The green dinosaur

Habitat: Wet Tropics lowland rainforest, Far North Queensland

Conservation status: Not listed, but many areas where it occurs are protected

Scientific name: Idiospermum australiense (it is the only species in the genus Idiospermum australiense). Common names include ribbonwood, 'green dinosaur', and 'idiot fruit tree', which is considered a mis-translation of the meaning of its genus name. Idiospermum derives from the Ancient Greek idios, which means individuality or peculiarity. Sperma means seed.

About

The 'green dinosaur' (Idiospermum australiense) is one of the world’s rarest and most primitive flowering plants. Twice thought to be extinct, its rediscovery in the Daintree Lowland Rainforest in the 1970s - inside the bellies of some dead cattle - was arguably Australia’s most significant botanical find.

Idiospermum - the green dinosaur

I. australiense belongs to a plant family with a lineage stretching back more than 120 million years, a time known as the Age of the Angiosperms. The oldest known fossils of the tree are 88 million years old and since then is has remained relatively unchanged. As such, the 'green dinosaur' is often referred to as a living fossil. It is the most famous of the primitive North Queensland rainforest plants and is of great significance to science and to our understanding of how flowering plants have evolved.

Sprouting from a seed the size of a human fist - one of the largest seeds of any Australian plant - this evergreen tree grows to 20-40m tall, up to 90cm in diameter. They are found only in lowland rainforest between near sea level and 260m above sea level. Simple leaves grow singly, in pairs or in whorls of 3-4. Their primitive flowers appear in June and July, and are about 4-5cm in diameter. The flower organs are spirally arranged and the petals are initially creamy white, changing colour as they age from pale pink to a deep crimson.

Unlike other Wet Tropics rainforest plants, the seeds of this plant are severely toxic to most animals, including cassowaries, and are predominantly dispersed by gravity. However there is evidence that the endemic musky rat-kangaroo disperses and burying some seeds. It is thought that during the Pleistocene, the seeds of the I. australiense may have been dispersed by the diprotodon, an ancient, extinct giant marsupial.

Distribution

The I. australiense exists only in three geographically distant populations. One population is in the Daintree Lowland Rainforest below Thornton Peak and the other two are some 150km to the south, in the foothills of the Bellenden Ker range and Mt Bartle Frere.

These populations thrive in ‘environmental refugia’, which are habitats located near rain-attracting mountains that have remained climatically stable for millions of years.

This stability has allowed for the preservation of an extraordinary diversity of plants, including many primitive species, which have retained anatomical and genetic features similar to those of ancestral flowering plants.

Germinating Idiospermum seed

Discovery and rediscovery

Timber-cutters were the first European-Australians to recognise the species south of Cairns in the late 1800s. It's likely they gave the tree one of its common names – the ribbonwood. The tree was then thought to be extinct, but was later brought to the attention of German botanist Ludwig Diels who described a species in 1902.

But when Diels returned to the location of his discovery, he found the vegetation had been cleared for a sugarcane farm and so the species was assumed, once again, to be extinct.

In 1971, a Daintree grazier by the name of John Nicholas called in the police as he thought his cattle were being poisoned. Government veterinarian Doug Clague responded to the call, and when he examined the cows’ stomachs, found relatively intact Idiospermum seeds that had been swallowed whole.

The seed’s apparent ability to kill cattle by inducing symptoms similar to strychnine poisoning – first by causing spasms and then paralysing the nerves - sparked curiosity among botanists. Queensland Herbarium botanist Stan Blake was sent specimens of the seed and recognised the outside shell of the fruit as belonging to the same plant which had been previously described by Diels as Calycanthus. This prompted Blake to request a detailed study of the plant.

Len Webb and Geoff Tracey were ecologists at the CSIRO Rainforest Ecology Research Unit at the time and they were charged with a research trip to locate a complete set of specimens. They located a number of trees in full flower amidst the riparian forest at Oliver Creek, just south of Cape Tribulation. They collected samples that confirmed assumptions about the plant’s identity, and that it represented a primitive line of angiosperms.

It was determined that the specimen was so different from the rest of the Calycanthaceae family that a new genus was warranted. Idiospermum was created, with Idiospermum australiense as its sole species.

Its rediscovery served to reinforce that the 1,200 square km region known as the Daintree Rainforest was deserving of its status as the oldest continuously existing rainforest in the world.

Idiospermum flowers (Image: CSIRO)

Pollination

Small beetles and thrips are attracted to the I. australiense flowers’ scent and colour. They crawl in and lay their eggs in the centre of the flower, which contains the flower’s pollen. This is then transferred to the next flower they visit.

While most modern flowering plants produce seeds which have one cotyledon (monocotyledons) or two (dicotyledons), the seedlings of the Idiospermum can have between two and five cotyledons. The I. australiense can also produce more than one shoot per seed (usually one per cotyledon). It is the only known species to display a continuous spiral of bracts, sepals, petals, stamens and staminodes.

Fruit and seeds of the Idiospermum

Fruits and seeds

Green dinosaur 'fruits' have very distinctive features and do not fit within the definition of true fruits as such: all the protective layers decay while still on the parent tree and each one released is an extremely large naked plant embryo.

These globular plant embryos or seeds can measure 8cm in diameter, thought to be the largest single seeds of any Australian plant. When they reach the ground, they split into three, four, and occasionally more, segments. The starchy reserves in the enormous seeds provide the energy for the seedlings to establish in the low light of the rainforest floor, reaching up to 1m in height.