Its appearance would have seemed familiar to us - but the Cretaceous creature that paleontologists are now reporting was not closely related to waterfowl: Natovenator polydontus proves that there were dinosaurs that were similarly adapted to a semi-aquatic lifestyle as diving bird species. In addition to other common features, the streamlined body in particular proves that this small creature could swim dexterously under water. The find extends earlier evidence of semiaquatic representatives of the so-called non-bird dinosaurs, say the scientists.
They were by no means "oldtimers" in the history of evolution: in recent decades, paleontology has shown more and more how extremely highly developed and diverse the dinosaurs were. Their success story would certainly have continued if they had not been violently swept off the stage of evolutionary history by the asteroid impact around 66 million years ago. Only one special lineage of dinosaurs survived the inferno: the ancestors of today's birds. To distinguish the groups, the "classic" representatives of the dinosaurs are often referred to as non-avian dinosaurs. It seems clear that the diverse representatives of this group had already adapted to many ecological niches that were later occupied again by mammals and modern birds.
Similar to waterfowl?
In 2017, a study provided evidence that this also applied to a lifestyle such as that of modern birds that hunt in the water: paleontologists had found characteristics in the dinosaur Halszkaraptor escuilliei that indicated a semi-aquatic lifestyle and waterfowl-like characteristics. In particular, his strikingly flattened forelimbs suggested that he might have used them like a penguin's flippers when diving. But the fossil remains did not allow any more precise conclusions to be drawn about the body shape of the animal. However, the researchers working with Sungjin Lee from Seoul National University are now reporting on a creature in which, among other things, this aspect also fits in with a semi-aquatic lifestyle.
The findings are based on a study of a fossil from Omnogovi Province, Mongolia, which is dated to the Late Cretaceous. It shows the skeleton of a theropod, which is only about 30 centimeters tall – a representative of the two-legged dinosaurs. According to the basic characteristics, it does not belong to the group of ancestors of birds. In addition to the skull, spine, a forelimb and hind legs, the fossil includes some rib bones. The paleontologists report that it was now possible to draw conclusions about the former body shape of the animal.
"Snappy" in the water on the go
Analysis of the remains showed that the little dino was apparently related to Halszkaraptor and also possessed features previously interpreted as indicating a predatory aquatic lifestyle. This also influenced the name that the scientists gave to the new species: Natovenator polydontus means something like "swimming hunter with many teeth". In his case, too, it can be assumed that he used his noticeably flattened front limbs like flippers when diving. With its long neck and sharp teeth, it may have been adept at hunting fish.
However, another feature is typical of today's animals with this way of life: A streamlined body ensures reduced water resistance when moving. This is exactly what Lee and his colleagues found evidence of in the case of Natovenator polydontus. The researchers noted that the animal's ribs were conspicuously oriented towards the back of the body, as is the case in diving bird species. As they explain, this creates a streamlined body shape. In addition, the features of the ribs also reflect a type of thoracic compression known from aquatic reptiles, the paleontologists report.
Overall, the findings indicate that Natovenator polydontus possessed adaptations that suggest life as a semiaquatic predator, Lee and his colleagues conclude. The scientists say that this is further evidence of the diverse forms and strategies that non-avian dinosaurs also produced.
Source: Communications Biology, doi: 10.1038/s42003-022-04119-9