, one could say that dinosaurs in general have the bad habit of losing their heads.
This is especially true for titanosaurs, the largest creatures to ever have walked the Earth. Only four skulls of these peaceful, long-necked herbivores have been found in one piece, due to their bones being as thin as a sheet of paper. And only two of these four skulls were found in perfect condition. Both come from the Argentine Patagonia and were classified this year by two groups of paleontologists.
Titanosaurs are a new species, so they still have no common name. Scientists have christened them with a combination of tributes to the place where they were found and the experts who worked on them: Sarmientosaurus mussacchioi and Tapuiasaurus macedoi.
“Although sixty species of the titanosaur have been discovered, their skulls are as rare as hens’ teeth,” said Matthew Lamanna, assistant curator at the Carnegie Museum of Natural History in Pittsburgh, and a specialist in dinosaurs from the southern hemisphere. “To truly understand what this animal was like, its evolutionary history, the food it ate, its intelligence, its senses, all of this depends on the structure of the skull and the soft tissue within it,” adds Lamanna, who is part of the international team that worked on the study of the Sarmientosaurus under the leadership of Dr. Rubén Martínez, who discovered the fossil in 1997 in the windy province of Chubut, in Argentina’s Patagonia.
“These two skulls are opening an unprecedented window into the biology of the titanosaur, which ironically belonged to the most successful group of dinosaurs and which we know less about precisely because of the lack of skulls,” said Diego Pol, renowned expert in titanosaurs from the Museum of Paleontology Egidio Feruglio, in Trelew, Argentina. Pol, together with Brazilian colleagues from the University of São Paulo, described the Tapuiasaurus. “Now comes the fun part, which is sitting down to compare both creatures,” adds Pol (co-discoverer of the world’s largest dinosaur: a giant titanosaur introduced to society in 2014).
For example, an interesting difference between the two skulls being studied is their teeth. While the teeth of the Sarmientosaurus are spoon-shaped or spatulate, and positioned along the entire jaw, in the other skull they are as small as cylinders that exist only in the front of the muzzle. This gives some clues about what they ate and how they chewed. Maybe they cut the grass using a scissor-like movement? Or they could have been eating flowering plants, the new food source that began to dominate the Cretaceous world?
It’s hard to resist the temptation to imagine a titanosaur with a bunch of flowers in its mouth, but we can’t rush to conclusions. “It makes sense to speculate about it, although you would almost have to find grains of pollen in some of their guts to be certain,” jokes Lamanna.
In addition to their teeth, paleontologists have discovered that both skulls are full of anatomical surprises. The Sarmientosaurus, for example, has an exquisitely delicate brocade of windows and columns and holes the size of hands that once housed enormous eyes, a modest cerebellum, and a brain the size of a lemon, too small for its bony vault. And buried deep within the bone, the greatest treasure of all: the structure of the internal ear.
“This has been a rare privilege because we could get inside the hearing abilities of the animal,” interjects Dr. Lawrence Witmer, palaeobiologist at Heritage College of Osteopathic Medicine at Ohio University in Athens, Ohio, an expert in the skulls of modern and extinct vertebrates. Thanks to CT scans made in Argentina, Witmer, who has worked with dozens of dinosaur fossils, was able to build digital 3D models of the entire skull, including the sizes and shapes of its complex interior architecture. “This dinosaur in particular was especially sensitive to low frequency sounds that can be heard over long distances without dissipating. Perhaps this creature could follow the footsteps of other members of its group without seeing them directly, just like whales in the ocean do.”
The internal ear also revealed something new about the animal’s posture. Based on the orientation of the semicircular canal and the position of the joint where the neck connects to the skull, Witmer suggests that the dinosaur normally had its snout oriented towards the ground.
“This tells us that it fed on low-lying plants on the ground floor, using its neck to draw an arc from side to side, almost without moving. Something like a vacuum cleaner.”
The eye sockets, though large in the titanosaur, are immense in the Sarmientosaurus. Witmer says, “We tried to put eyes inside it. It’s one of the things that my lab does, beyond just understanding the bones. And it seems that this animal had pretty large eyes, compared to other titanosaurs.” In this regard, he thinks that the sense of sight was very important for this dinosaur, as important as other senses like the ability to hear low frequency sounds. And this is interesting because these characteristics were less important in the more advanced titanosaurs. “That’s something that we did not expect and it’s exciting to witness this change in the anatomy of the senses, where a dinosaur goes from depending on his eyes to depending on his ears. It’s like we’re clearly seeing how evolution works.”
This kind of opportunity rarely occurs in paleontology, because our planet is a dynamic and violent environment that tends to erode, crush, or flood the windows to the past. But now a portal to the world of the giants with crystal skulls is opening and scientists like Matthew Lamanna are rushing to get their foot in the door.
“Finding the head of a titanosaur, finding it so well-preserved, and finding it in such an interesting animal, is something that literally happens once in a lifetime. I wouldn’t be surprised if I never work with a fossil as important as this one again.”