A freshwater dolphin found only in China is now "likely to be extinct", a team of scientists has concluded.
The researchers failed to spot any Yangtze river dolphins, also known as baijis, during an extensive six-week survey of the mammals' habitat.
The team, writing in Biology Letters journal, blamed unregulated fishing as the main reason behind their demise.
If confirmed, it would be the first extinction of a large vertebrate for over 50 years.
The World Conservation Union's Red List of Threaten Species currently classifies the creature as "critically endangered".
Sam Turvey of the Zoological Society of London (ZSL), one of the paper's co-authors, described the findings as a "shocking tragedy".
"The Yangtze river dolphin was a remarkable mammal that separated from all other species over 20 million years ago," Dr Turvey explained.
"This extinction represents the disappearance of a complete branch of the evolutionary tree of life and emphasises that we have yet to take full responsibility in our role as guardians of the planet."
The species (Lipotes vexillifer) was the only remaining member of the Lipotidae, an ancient mammal family that is understood to have separated from other marine mammals, including whales, dolphins and porpoises, about 40-20 million years ago.
The white, freshwater dolphin had a long, narrow beak and low dorsal fin; lived in groups of three or four and fed on fish.
The team carried out six-week visual and acoustic survey, using two research vessels, in November and December 2006.
"While it is conceivable that a couple of surviving individuals were missed by the survey teams," the team wrote, "our inability to detect any baiji despite this intensive search effort indicates that the prospect of finding and translocating them to a [reserve] has all but vanished."
The scientists added that there were a number of human activities that caused baiji numbers to decline, including construction of dams and boat collisions.
"However, the primary factor was probably unsustainable by-catch in local fisheries, which used rolling hooks, nets and electrofishing," they suggested.
"Unlike most historical-era extinctions of large bodied animals, the baiji was the victim not of active persecution but incidental mortality resulting from massive-scale human environmental impacts - primarily uncontrolled and unselective fishing," the researchers concluded.