Leaving a trail

Photo: Otago Museum.
Photo: Otago Museum.
Fossil leaves secure our place in an ancient story, writes Emma Burns.

In the 1820s, French geologist Adolphe-Theodore Brongniart examined a collection of fossilised leaves of long-extinct plants sent to him from India. One fossil among them was the first described specimen of a very special group of  hardy Permian plants: he named it Glossopteris.

Glossopteris was a seed-fern tree, an unusual plant by today’s standards. Flowerless, it bore pine-like seeds and had broad tongue-shaped leaves with distinctive vein patterns similar to living ferns.

During the Permian, the landscape was changing from slush to lush as the giant swamp forests of the Carboniferous began to dry out and the temperatures rose.  These trees grew in thick groves in unfrozen areas alongside southern ice sheets and their roots were well suited to cold, boggy conditions.

Given the quantities of leaves found in fossil deposits, Glossopteris were likely deciduous. Fallen leaves accumulating with sediments on the boggy ground would, with time and burial beneath modern deposits slowly transform into coal.

Geologists involved in the colonial quest for coal  through the 19th century and into the 20th century came to recognise the familiar leaf structure of Glossopteris in Permian coal and fossil beds. After the early finds in India, fossil Glossopteris discoveries followed in Australia, South Africa, South America and Tasmania.

With the growing number of finds, the similarities in these fossil assemblages distributed across different continents began to grow in importance as evidence for the theory that lands now separated were once joined. An interconnected landmass of Madagascar, India and Africa was coined "Gondwanaland" by Austrian geologist Eduard Suess after the Gondwana region of India.

The name stuck, and over time the southern supercontinent’s membership grew as geological evidence connected other continents. With German physicist Alfred Wegener’s work on the theory of continental drift, Glossopteris finds became one of the key fossil markers providing evidence to more accurately work out how the sections of Gondwana pieced together.

During  Robert Falcon Scott’s fatal return journey from the South Pole in 1912, the party explored the rock formations of the Beardmore Glacier. Later that year the journals, records, and  16kg of rock samples were recovered, among them the first record of Glossopteris collected from the Antarctic, and the hardest won.

In New Zealand, fossil-bearing Permian rock is rare. Remarkably, a small number of partial leaves of Glossopteris were found alongside fossil bivalve molluscs in a Permian marine deposit at Productus Creek in Southland. These represent the earliest plant fossils in New Zealand’s record, and are proof of New Zealand’s place in Permian Gondwana.

The Permian period came to a close about 252 million years ago with the world’s largest extinction. Some cite an impact event, others volcanic catastrophe. Glossopteris hung on, but dwindled and had vanished by the mid Triassic, about 25 million years later.

Otago Museum is hosting a travelling exhibition, Life before Dinosaurs: Permian Monsters, which brings the Permian to life, focusing on the animals that ruled the Earth millions of years before the dinosaurs.

The museum displays a Glossopteris fossil collection, on loan from the University of Otago’s Geology Museum, in the Southern Land, Southern People gallery. Although perhaps not as impressive as animatronic monsters, their importance is enormous.

- Emma Burns is curator, natural science at Otago Museum.

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