Cleverly adapted to fishing: When the spinosaur Irritator challengeri opened its snout while hunting, the lower jaw structures spread and thus widened its pharynx, a digital skull reconstruction suggests. The anatomical clues also show that Irritator kept his head down while hunting and snapped comparatively weakly but quickly. In this way he apparently captured small aquatic animals in Cretaceous South America, the paleontologists say.
Bizarre, spectacular and mysterious: Due to their unusual characteristics, the spinosaurs became celebrities of the Cretaceous animal world. The history of their discovery dates back to 1912: Spinosaurus aegyptiacus from North Africa was the first to enter the paleontological scene. This predatory dinosaur was over 16 meters long, had long appendages on the vertebrae and a snout that was elongated like a crocodile. In the meantime, it has been shown that it was an extremely large representative of a whole group of predatory dinosaurs, which it gave its name to: In addition to North Africa, Cretaceous predatory dinosaur species that can be assigned to the group of spinosaurs were also discovered in Europe, Australia and North and South America .
Various studies have already shown that the unusual features of this group of dinosaurs are due to adaptations to life on or in the water. They apparently fed on fish and other aquatic creatures. So far, however, skull fossils have been able to provide comparatively little information about these animals, because there are only very few finds of these structures. The best-preserved skull is a find from an approximately 115-million-year-old rock layer in eastern Brazil, which is kept in the State Museum of Natural History in Stuttgart. The fossil, about 85 centimeters long, comes from the Spinosaurus species Irritator challengeri. With a length of about 6.50 meters, it was comparatively small. However, it had basic characteristics similar to those of the gigantic Spinosaurus aegyptiacus.
X-rayed and biomechanically analyzed
However, even the most complete known spinosaur skull fossil has remained cryptic. Because the poor state of preservation has so far not allowed any detailed insights. An international team of paleontologists led by Marco Schade from the University of Greifswald has now examined the remains of the skull with the help of X-ray computer tomographs used in medicine and materials research. In this way, the scientists were able to work out the individual bones and draw conclusions about missing parts. They then used the results to develop a three-dimensional reconstruction of the skull. This also reflected former muscle structures.
As the paleontologists report, the skull model provided interesting information about the behavior and adaptations of the animal. The biomechanical models therefore suggest that when Irritator opened its mouth, certain bone structures in the lower jaw spread outwards due to the shape of the jaw joint. In this way, the animal's pharynx expanded, the scientists explain. Although the mechanism differs, the concept is thus similar to the expansion of the gular pouch in pelicans, writes the University of Greifswald in its communication on the study.
Adaptations for hunting in water
The biomechanical evidence also shows that Irritator had weak bite power compared to other large predatory dinosaurs. However, the results suggest that the musculature and the structure of the mouth probably enable very rapid movements of the jaw. The researchers report that the anatomy of the skull also gives an indication of the animal's head position: when hunting, Irritator apparently tilted its snout downwards by about 45 degrees in order to be able to see three-dimensionally in front of it. Because only with this posture no disturbing structures impede the field of vision.
As the team summarizes, the results again show adaptations to the aquatic lifestyle in this representative of the spinosaurs. It apparently preyed on small aquatic prey such as fish, which its anatomical features and mechanisms enabled it to grasp effectively and swallow quickly. Findings from the site where Irritator was found also fit in with this: the dinosaur lived there in a lake landscape in which numerous species of fish were found.
The information on skull anatomy also provided new clues about the evolutionary history of spinosaurs, the paleontologists report. Because it opened up new possibilities for comparison with other predatory dinosaurs. "The new study provides a better understanding of how spinosaurs lived and shows that, relative to other carnivorous dinosaur lineages, they developed anatomical characteristics quite rapidly during their evolution, which ultimately led to them becoming particularly specialized and different dinosaurs," writes the Graduated from the University of Greifswald.
Source: University of Greifswald, specialist article: Palaeontologia Electronica, doi: 10.26879/1242