A pair of apparently conjoined whale calves have been found dead off the coast in Mexico, in what scientists described as "without precedence" in the region.

The whales - which washed up on January 5 - weighed about half a tonne and measured 4 meters (13 feet) long and were linked at the mid-section.

A marine biologist from the National Natural Protected Areas Commission told Reuters conjoined whales or "siamese" whales have never been logged before in Mexico.

It would have been virtually impossible for the whales to survive, indicating they were probably born prematurely or stillborn, scientists added.

This video of the gray whales, with two visible heads and two tails, was taken by Krystian Abundez in the Ojo de Liebre lagoon, which leads into the Pacific Ocean along the Baja California peninsula, an important breeding ground for gray whales.

Scientists who found the whales told Reuters the creature was female, but said they did not know yet whether the twins shared internal organs, because it was not opened up for examination.

Tissue samples have been taken for further investigation. The carcass has been buried at a beach close to the lagoon, where it was found. Scientists will wait for the carcass to decompose in order to recover the skeleton for further scientific studies.

The gray whale migrates along North America's Pacific Coast between arctic seas and the lagoons off of Mexico's Baja California.

They are about 46 feet long (14 meters) and weigh up to 40 tonnes. Gray whales feed off the sea bottom, scooping up mud and eating small crustaceans and tube worms found in sediments.