The dinosaur, called Natovenator polydontus - meaning swimming thief - lived about 72 million years ago during the Cretaceous Period and was built like a diving bird with a streamlined body, while possessing a goose-like elongated neck and a long flattened snout with a mouth bearing more than 100 small teeth, scientists said. It almost surely was covered in feathers, they added.

"Natovenator has many peculiar characteristics," said palaeontologist Yuong-Nam Lee of Seoul National University in South Korea, the lead author of the research published in the journal Communications Biology.

While it was a cousin of the speedy little predator Velociraptor, Natovenator was adapted to a semi-aquatic lifestyle in a freshwater ecosystem, perhaps floating on rivers and lakes, paddling with its front limbs, and using its flexible neck to catch fish and insects, or diving underwater to capture its prey, the researchers said.

Its well-preserved remains - a skeleton about 70% complete - were unearthed in the Gobi Desert, which over the decades has been a treasure trove for dinosaur fossils.

Not many of the dinosaurs called "non-avian" - in other words, not the birds - are known to have lived a semi-aquatic lifestyle. A close relative of Natovenator named Halszkaraptor, described in 2017, lived a similar lifestyle at roughly the same time in the same region. Both had a very bird-like appearance and were closely related to the bird lineage.

Natovenator measured about 45 cm long, with a skull about 7 cm long. Its front limbs appeared somewhat flattened, perhaps as an adaptation for paddling and swimming. The streamlining of its body is shown by ribs that point toward the tail, as in diving birds, an arrangement that reduces drag in the water and allows efficient swimming.

"Natovenator is an amazing little animal for several reasons. First, it is small and delicate. When we found it, we were uncertain as to its identification because it looked more like a lizard or mammal skeleton than a dinosaur. Once it was prepared, we realised it was a theropod dinosaur, but what kind? Finally, it made sense once Halszkaraptor was described," Currie said.

"It is very specialised for living in an environment not typical for an animal related to the Velociraptor and its other relatives. Most people think of dinosaurs as specialised land animals, not competing with crocodiles in the water," Currie added.

There were various diving birds during the Cretaceous period, including North America's Hesperornis, which reached about 1.8m long, but none are known from the area Natovenator inhabited.

Reuters