Hard of hearing, but a good head
African dinosaur was hard of hearing but had agile skull movement for self defense
Using computed tomography to examine the inner-ear anatomy of a 150 million-year-old dinosaur fossil, a group of paleontologists from the Museum of Natural History in Berlin, Germany—among them a Brazilian—was able to infer the specimen’s hearing range and some features of its movements. Their study shows that Dysalotosaurus, a small bipedal herbivore, heard a narrow range of low-frequency sounds, similar to what alligators and primitive birds heard. This means that it would not have been able to hear most human conversation, for example.
Its senses were probably sufficiently refined, however, to discern approaching predatory theropods such as the agile Elaphrosaurus, which was a little larger than Dysalotosaurus, and the gigantic Allosaurus, a carnivore similar to Tyrannosaurus rex. While grazing in herds, Dysalotosaurus paid attention to its surroundings with its nose inclined slightly upward. The paper also suggests that the side-to-side movements of its head were important to the animal’s survival. “We don’t just talk about bones in our paper; we were able to tell something about the animal’s life,” says Johannes Müller, a paleontologist who supervised the detailed description of the fossil skull that was published in the September 2012 issue of the Journal of Vertebrate Paleontology. “It’s as if Dysalotosaurus were taking a stroll in our back yard.”
Since 2010, Müller has been advisor to Gabriela Sobral, a Brazilian doctoral candidate whose dissertation is funded through an agreement between the Coordinating Agency for the Improvement of Higher Education Personnel (Capes) and the German Academic Exchange Service (DAAD). They are researching the evolution of the archosaurs, a group of animals that includes many extinct dinosaurs and pterosaurs, as well as present-day birds and crocodiles. The hearing of crocodilians is similar to that of birds, which are in fact descendants of the only group of dinosaurs that survived, the theropods. There is still considerable debate among researchers as to which of these characteristics of the hearing of birds and crocodiles were inherited from a common ancestor, and which of them are the product of independent evolutionary histories that led to similar results. Ascertaining the evolution of the inner ear of these dinosaurs could help shed light on such questions. Müller and Sobral’s preliminary conclusions, however, point to a history that is more complex than anyone had imagined.
Before she began her research in Germany, Sobral had done in Brazil only theoretical studies on evolution. “I wanted to learn more about anatomy, and I needed practical experience,” she recalls. That need coincided with the purchase by Berlin’s Museum of Natural History of a Nanotom, an apparatus built on the same principle of operation as tomographs used in medicine. The machine emits X-rays that pass through an object in several directions and are then detected by sensors. The resulting data are processed by a computer to create a three-dimensional model of the object’s internal structure. “It’s an advanced technique for studying parts of the anatomy that cannot be seen by the naked eye,” explains Max Langer, a paleontologist from the University of São Paulo campus in Ribeirão Preto. While the resolution of a medical tomograph is on the order of hundreds of micrometers (thousandths of a millimeter), the Nanotom has a resolution of nearly five micrometers. “Our machine is used to examine small objects ranging from the size of a fist to minutiae like insect genitalia,” Sobral notes.
While searching for some interesting material to examine with the Nanotom, Sobral found, half-forgotten in the museum’s collection, the skull of a dinosaur of the order Ornithischia (despite its name, the group did not give origin to birds). The bones of the specimen, from the species Dysalotosaurus lettowvorbecki, were disconnected but well preserved. The skull was part of what little remained from a pile of Dysalotosaurus fossils that the Berlin-based museum had lost during the Second World War. The fossils had been unearthed along with more than 200,000 kilos of bones between 1909 and 1913 during expeditions conducted by paleontologists from the museum at the foot of Mount Tendaguru in Tanzania. A number of famous dinosaur discoveries occurred there, including the long-necked sauropod Brachiosaurus and the spiny ornithischian Kentrosaurus, which coexisted with Dysalotosaurus during the Upper Jurassic period 140 to 160 million years ago.
Other researchers—one in 1955 and one in 1989—had already described the skull of D. lettowvorbecki based on the fossil remains and on drawings and photographs of the lost material. However, they did not have any way to examine the internal cavities of the walls of the skull—which were filled with petrified sediment—without damaging them.
