An exotic plant has produced male and female cones outdoors in Britain for what is believed to be the first time in 60m years. Botanists say the event is a sign of global heating.

Two cycads (Cycas revoluta), a type of primitive tree that dominated the planet 280m years ago, have produced cones on the sheltered undercliffs of Ventnor Botanic Garden on the Isle of Wight.

The species is native to Japan and usually only found indoors as an ornamental plant in Britain, but one of the garden’s plants has produced what is believed to be the first outdoor female cone on record in the UK.

Cycads previously lived in what is now Britain millions of years ago, with fossils of the plants found in the Jurassic strata of rock stretching from the Isle of Wight to the Dorset coast, an era when the Earth’s climate had naturally high levels of carbon dioxide.

A plant growing outside at Ventnor first produced a male cone seven years ago, but this year different plants have produced flower-like male and females cones, giving botanists the opportunity to transfer pollen and generate seed.

“For the first time in 60m years in the UK we’ve got a male cone and a female cone at the same time,” said Chris Kidd, the curator of Ventnor Botanic Gardens. “It is a strong indicator of climate change being shown, not from empirical evidence from the scientists but by plants.”

According to Kidd, last summer’s heatwave and this year’s record-breaking temperatures have caused the plant’s production of cones, with a run of milder winters also helping. He said records kept at the botanic garden show that the highest average temperatures for January 100 years ago were lower than today’s lowest average for the same month. As a result, the 27-hectare (67-acre) garden, with a climate milder than any other part of Britain except the Isles of Scilly, is growing temperate plants that would have once been unable to survive through a British winter.

“It’s not something that’s happened with a short-term mild spell. It’s a longer-term warming which is making these things happen,” he said. “The [cycad] plant will have made the decision to commit to cone production [in summer 2018], and that production is set in place to run through over winter and produce the following year.

“Thirty years ago we couldn’t have grown them. But these plants have been growing out of doors here in the gardens for 15 years, going through their natural cycles.”

Cycads are a relic from a time before flowering plants. In its native Japan, Cycas revoluta is believed to be pollinated by beetles. In the botanic garden, the plant with a male cone is some distance from the female and so the pollen is to be transferred by hand in about a week.

“By crossing them and making seed from them we’re doing something that has never been done before in the UK,” said Kidd. “We see the undercliff here as being a predictor for the wider British landscape in 20 to 30 years time.”

Cycad species are composed of three families, the only surviving members of an ancient and largely extinct lineage that has changed little since the Jurassic period, and so are considered “living fossils”. All cycads are native to warmer parts of the world, but are naturally absent from Europe and Antarctica. Jurassic-era fossils of specimens strikingly similar to the present-day plants have been found on both continents.