A

EVERYBODY knows that the dinosaurs were killed by an asteroid. Something big hit the earth 65 million years ago and, when the dust had fallen, so had the great reptiles. There is thus a nice if ironic, symmetry in the idea that a similar impact brought about the dinosaurs’ rise. That is the thesis proposed by Paul Olsen, of Columbia University, and his colleagues in this week’s Science.

B

Dinosaurs first appeared in the fossil record 230m years ago, during the Triassic period. But they were mostly small, and they shared the earth with lots of other sorts of reptile. It was in the subsequent Jurassic, which began 202 million years ago, that they overran the planet and turned into the monsters depicted in the book and movie “Jurassic Park”. (Actually, though, the dinosaurs that appeared on screen were from the still more recent Cretaceous period.) Dr Olsen and his colleagues are not the first to suggest that the dinosaurs inherited the earth as the result of an asteroid strike. But they are the first to show that the takeover did, indeed, happen in a geological eyeblink.

C

Dinosaur skeletons are rare. Dinosaur footprints are, however, surprisingly abundant. And the sizes of the prints are as good an indication of the sizes of the beasts as are the skeletons themselves. Dr Olsen and his colleagues, therefore, concentrated on prints, not bones.

D

The prints in question were made in eastern North America, a part of the world the full of rift valleys to those in East Africa today. Like the modern African rift valleys, the Triassic/Jurassic American ones contained lakes, and these lakes grew and shrank at regular intervals because of climatic changes caused by periodic shifts in the earth’s orbit. (A similar phenomenon is responsible for modern ice ages.) That regularity, combined with reversals in the earth’s magnetic field, which are detectable in the tiny fields of certain magnetic minerals, means that rocks from this place and period can be dated to within a few thousand years. As a bonus, squishy lake-edge sediments are just the things for recording the tracks of passing animals. By dividing the labour between themselves, the ten authors of the paper were able to study such tracks at 80 sites.

E

The researchers looked at 18 so-called ichnotaxa. These are recognizable types of the footprint that cannot be matched precisely with the species of animal that left them. But they can be matched with a general sort of animal, and thus act as an indicator of the fate of that group, even when there are no bones to tell the story. Five of the ichnotaxa disappear before the end of the Triassic, and four march confidently across the boundary into the Jurassic. Six, however, vanish at the boundary, or only just splutter across it; and there appear from nowhere, almost as soon as the Jurassic begins.

F

That boundary itself is suggestive. The first geological indication of the impact that killed the dinosaurs was an unusually high level of iridium in rocks at the end of the Cretaceous when the beasts disappear from the fossil record. Iridium is normally rare at the earth’s surface, but it is more abundant in meteorites. When people began to believe the impact theory, they started looking for other Cretaceous-and anomalies. One that turned up was a surprising abundance of fern spores in rocks just above the boundary layer – a phenomenon known as a “fern spike”.

G

That matched the theory nicely. Many modern ferns are opportunists. They cannot compete against plants with leaves, but if a piece of land is cleared by, say, a volcanic eruption, they are often the first things to set up shop there. An asteroid strike would have scoured much of the earth of its vegetable cover, and provided a paradise for ferns. A fern spike in the rocks is thus a good indication that something terrible has happened.

H

Both an iridium anomaly and a fern spike appear in rocks at the end of the Triassic, too. That accounts for the disappearing ichnotaxa: the creatures that made them did not survive the holocaust. The surprise is how rapidly the new ichnotaxa appear.

I

Dr Olsen and his colleagues suggest that the explanation for this rapid increase in size may be a phenomenon called ecological release. This is seen today when reptiles (which, in modern times, tend to be small creatures) reach islands where they face no competitors. The most spectacular example is on the Indonesian island of Komodo, where local lizards have grown so large that they are often referred to as dragons. The dinosaurs, in other words, could flourish only when the competition had been knocked out.

J

That leaves the question of where the impact happened. No large hole in the earth’s crust seems to be 202m years old. It may, of course, have been overlooked. Old craters are eroded and buried, and not always easy to find. Alternatively, it may have vanished. Although the continental crust is more or less permanent, the ocean floor is constantly recycled by the tectonic processes that bring about continental drift. There is no ocean floor left that is more than 200m years old, so a crater that formed in the ocean would have been swallowed up by now.