Upon scanning the pieces of skull with X-rays at a high enough power to distinguish sediment from bone, Sobral confirmed that the millimetric cavities of the inner ear, both inside the lateral wall of the skull and inside the roof of the braincase, were intact.
Working with Müller and Christy Hipsley, an American researcher from the museum, Sobral compared the inner ear of Dysalotosaurus with that of extinct and living species. They paid special attention to the cavity that houses the cochlea, the part of the ear containing the cells that discriminate sounds.
The human cochlea, as in all mammals, is spiral-shaped like the shell of a snail. But in the other vertebrates its tissue extends in a straight line through the bony canal. In 2009, a study conducted by paleontologist Stig Walsh of London’s Natural History Museum compared several species of living reptiles and birds. The study showed that it is possible to deduce the sound frequencies that animals can hear, based on the size of the base of the skull and the length of the cochlear canal. “The longer the cochlea, the better the animal is able to discriminate between high-and low-frequency sounds,” Sobral explains.
The nearly 10-millimeter cochlea of Dysalotosaurus, regarded as short, allowed it to distinguish a relatively narrow range of sound frequencies, between 350 and 3,850 Hertz (Hz)—in other words, neither very low nor very high. This range is similar to that of the crocodile species with the best hearing capability, such as the alligator, and the species of living birds closest to the dinosaurs, such as the heron and the ostrich.
The conclusion agrees with that of other studies suggesting that dinosaurs in general did not hear very high sounds. In 2007, researchers inferred from the animals’ weight that Brachiosaurus and Allosaurus had better hearing, ranging between 100 and 1,000 Hz. “But weight estimates of animals is a rather controversial topic in paleobiology,” Sobral points out.
The paleontologists’ attention was also drawn to another region of the hearing, the cavities of the semicircular canals. Present in one form or another in all vertebrates, these three canals are the part of the ear responsible for ensuring that the animals maintain their balance when they move around (see the image). The three canals are approximately perpendicular to one another. Fluids moving within each of these canals send the brain information about the body’s movement in the three spatial dimensions. The size of the canals indicates the animal’s sensitivity to movement in a specific plane.
The lateral semicircular canal of Dysalotosaurus was slightly larger than the other two. The discovery was a surprise, because in dinosaurs studied previously—a few theropods and an ornithischian—the anterior semicircular canal is the largest, the same as in birds.
Paleontologists also know that the inclination of the lateral semicircular canal is linked to the animal’s alert posture, that is, the way it stands when paying attention to its surroundings. Animals on alert tend to position their head so that the lateral semicircular canal is parallel to the horizon line.
In alert posture, Dysalotosaurus likely held its head slightly raised, with its nose pointing 17° above the horizon. Similar studies are still few, but of the known dinosaurs, only Dysalotosaurus and the sauropod Massospondylus had an alert posture with the head inclined upward. Most dinosaurs held their head horizontally, like Allosaurus, or slightly downward, like Tyrannosaurus, supposedly to broaden their field of vision by combining the view from both eyes, which are generally quite far apart from one another in archosaurs. From this, the researchers conclude that Dysalotosaurus did not have well-developed binocular vision.
The paleontologists were also surprised to find a cavity in the ear of Dysalotosaurus known as the pseudo-round window. Present in birds, crocodiles and a few other reptiles, this cavity is covered by the secondary tympanic membrane, a structure that also evolved independently in the ear of mammals. The role of this membrane—regarded as the mark of refined hearing—is to facilitate the movement of sound waves in the cochlea, thus increasing its sensitivity.
The presence in a single species of characteristics regarded as primitive, such as a short cochlea, along with modern characteristics such as a secondary tympanic membrane, complicates the story of the evolution of the ear. “The ear of Dysalotosaurus shows that the hearing structures each evolved independently rather than all together,” Sobral explains.
Sobral, G. et al. Braincase redescription of Dysalotosaurus lettowvorbecki (dinosauria, ornithopoda) based on computed tomography. Journal of Vertebrate Paleontology. 32(5). Sept. 2012.