Science Spin May 2009

Solving the mystery of the Triassic mass extinction

By Anthony King

Dinosaurs, scientists believe, departed the Earth following the impact of a huge asteroid which devastated the Earth and cooled its climate. Less is known about how the curtain rose on their long reign. What gave these massive creatures the opportunity to rise and dominate the Earth? Researchers believe that dinosaurs got their big break 200 million years during a period of 'mass extinction'. It seems possible now that good luck was the main reason the dinosaurs, rather than a rival group, rise to dominance in the Jurassic.

However, by the time dinosaurs got the chance they had been waiting for, they had already been around and evolving for about 30 million years, diversifying from one type into a range of shapes and sizes. But, during the geological period known as the Triassic - between 248 and 213 million years ago - the dinosaurs were still only 'bit players'.

A group of crocodile ancestors, called the crurotarsans, were in a more dominant role than dinosaurs at this time. The cause of the mass extinction which killed off these pre-historic crocodile relatives, and many other species, is still under debate, but whatever the factors were that caused it to happen, these extinctions opened the door to the dinosaurs.

Triassic

If we could travel back in time to a period around the closing stages of the Triassic, we would be able to see bizarre creatures - to our modern eyes at least - such as two-legged crurotarsans with beaks rather than teeth walking around, sharing the landscape with large herbivorous crocodiles covered in protective armour, as well as strange creatures in trees, called monkey lizards, with grasping hands and spines at the end of their tails.

As well as these rather odd creatures we could encounter gliding reptiles, giant pig-like reptiles with jaws equivalent to those of a grizzly bear, and swift predatory crurotarsans with greyhound proportions and pencil-thin legs. In this world, there are also fish-eating phytosaurs that look very like modern crocodiles, while the top line predator to be found are the rauisuchians - a quite fearsome group of carnivores with deep skulls, powerful jaws and long steak-knife teeth. This diverse, odd community of animals also included insects, primitive mammals, amphibians, early dinosaurs and flying pterosaur reptiles.

The mass extinctions when they came, as with most mass extinctions of life in the Earth's history was quite rapid, not gradual. "Ecosystems are functioning well, life is diverse, then 'wham', along comes the extinctions," said Professor Mike Benton, dinosaur expert at Bristol University. Professor Benton has sought to explain why the dinosaurs won out at the end of the Triassic.

The notion that dinosaurs gained the upper hand over the crurotarsans by virtue of being warm-blooded is unlikely, said Prof Benton, given that dinosaurs were closely related and, therefore, similar to them. In fact, many crurotarsans were such ringers for dinosaurs that they were mistakenly identified as such in the past.

In any case, scientists are unsure of whether dinosaurs were cold-blooded or not, as in recent years the idea that they were warm blooded has gained ground. It has been suggested by some researchers that dinosaurs were warm blooded and the crurotarsans - being crocodile relatives - were cold blooded, and this is why the dinosaurs won out.

However, Prof Benton's position is that the crurotarsans and dinosaurs were so closely related at the end of the Triassic that they could not have major differences in their physiology, such as one group being warm blooded and the other cold blooded.

At the end of the Triassic, the crurotarsans would have been favourites to emerge as dominant in the Jurassic, rather than the dinosaurs, said Prof Benton. This conclusion was based on comparisons of the anatomical characteristics and body forms of the two groups by Prof Benton and research student Steve Brusatte reported in the journal Science.

The evidence the researchers found indicated that in the run up to the mass extinction, dinosaurs were not evolving faster than crurotarsans, and the crurotarsans had a larger range of body types, diets, lifestyles and were more abundant. So, what was happening?

Prof Benton believes that the dinosaurs simply got lucky. "The extinction cleared out the majority of the carnivore groups, and allowed the carnivorous dinosaurs to radiate and expand," he said. Otherwise, the Jurassic, which followed the Triassic and has become synonymous with the reign of the dinosaurs, could have belonged to the crurotarsans.

Fossils

Dr Jenny McElwain, a Palaeobotanist at UCD travelled to Greenland in the summer of 2002 on a fossil quest. Dr McElwain and her colleagues searched for plant fossils in sedimentary rocks lain down along the margin of a large lake during the Late Triassic-Early Jurassic geological period.

The team wielded that vital instrument of palaeontology - the hammer - to discover the best fossils in what are today river cuttings along a glacial valley. They then excavated metre-by-metre sections, collecting everything of value. "You collect it, label it, wrap it, and then ship it back to the lab," said Dr McElwain. There was another fossil trip in 2004 to the same location that yielded a massive tonne of material.

The various fossil collections yielded nine snapshots of Greenland as it existed from 210 to 195 million years ago, showing it to be a land of luxurious forests of palm-like cycads and extinct bennettitale trees. Nothing like it exists today, but it is best described - in modern terms - as a cross between the Florida everglades, the cypress swamps of the southern United States, and the broad leaf conifer forests of New Zealand.

The fossil plants recovered proved their worth when Dr McElwain previously reported a dramatic drop in the number of breathing pores called 'stomata' on leaf surfaces from the Triassic-Jurassic time 'boundary' - representing 'ground zero' for the extinction.

The drop off in breathing pores was important as it indicated that elevated carbon dioxide levels in the atmosphere: plants take in this vital gas through their stomata, and have fewer stomata when carbon dioxide levels are high, in order to reduce water loss.

