Ants 50 million years older than any known before have been found trapped in a piece of amber, scientists disclosed on Wednesday.

The four specimens are preserved in 92-million-year-old amber from New Jersey, USA.

Ants are only distinguished from other similar insects by the presence of a gland above the hind leg.

In other specimens of a similar age the gland has not been visible, so scientists could not be sure they were ants, but it can clearly be seen in the New Jersey insects.

Scientists now believe ants evolved in the earliest part of the Cretaceous age, more than 130 million years ago.

The findings were published in the journal Nature by Donat Agosti and colleagues from the American Museum of Natural History in New York.