IT TAKES a skilled eye to spot the moment where the world changed in the cliffs at Wasson Bluff in Nova Scotia, Canada. The rocks are a geological jumble, layers of reddish sandstone interspersed with grey basalt spewed from deep inside the Earth as North America and Africa split apart 200 million years ago. But according to Paul Olsen of Columbia University’s Lamont Doherty Earth Observatory, these cliffs contain a record of one of the most devastating events to hit the Earth in the past half-billion years.
It’s the red rocks that capture the attention of the busload of palaeontologists hiking along the shore in a steady drizzle. The sandstone layers preserve traces of ancient life: bones, teeth, footprints, leaf impressions and pollen. At one point, Olsen says, the fossils change dramatically, marking the transition between the Triassic period, which ended around 200 million years ago, and the Jurassic period that followed.
The eye sees no clear marker, but the fossils tell a different story. The Triassic rocks contain the remains and footprints of a bewilderingly diverse assortment of reptiles, and so do the Jurassic rocks. That’s where the similarity ends; the two periods have almost nothing in common. Amazingly, Olsen estimates that this changing of the guard took less than 30,000 years – a blink of the eye in geological terms. Fossil pollen grains record an even more dramatic shift in plant life that took no more than 1000 years.
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Those changes record one of the five most devastating mass extinctions of the past 500 million years (see Diagram). It is one that has attracted little public attention, partly because it claimed no charismatic victims. “Nothing as newsworthy as dinosaurs or trilobites went extinct,” says palaeontologist Michael Benton of the University of Bristol in the UK. Yet he says the extinction toll is comparable to the better-known event that wiped out the dinosaurs 65 million years ago. Olsen says that by one measure – the number of families of land animals that died out per million years – the Triassic-Jurassic extinction might be the biggest of all, surpassing even the great extinction that ended the Permian some 250 million years ago and which wiped out around 95 per cent of marine species (żěè¶ĚĘÓƵ, 26 April 2003, p 38).
Olsen’s view is palaeontological heresy in some quarters, but it’s starting to get a serious hearing as new evidence shows that we seriously misjudged the Triassic. The period was once thought of as little more than a rehearsal for the age of dinosaurs – a kind of “Jurassic-lite” – and the extinction as a minor setback for a group of land animals that were already well on their way to global dominance.
Wild and wonderful
We now know that dinosaurs played only a bit part in a world much more alien than we once thought. The Triassic was not the dawn of the dinosaur age at all; it was “a wild and wonderful world with all kinds of strange and weird creatures”, says Olsen.
That world evolved over the course of 50 million years from an unpromising beginning. The Permian extinction left bleak and empty landscapes over the vast single continent of Pangaea. For the first few million years of the Triassic, the only plants to speak of formed scrub vegetation along rivers and streams. As for animals, the extinction effectively cleared the stage for the reptiles that survived to evolve and take over.
The first to take advantage of the empty world was a group of reptiles called the synapsids. They dominated the early Triassic, and gave rise to mammals. By the middle of the Triassic a second group of Permian survivors called the diapsids were starting to take over. That’s when things began to get interesting.
“The real evolutionary action was in a group called the archosaurs”
Some of these beasts took to the water and evolved into ichthyosaurs, plesiosaurs and other marine reptiles. Others stayed on land and evolved into snakes and lizards, but the real evolutionary action was taking place in a group of land animals called the archosaurs – the “ruling reptiles”.
The classic view is that archosaurs turned up in the middle Triassic and evolved into a dazzling array of large creatures, including dinosaurs, crocodiles and pterosaurs. They produced a few assorted “others” too, but these were of no great significance. In this view the late Triassic was the “dawn of the dinosaurs”: thanks to superior evolutionary adaptations, these great creatures became the dominant land animals. The end-Triassic extinction pruned a number of dinosaurs, but the group as a whole marched on.
Yet in the past few years palaeontologists have realised that the late Triassic wasn’t the dawn of the dinosaurs at all. Far from being a supporting cast, the assorted “others” were in fact the stars of the show, and dinosaurs hardly got a look-in. When the extinction came, it was the others that were hit hardest. All sorts of large, bizarre reptiles disappeared forever. And much as the death of the dinosaurs cleared the way for the rise of mammals, the Triassic reptiles’ demise heralded the age of the dinosaurs. “The late Triassic was basically the heyday of the archosaurs,” says Sterling Nesbitt, one of Olsen’s students.