The fossilised leaves showed that carbon dioxide levels before the boundary were about three times higher than today, but five to seven times higher at the peak of extinction. As a result, global temperatures increased by up to 7 degrees C around the extinction peak, causing stress to plants. As the Triassic came to an end, the number of species and diversity in the Greenland forests fell, the mix of plants changed, and the canopy became simpler and less layered.

Volcanism

Dr McElwain's work had, thus, put global warming at the scene of the crime in terms of what caused the mass extinctions. But, what stoke this worldwide rise in temperature in the first place? The Triassic world was geographically simple, with one huge continent connecting all the land on Earth, called 'Pangea'.

The break-up of Pangea at the end of the Triassic led to an era of extreme volcanism, as pieces of the super continent - which are now the seven continents - broke free of one another, moving over, under, sliding past, and smashing into one another. Large swaths of the planet was sundered with volcanoes, spewing out lava and enormous quantities of carbon dioxide, sulphur dioxide and other toxic gases. These gases would have retained heat in the atmosphere, thereby creating what's been called the Triassic hothouse.

About two and a half million cubic kilometres of hot magma - an absolutely enormous quantity in today's terms - flowed onto the super continent forming what is now called the 'Central Atlantic Magmatic Province'. Surviving fragments from this province were pieced together by geologists just in the last decade, when rocks in the eastern US, Morocco and South America were dated to around this time. Only then was the huge scale of the volcanism at the time of the break-up of Pangea recognised.

Despite these findings, the reason for extinctions is still debated. "The majority of experts are happy to pin the blame on volcanism, but how it did so is far from understood," said Paul Wignall, Professor of Palaeoenvironments at the University of Leeds. "I've worked on all mass extinction events, but the Triassic-Jurassic extinction is the most enigmatic and hardest to understand because there is no obvious kill mechanism. One idea is global warming, but warming doesn't kill much; it's the environmental change itself."

Atmosphere

It is something of a Triassic mystery, therefore, that Dr McElwain and her team at UCD are attempting to solve. Their approach to the problem is to try and revive the ancient atmospheric environment in state-of-the-art growth chambers. Each chamber is a large room where plants are grown, and where levels of carbon dioxide and sulphur dioxide can be adjusted. Temperature, humidity and night and day cycles can also be controlled in the ultra-modern chambers, built with the help of a €1.7 million EU Marie Curie grant.

"The idea is to change the atmosphere and the environment to those we believe existed at the Triassic-Jurassic boundary," said Dr Matthew Haworth, a post-doctoral researcher on the project. "We then put relict plants, those with a long evolutionary history, into the growth rooms and look at how they respond." Cycads, ancient conifers, ferns and the maidenhair tree will be grown and the results compared to the Greenland fossils.

Torrents of acid rain washed over the Triassic lava fields and the researchers will be able to simulate acid rain by misting the chambers with water and sulphur dioxide. They will check the effect of a spike in sulphur dioxide, a potent pollutant released by volcanoes, on plants.

Another project will test the 'thermal damage hypothesis'. Research has shown that large leaves in high carbon dioxide environments heat rapidly and cooling is impeded due to the low density of leaf pores. Decreased heat loss cause plant temperatures to rise to levels where proteins become damaged and photosynthesis is impaired.

Injury from high temperatures in plants, would, thus, be size dependent and there is evidence for this in the fossils. "What we see across the Triassic-Jurassic boundary in early data is a pattern in which large leaf species get replaced by species with dissected leaves," said Dr McElwain. Thermal image cameras will detect minute changes in leaf surface temperature and the simulated atmospheres will reveal how different gases influence plant physiology and leaf temperature.

If we see a change in leaf shape in the growth rooms, said Dr Haworth, and we observe a similar change in the fossils, we can extrapolate that maybe these were the conditions that occurred during the mass extinction.

The investigation of palaeo 'wild fires' and oxygen levels is another area being investigated by Dr Claire Belcher, UCD palaeobotanist. Dr Belcher has reported that oxygen levels at the end of the Triassic were not as low as some thought, and were most likely above 15 per cent. These results were published in the journal Science last year.

Debate

Some experts doubt whether volcanism is the cause of the Triassic-Jurassic extinctions and one of those is Paul Olsen, Professor of Earth Sciences at Columbia University, in New York City. "I've taken the view that the extinction may have been caused by the impact of a large asteroid, similar to that at the KT boundary (the time period when the dinosaurs died out)." But, Prof Olsen does admit that the evidence for his proposal is - so far at least - weak.

The work at UCD in addressing the mystery of the Triassic Jurassic extinctions has been very valuable, indicated Prof Olsen, particularly the work showing the dramatic changes in temperatures associated with plant extinctions.

"It is wonderful stuff, and some of the best work done on the subject," said Prof Olsen. The increased temperatures at that time, he said, likely exaggerated the hydrological cycle, with increased wet periods and dry periods. This might have meant that plants had access to plenty of water, but they also had to suffer intense heat. "It may have been lethally hot in the continental interiors at that time," said Prof Olsen.

The question remains, what kind of disaster did the dinosaurs manage to survive? Was it overheated ecosystems, retreating oceans, poisonous gases, depleted oxygen, raging fires or an asteroid? Whatever the denouement, it took a very long time for life on Earth to rebound, a salutary lesson considering global warming today, and declining biodiversity.