The creature snapped up fish with a scissors-like beak as it skimmed over the water and had a head crowned by a huge, bony crest.
Brazilian scientists Alexander Kellner and Diogenes de Almeida Campos Thursday described a previously unknown type of pterosaur, winged reptiles that were cousins of the dinosaurs.
The find is important both for the oddity of its cranial crest and for the insight that the animal offers into how pterosaurs hunted for food, the researchers said.
They named it Thalassodromeus sethi meaning "sea runner" and "Seth," for the ancient Egyptian god of evil and chaos.
Kellner said Thalassodromeus, which lived 110 million years ago, had a head that measured 4½ feet long due to the size of its crest, a wingspan of nearly 15 feet and a body length of about 6 feet.
"If you didn't have the fossils, you wouldn't believe that such an animal would have ever lived," Kellner said in a telephone interview from Rio de Janeiro.
"Can you imagine such an animal just cruising over the water and skimming over the surface in your direction? It must have been, really, a vision of hell," added Kellner, of the National Museum in Rio.
Searching for food, Thalassodromeus probably glided low over the water in a brackish inland lagoon, its lower jaw skimming the surface of the water, ready to nab any tasty fish or crustaceans it encountered, said Kellner, whose findings were published in the journal Science.
Similarities between this pterosaur's flattened jaws, which end in a scissors-like beak, and the beak of a type of living bird called Rynchops prompted the belief that Thalassodromeus, like these so-called skimmer birds, skimmed over the water's surface, with the lower jaw slightly submerged, Kellner said.
"The new pterosaur from Brazil gives us important information about the feeding strategy of pterosaurs," Fabio Dalla Vecchia, a pterosaur expert at the Paleontological Museum of Monfalcone, Italy, told Reuters.
The most eye-popping characteristic of Thalassodromeus is its large, thin, cranial crest that looks with its V-shaped end like a giant spearhead or knife blade.
The bony crest makes up about three-quarters of the animal's head. Proportionately, it is the largest such crest of any known extinct or living vertebrate, with the exception of one other type of pterosaur.
"This is pretty close to the far end of weird," said Christopher Bennett, a pterosaur expert at the University of Bridgeport in Connecticut who has seen the new specimen. "But pterosaurs are really weird animals."
The crest is covered by a network of grooves that Kellner said represented an extensive system of blood vessels that the pterosaur may have employed to regulate its body temperature -- in this case, cooling off.
Bennett called this "a reasonable conclusion," but said there is "an awful lot of evidence to suggest that crests were used for sexual display" in other pterosaurs.
Alan Feduccia, a dinosaur expert at the University of North Carolina at Chapel Hill, said the authors "have done a splendid job of bringing this large and remarkable ... pterosaur back to life."
He agrees that the animal probably was a skimmer, but questions the authors' interpretation that Thalassodromeus' large crest was for body cooling. Feduccia said there are modern birds, such as the hornbill, with large bony crowns that contain blood vessels used to nourish the head and not for cooling.
But despite the disagreement, Feduccia marvels: "One can only imagine the incredible sight of these flying reptiles skimming the Araripe lagoon some 110 million years ago."
Pterosaurs were not dinosaurs, although both were highly successful types of reptiles. Both appeared about 225 million years ago during the Triassic Period and flourished until 65 million years ago, when an asteroid or other big extraterrestrial object slammed into Earth. Some fossils suggest that pterosaurs had a fur-like body covering.
Pterosaurs were the Earth's first flying vertebrates, appearing many millions of years before birds or bats.
Thalassodromeus lived in the middle of the Cretaceous period -- the final chapter of the age of dinosaurs.
Little is known about pterosaurs because their lightly built bones do not lend themselves to fossilization. Kellner describes Thalassodromeus in the journal Science based on a well-preserved skull found in 1983 at the fossil-rich Santana Formation in northeastern Brazil. He said bones from other parts of the body have been found there, allowing him to determine the animal's wingspan and body size.