The illusion of dinosaur dominance stemmed from the fact that fossils of Triassic land animals are rare and usually incomplete. Nature rarely provides whole articulated skeletons, so palaeontologists usually identify fragmentary remains by looking for familiar patterns in teeth or distinctive bones. When they found Triassic fossils that looked like they came from dinosaurs, they logically assumed that they were dinosaurs.
One example of over-hasty classification was Revueltosaurus. Its spade-shaped teeth with serrated edges, found in late Triassic rocks at Revuelto Creek, New Mexico, in 1989, strongly resembled the distinctive teeth of plant-eating dinosaurs such as stegosaurus and triceratops. These dinosaurs, members of the order known as the ornithischians, were thought to have appeared around the middle Triassic, so researchers simply assumed that Revueltosaurus must have been an ornithischian dinosaur too.
Things changed in 2004, when Randall Irmis, a graduate student at the University of California, Berkeley, uncovered a complete Revueltosaurus skeleton, plus fragments of dozens more, in the Petrified Forest National Park in Arizona. The teeth were unmistakable, and so was the skeleton. It was clear that the animal wasn’t a dinosaur at all – it was much more closely related to crocodiles. A clutch of other so-called Triassic ornithischian dinosaurs, all grouped according to tooth or jaw fragments, have met similar fates – Tecovasaurus, Lucianosaurus and Technosaurus to name just a few. In fact Irmis believes there might not be any Triassic ornithischians at all.
Even Triassic species whose dinosaur credentials once appeared secure are now under suspicion. Take Eoraptor and Herrerasaurus, which lived 230 million years ago in what is now Argentina. For many years they were thought to be the earliest-known dinosaurs, members of a group of two-legged predators called theropods. But Irmis says they probably weren’t theropods, and some research suggests they were not dinosaurs at all.
That’s not to say there were no dinosaurs in the Triassic. Good records exist of theropods later in the period, such as Coeleophysis from Arizona. And the plant-eating prosauropods, a group that later gave rise to the most massive of all dinosaurs, the sauropods, are abundant in Europe, South America and southern Africa. “There were a lot fewer Triassic dinosaurs than we thought,” Olsen says. “There’s only one or two species [of theropod or prosauropod] at any one point in time, not a great diversity,” Irmis adds.
So if the late Triassic wasn’t a dinosaur world, what did it look like? It turns out that the archosaurs split into two fast-diverging lines, one leading to dinosaurs and pterosaurs, and the other to a diverse array of weird and wonderful creatures (see Diagram). And it was these that ruled the roost for millions of years. In evolutionary terms those reptiles were closer to crocodiles than to dinosaurs, but the relationship wasn’t immediately obvious.
Take the ornithosuchians, for example. To the untrained eye these look exactly like large predatory dinosaurs in the allosaurus mode, but they were a distinct lineage. There were also the rauisuchians, long-legged predators more like a bear or lion than a crocodile. Some rauisuchians may have been bipedal, but most probably walked on all fours. The largest stretched 7 metres to the tip of their thick tail, though most were 2 to 5 metres long. Some were bizarre, such as the sail-backed Arizonasaurus, discovered by Sterling Nesbitt, one of Olsen’s graduate students.
The most common plant-eaters of the late Triassic were not dinosaurs either. Aetosaurs were low-slung animals 1 to 5 metres long with small heads and armoured bodies, built like the ankylosaurs of the dinosaur age or the extinct, car-sized armadillos that died out around 10,000 years ago.
The crocodile kinship was most obvious in phytosaurs, long-bodied predators with narrow jaws that looked like modern gharials but with nostrils just in front of their eyes, not on the tips of their noses. But phytosaurs did not give rise to modern crocodiles: the lineage died out at the end of the Triassic. Ironically, the true ancestral crocodiles were, if anything, the least crocodilian of the lot – toothy, two-legged predators with long tails, built more like a small dinosaur than a crocodile.
No one now disputes that something drastically pruned the archosaur family tree at the end of the Triassic, removing the dominant land animals and leaving just dinosaurs, pterosaurs and a handful of ancestral crocodiles. The big questions are when the extinctions happened, what they affected, and what caused them.
“Clearly there’s a Triassic-Jurassic extinction of some importance,” says Michael Parrish of Northern Illinois University in DeKalb. But whatever happened, it didn’t affect everything. Plenty of groups, including small predatory dinosaurs, the early mammals, and some crocodile relatives survived into the Jurassic. Yet large groups of archosaurs mysteriously vanished at the end of the Triassic. “It really isn’t obvious why the non-dinosaurs get hammered,” says Olsen.
