It lived at the edge of a lake or river 370 million years ago on the fringes of a flooded forest. It was about a metre long and as a half-fish, half-amphibian with four limbs it could slither about in pursuit of small fish in shallow tidal pools.

It mightn't sound like your next of kin, but the newly discovered creature, yet to be named, could be a missing link that connects all 25,000 modern species of land animals, including humans, with an ancient common ancestor that bridged the gap between life on the land and life in the sea.

The discovery was made on the slimmest of evidence, two small bits of fossilised lower jaw both barely an inch long. The details they reveal, however, fit neatly into a fossil framework that includes a fish Panderichthys that lived 375 million years ago and the Acanthostega, a four-limbed, eight-toed beast that lived 365 million years ago.

Experts at the Natural History Museum in London who announced details of the fossil finds this week believe the two jaw pieces come from the same species of tetrapod. Tetrapods were primitive four-limbed amphibians which were among the first marine animals to creep out of the water and test the quality of life on land.

The oldest confirmed tetrapod fossil was found in Scotland and is thought to be about 363 million years old. Ireland has some of the oldest known tetrapod footprints, hardened into solid rock on the Kerry coastline near Valentia.

Tetrapods are credited with being our earliest known vertebrate land ancestors and they began a line of descent that includes dinosaurs, reptiles, early mammals and eventually humankind. If the unnamed fossils are eventually confirmed to be from a tetrapod species then it will be the oldest yet identified.

"This new species was an early ancestor of people and all back-boned animals living on the land including the dinosaurs," said Dr Per Ahlberg of the Natural History Museum, who made the fossil discovery.

"This discovery tells us something about where we came from and who we are. What emerged from this was not just ourselves, but 25,000 living forms," he said.

The discovery is remarkable in that it was made at all. One fossil was dug up in Estonia two years ago. Another was found some years ago by an amateur collector near Ligatne, Latvia, and was kept in Riga until identified by Dr Ahlberg as from the same class of tetrapod as the first.

While both came from the Baltic coastline, the creatures that produced the fossils lived when Latvia and Estonia were tropical wetlands located somewhere near the equator. And the fossils now occupy a place between older fishlike ancestors with gills and fins and younger crocodile-like descendants with legs and toes.

The common connection between these three groups can be seen in their jaws, Dr Ahlberg said. "Of all the bones in a skeleton that we could have found, the jaw is exactly the right piece, because it carries the signature of the creature. The jaws of fish and land animals are very different and change dramatically during the evolution process."

Many questions remained, he admitted. "The question we can't yet answer is whether this creature had paired fins or legs. A whole fossilised skeleton of one of these animals would be one of the great discoveries of the century."

He was happy, however, to speculate on how it might have lived as it sought food in its traditional watery hunting grounds, but also in its brave if somewhat dry new world.

"There is every reason to believe it is really spending most of its time in the water," he said. "No doubt it has some terrestrial ability, but we are looking at an animal that essentially lives in water and produces its crawling capacity to haul itself over mud-banks.

"Imagine a flooded forest at the edge of a lake or river. There are small fishes living in there which you want to catch. If you have the ability to crawl and slither through this stuff, you are set up to exploit this. It's not hard to see how you could get from that to more truly terrestrial animals."

Although given celebrity status among the world's palaeontologists, the creature has yet to acquire a public name. This is to be kept secret until a paper on the discovery is published next August in the journal Palaeontology.