Researchers have discovered that a 40-million-year-old whale fossil, once thought to be the heaviest animal of all time, is now second to the blue whale.

The huge, 17-meter whale, which is believed to have very thick bones, was originally thought to have weighed up to 340 metric tons, the equivalent of 150 cars stacked on top of each other.

But US researchers have now reduced its weight, saying it would not have been able to survive or even swim if it weighed as much as previously predicted.

In a new study published in the journal PeerJ, this ancient animal was put back on the scale and slimmed down to half the weight of the largest known blue whales.

The skeleton of a fossil whale known as Perucetus colossus was once thought to be the heaviest animal to have ever lived on Earth.

Researchers estimate it lived about 39 million years ago and belongs to an extinct group of early whales called basilosaurids.

Researchers found out that the bones of this oldest animal were unusually thick.

The giant fossil whale Perucetus colossus has been found to have both voluminous internal bones and extra bone growth on the outside—a condition called pachyostosis, which is also seen in some modern-day aquatic mammals such as manatees.

Based on a number of assumptions, researchers initially estimated this whale's body mass to be about 180 metric tons, but peaked at 340 metric tons.

According to these calculations, the Perucetus colossus is as heavy as the largest blue whale ever known, despite being considerably shorter at just 17 meters compared to blue whales, which can reach an incredible 33 meters.

Researchers now estimate that the 17-meter-long Perucetus colossus actually weighed between 60 and 70 tons.

According to new estimates, a Perucetus of up to 20 meters can weigh more than 110 tons, which is still well below the 270 tons of the largest blue whales.

At the new weight, the whale was able to swim to the surface and stay there, breathing and recovering after diving, as most whales do, the researchers said.