“It’s not at all obvious why the non-dinosaurs get hammered”
Timing is a major matter of debate. Olsen thinks that a single event at the very end of the Triassic wiped out what he calls “a fully mature world”. Parrish is more cautious. The rarity of many species makes it very hard to tell when they disappeared, he says. Benton believes there were at least two events, one at the end of the Triassic, and one about 17 million years earlier that wiped out three major herbivore families including the once-dominant rhynchosaurs. And Spencer Lucas of the New Mexico Museum of Natural History and Science in Albuquerque says, “There was no single end-Triassic extinction. There was a period of elevated extinction.” This lasted between 5 and 15 million years, he reckons.
Olsen focuses on the red sediments that stretch from Nova Scotia down the east coast of the US and across the Atlantic to the Argana basin of Morocco. He says he can see the big change happening simultaneously in all of them. Not many bones get preserved, but the rocks are rich in pollen, leaf impressions and footprints. The pollen shows that plants changed dramatically at the end of the Triassic in a millennium or less. What is more, the footprints show that dinosaurs were few and far between at this time, though they dominated the early Jurassic. The biggest dinosaur footprints jumped from 25 to 35 centimetres long within just 30,000 years. This means that whatever made those tracks doubled in size over this time. The once-common non-dinosaurian reptiles vanished, and in the Jurassic, Olsen says, we really do see the dawn of the age of dinosaurs.
Rock solid
No obvious geological change in the rocks marks the boundary layer at Wasson Bluff, and Olsen says that backs up his theory that some kind of extinction event happened. A change in rocks means a change in environment, which can drive species elsewhere and be mistaken for a rapid global extinction.
Others say that Olsen is focusing too closely on the end of the Triassic on land. Fossils of marine invertebrates are far more common than those of land animals, and Lucas argues they show a series of extinctions earlier on in the Triassic. For example, ammonites – extinct marine animals with coiled shells that were very similar to today’s nautiluses – suffered heavily in the late Triassic, with only one of the 66 genera that lived in the period surviving into the Jurassic. Lucas says that occurred via a series of extinctions, which winnowed them down to only three genera surviving at the very end of the Triassic.
The Triassic as a whole has been difficult to study. “There are great gaps of time not well represented by fossils,” Benton says. “We’re still at a fairly undeveloped stage in terms of geological knowledge.” Land animals were spread unevenly over Pangaea, and their fossils are rare, making it hard to be sure when they appeared and disappeared. Also it’s hard to match dates from different sites. Benton says the change Olsen sees in Nova Scotia may not be between the Triassic and Jurassic at all, it might actually mark the boundary between two stages of the Triassic four million years earlier. Correlating marine and land deposits is notoriously tricky too, and identifying species can be a tough job when fossils are only fragmentary.
The cause of the extinctions is another enigma, and a hotly contested one too. Olsen, who believes the change was quick and massive, has always favoured an asteroid impact, but admits there’s no compelling evidence for one at the end of the Triassic. The 70-kilometre-wide Manicouagan Crater in Quebec, Canada, seemed a logical suspect until it was found to be far too old, predating the end of the Triassic by 14 million years.
Another possibility is climate change. Massive volcanic eruptions split Pangaea around 200 million years ago, separating what are now North America and Africa. The carbon dioxide and other gases produced by the eruptions could have been enough to change the world dramatically.
Recently Larry Tanner of Le Moyne College in Syracuse, New York, found elevated levels of iridium in rocks from the Triassic-Jurassic boundary at Wasson Bluff. This element is often a signature of an asteroid impact. However, the rocks contain an order of magnitude less iridium than rocks from the end of the Cretaceous 65 million years ago, when the famous asteroid impact that wiped out the dinosaurs occurred. Also missing are other telltale signs of impact such as shocked quartz or microspherules of splattered rock. Tanner reckons the iridium might have come from the massive volcanic eruptions. “It’s a puzzle that still requires some pieces to put it all together.”
It’s sure to take many more years to finish the jigsaw puzzle completely, but already the overall picture is becoming clear. No longer can we think of the Triassic as just the kindergarten of the dinosaurs: it was a weird and wonderful episode in the history of life, the likes of which has never been seen before or since. Walking With Archosaurs, anyone?
