Antarctic prehistory

A scientist studies a fossil dolphin skeleton at Marine Plain in the Vestfold Hills
A fossil dolphin skeleton at Marine Plain in the Vestfold Hills (1988). (Photo: Patrick Quilty)
Image of Gondwana progressively becoming the continents we know todayMap of GondwanaFossil leaf embedded in rock

Life has been on earth for about 3.5 billion years. Until about 575 million years ago, all life was microscopic plant life.

Before that time, life in the Antarctic was very different from the way it is now. Antarctica was part of the supercontinent Gondwana and was fully vegetated with all the animal and plant life that existed elsewhere on the supercontinent. Of course there were regional differences, just as there are differences between the animals and vegetation in parts of Australia.

When animal life burst forth on earth, initially all in the marine environment, Antarctica lay along what is now Australia's southern margin, but the pair was rotated anticlockwise about 90° and the equator ran though both continents. About 400 million years ago Gondwana moved dramatically and Antarctica took up its place over the South Pole, moving a little from time to time. Thus, the South Pole has not always been within Antarctica, but always very close to it.

When animal life evolved, the carbon dioxide content of the atmosphere was much higher than now and some estimates put it at 15–20 times higher. As the oxygen content grew it eventually reached the stratosphere, where it was converted to ozone, providing protection from ultraviolet rays. When this happened, plant life moved from the oceans to the land and then land animals evolved to feed on the plant life.

About 130 million years ago Gondwana really began to disintegrate, with India and South America-Africa departing. The links between Australia and Antarctica remained very strong until about 55 million years, when Australia began moving north quickly, allowing Antarctica to become isolated for the first time since animals evolved. At about 30 million years the Antarctic Circumpolar circulation evolved, the Southern Ocean began to circulate around the southern world, and the modern Antarctic environment evolved.

Fossil record 

Until 55 million years ago, the history of life on Antarctica was very similar to that on other southern continents, especially Australia. The fossil record is very similar, with abundant plants and animals, including those that lived on land. There are abundant plant fossils in many areas of Antarctica from this period.

Perhaps historically, the most significant evidence of earlier vegetation, well preserved in Antarctica, is the leaf form Glossopteris, which was at the heart of the concept of Gondwana - an hypothesis first proposed to explain the widespread distribution of glacial rocks and Glossopteris. When Robert Falcon Scott and his men were found dead in their tent on the way back from the South Pole, specimens of Glossopteris from the Beardmore Glacier region were found with them. While they had jettisoned almost everything else on the way back, they recognised the immense scientific importance of Glossopteris and refused to leave the specimens behind. These specimens are some 280–300 million years old.

There is a major dispute about when Antarctic vegetation died out. It is clear that a good vegetation cover - containing many elements that still exist in some southern hemisphere cold temperate rainforests such as Tasmania, New Zealand and southern South America - continued in Antarctica until about 35 million years ago. However, there are other tantalising pieces of evidence to suggest it continued until even much more recently.

For example, abundant remains of the southern beech Nothofagus (N. beardmorensis) have been found at 86°S, only 400 kilometres from the South Pole. The age is contentious but may be as young as 2–3 million years, suggesting that the modern ice sheet-covered environment of Antarctica may be a more recent development than previously thought. The leaves occur abundantly in lake sediments which are in turn under- and over-lain by glacial sediments. The leaves are very similar but larger than the Tasmanian species Nothofagus gunnii, also known as the Deciduous Beech or Tanglefoot. Study of the wood suggests that the plant grew as a straggly shrub, similar in growth form to the Arctic willow (Salix arctica). 

Dinosaurs lived in Antarctica and are particularly well known from the northern tip of the Antarctic Peninsula although few have been described formally. They include ankylosaurs (the armoured dinosaurs), mosasaurs and plesiosaurs (both marine reptilian groups).

Seymour Island, just off the Antarctic Peninsula, is one of the most important fossil sites on earth. It contains a bewildering array of different fossils, representing a great array of different environments, and also includes one of the very few uninterrupted sections across the Cretaceous/Tertiary boundary, the time when dinosaurs and many other life forms died out, probably due to meteorite or asteroid impact.

Marine Plain in the Vestfold Hills is another important locality which is yielding a great diversity of fossils from 4.5–4.1 million years ago. Perhaps the best known are the fossil dolphins and whales. About 5 or 6 species are known but only two dolphins have been studied in detail. The dolphins are new to science and are very different from modern dolphins. They are quite common and occur as almost complete skeletons as if they died and sank gently into the mud without being disturbed by predators or rough sea conditions. They died in an embayment which was less than 40 metres deep. They consist of two species, one about 4.5 m long and the other about 8–9 m long. They are unusual in lacking teeth, and one, in being so large for a dolphin. They may have consumed squid by sucking them into their very long, slim jaws and locating them by echo-location devices. Other species of cetaceans are represented by fragments only.

An interesting fossil from Marine Plain is a 'palinurid' crustacean - a small lobster. This family includes the modern lobsters of commerce around Australia. The group is now extinct from the Antarctic. The palinurids have another interesting aspect. They belong formally to the Family Palinuridae which includes Palinurus, Panulirus, and Linuparus. These names are all anagrams of each other and suggest a sense of humour among the group of biologists that named them. Palinurus in the ancient world, was the navigator in Virgil's Aeneid.

Marine Plain fossils occur where they died, in contrast with most other localities where fossils of the age are known and this is where the value of the deposit will eventually be documented.

It is clear that a major extinction event occurred around Antarctica in the period 2–3 million years ago, but it is now becoming evident that it may be part of a global extinction event that is now being documented. The cause is debated but one strong contributor probably was the closure of the Isthmus of Panama, which caused the flow of water from the Atlantic to Pacific Ocean to stop, thus causing the Gulf Stream and northern hemisphere glaciation to begin. Another contributor may have been the Himalayas which were growing at the time and influenced the circulation of the atmosphere in the northern hemisphere.

How do we work out the story of Antarctic fossil life when there is so little outcrop and so few workers in the field? Modern biochemical methods are allowing us to work out the general trend of evolution, even in some groups which do not leave fossils. Two groups where these techniques have been particularly successful are in the Euphausiacea (including Antarctic krill such as Euphausia superba) and the Antarctic fish that belong to the Notothenioidea. Both groups have no fossil record. For the Euphausiacea this is true globally and yet it constitutes one of the world's great biomass features.

One key approach is to examine DNA from mitochondria in modern cells. This DNA is believed to be almost entirely maternal in origin and thus not part of the reproduction process involving nuclear DNA. Mitochondrial DNA accumulates mutations at a rate that is known roughly. The difference in mitochondrial DNA in two similar organisms is a function of the time since they evolved from a common ancestor. Biochemical techniques now allow estimates of this time to be made and a rough evolutionary tree to be established. The evolution of krill and fish is now roughly worked out and the times of change coincide quite well with oceanographic events that are identifiable in the geological record.