WASHINGTON Early whales gave birth - and probably rested and mated - on land, according to a study published Wednesday that examined 47.5 million-year-old fossils discovered in Pakistan, AFP reports.

The `stunning discovery` reinforces the belief that modern sea-dwelling mammals originated from terrestrial ancestors, said H. Richard Lane, director of the National Science Foundations paleontology program, which funded the research.

The team that discovered the Pakistan fossils in 2000 and 2004 were initially baffled when they found whale skeletons so close together, said team leader Philip Gingerich of the University of Michigan.

`When we first saw the small teeth, we thought we were dealing with a small adult whale, but then we continued to expose the specimen and found ribs that seemed too large to go with those teeth,` he said.

`By the end of the day, we realized we had found a female whale with a fetus.` The fetus was positioned for head-first delivery, like land mammals but unlike modern whales, according to the study, published in the online journal PLoS.

The positioning indicated the whales still gave birth on land, said Gingerich.

Other clues -- such as the whales big teeth that would have been well-suited to catch and eat fish -- suggested to researchers that the mammals lived most of their time in the sea, but came on land to rest, mate and give birth.

The primitive whale couldnt travel far on land, although `they could support their weight on their flipper-like limbs,` said the studys authors.

`They clearly were tied to the shore,` according to Gingerich. `They were living at the land-sea interface and going back and forth.` The discovery gives unparalleled insight into how the early whale gave birth and how it transitioned from land to sea, according to the researchers.

The fossils species `occupies an intermediate position on the evolutionary path that whales traversed as they made the transition from full-time land dwellers to dedicated denizens of the deep,` the authors said in a statement.

`Specimens this complete are virtual `Rosetta stones,`` Gingerich said.

They provide `insight into the life history of extinct animals that cannot be gained any other way.`