K

There is a third possibility, however. This is that the crater is known, but has been misdated. The Manicouagan “structure”, a crater in Quebec, is thought to be 214m years old. It is huge – some 100km across – and seems to be the largest of between three and five craters that formed within a few hours of each other as the lumps of a disintegrated comet hit the earth one by one.

A. Between the Inishowen Peninsula, north west of Derry, and the Glens of Antrim, in the east beyond the Sperrin Mountains, is found some of Western Europe’s most captivating and alluring landscape.

B. The Roe Valley Park, some 15 miles east of Derry is a prime example. The Park, like so many Celtic places, is steeped in history and legend. As the Roc trickles down through heather bogs in the Sperrin Mountains to the South, it is a river by the time it cuts through what was once called the ‘garden of the soul’ – in Celtic ‘Gortenanima’.

C. The castle of O’Cahan once stood here and a number of houses which made up the town of Limavady. The town takes its name from the legend of a dog leaping into the river Roe carrying a message, or perhaps chasing a stag. This is a wonderful place, where the water traces its way through rock and woodland; at times, lingering in brooding pools of dark cool water under the shade of summer trees, and, at others, forming weirs and leads for water mills now long gone.

D. The Roe, like all rivers, is witness to history and change. To Mullagh Hill, on the west bank of the River Roe just outside the present day town of Limavady, St Columba came in 575 AD for the Convention of Drumceatt. The world is probably unaware that it knows something of Limavady; but the town is, in fact, renowned for Jane Ross’s song Danny Roy, written to a tune once played by a tramp in the street. Limavady tow n itself and many of the surrounding villages have Celtic roots but no one knows for sure just how old the original settlement of Limavady is.

E. Some 30 miles along the coast road from Limavady, one comes upon the forlorn, but imposing ruin of Dunluce Castle, which stands on a soft basalt outcrop, in defiance of the turbulent Atlantic lashing it on all sides. The jagged­-toothed ruins sit proud on their rock top commanding the coastline to east and west. The only connection to the mainland is by a narrow bridge. Until the kitchen court fell into the sea in 1639 killing several servants, the castle was fully inhabited. In the next hundred years or so, the structure gradually fell into its present dramatic state of disrepair, stripped of its roofs by wind and weather and robbed by man of its caned stonework. Ruined and forlorn its aspect may­be yet, in the haunting Celtic twilight of the long summer evenings, it is redolent of another age, another dream.

F. A mile or so to the east of the castle lies Port na Spaniagh, where the Neapolitan Galleas, Girona, from the Spanish Armada went down one dark October night in 1588 on its way to Scotland, of the 1500-odd men on board, nine survived.

G. Even further to the east, is the Giant’s Causeway stunning coastline with strangely symmetrical columns of dark basalt – a beautiful geological wonder. Someone once said of the Causeway that it was worth seeing, but not worth going to see. That was in the days of horses and carriages, when travelling was difficult. But it is certainly well worth a visit. The last lingering moments of the twilight hours are the best lime to savour the full power of the coastline s magic; the time when the place comes into its own. The tourists are gone and if you are very lucky you will be alone. A fine circular walk will take you down to the Grand Causeway, past amphitheatres of stone columns and formations. It is not frightening, but there is a power in the place – tangible, yet inexplicable. The blackness of some nights conjure up feelings of eeriness and unease. The visitor realises his place in the scheme of the magnificent spectacle. Once experienced, it is impossible to forget the grandeur of the landscape.

H. Beyond the Causeway, connecting the mainland with an outcrop of rock jutting out of the turbulent Atlantic, is the Carrick-a-Rede Rope Bridge, when first constructed, the bridge was a simple rope handrail with widely spaced slats which was used mainly by salmon fishermen needing to travel from the island to the mainland. In time, the single handrail was replaced with a more sturdy caged bridge, however, it is still not a crossing for the faint- hearted. The Bridge swings above a chasm of rushing, foaming water that seems to drag the unwary- down, and away. Many visitors who make the walk one way are unable to return resulting in them being taken off the island by boat.

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Great thanks to volunteer Uc Bu who has contributed these explanations and markings.

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