The large group of dinosaurs, which included large predators like the T. rex, also included a number of unusual beasts, oddballs and outlaws, and a newly described dinosaur from Mongolia, about the size and somewhat resembling a goose, fits that description.

The dinosaur, named Natovenator polidontus, lived about 72 million years ago during the Cretaceous period and was built like a diving bird with a streamlined body, an elongated goose-like neck and a long, flattened snout with a mouth of more than 100 small teeth, scientists said in Thursday.

It was almost certainly covered in feathers, they added.

“Natovenator has many distinctive characteristics,” said paleontologist Yuong-Nam Lee of Seoul National University in South Korea, lead author of the study published in the journal Communications Biology.

Although it was a relative of the fast small predator Velociraptor, Natovenator was adapted to a semi-aquatic lifestyle in a freshwater ecosystem, perhaps floating in rivers and lakes, paddling with its forelimbs and using its flexible neck to grab fish and insects or dive underwater to catch prey, they said are scientists.

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

Natovenator is part of a group of dinosaurs called theropods that share traits including being bipedal.

The group is best known for large carnivores including Tyrannosaurus, Tarbosaurus and Giganotosaurus.

But theropods, many of which were feathered, branched out in unusual directions with examples such as the sloth-like long-clawed Therizinosaurus, the ostrich-like Struthiomimus, the termite-eating Mononykus, and the entire bird lineage.

“The diversity of theropod dinosaurs by the end of the Cretaceous is absolutely incredible,” said paleontologist and study co-author Philip Currie of the University of Alberta.

“I believe there will be more discoveries of fascinating, bizarre theropods in the future,” Lee added.

It is not known that many of the dinosaurs that we call “non-avian”, meaning they were not birds, lived a semi-aquatic lifestyle.

A close relative of Natovenator called Halszkaraptor, described in 2017, lived a similar lifestyle around the same time in the same region. Both animals had a very bird-like appearance and were closely related to the bird lineage.

Natovenator was about 45 cm long, with a skull about 7 cm long.

Its forelimbs appeared somewhat flattened, perhaps as an adaptation for paddling and swimming.

The ribs point toward the tail, as in diving birds, an arrangement that reduces drag in the water and allows for efficient swimming.

“Natovenator – which means ‘swimming thief’ – is an amazing little animal for several reasons. First, it’s small and delicate. When we found it, we weren’t sure of its identification because it looked more like a lizard or mammal skeleton than a dinosaur. After was processed, we realized it was a theropod dinosaur, but what kind? It finally made sense after Halszkaraptor was described,” Currie said.

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

During the Cretaceous there were a variety of diving birds, including the North American Hesperornis, which reached 1.8 meters in length, but no such animal is known from the area where Natovenator lived.

“More than 30 different tetrapod lineages have independently moved into aquatic ecosystems,” Lee said. “Why not dinosaurs?”