IF IT’S September, then it must be time for Paul Sereno to leave civilisation behind and go looking for dinosaurs. This year, the University of Chicago palaeontologist is contemplating four months in Niger’s Ténéré desert, a vast arc of wilderness stretching from the Algerian border to the heart of the Sahara. Ténéré is so remote that even geographers call it “the desert within a desert”. To get there, Sereno’s team will have to strike out across open dunes in four-wheel-drive trucks, tracking their progress by global positioning satellites and hauling their water supply behind them. It’s hardly well-worn dinosaur territory, but that’s the point. Sereno isn’t looking for well-worn dinosaurs. He’s hoping for oddballs, like the three he dug up last time he went to Niger. And he reckons he’ll find them, no trouble.
Sereno is not alone. From South America to Madagascar, dinosaur hunters are unearthing creatures from the Cretaceous period-the third and final part of the dinosaur era, from 144 to 65 million years ago-that are startlingly different from those we have come to think of as “normal”. The animals they’re digging up-and the ones that don’t seem to be there to be dug up-are giving us a much clearer idea of what the world was like as the age of the dinosaurs drew to a close. And it turns out that what most of us think of as classic Cretaceous dinosaurs were actually nothing of the sort. In fact, they were regional specialities, confined to a small corner of the northern hemisphere. It’s as if today’s zoologists had focused on Australia and concluded that kangaroos and koalas were the dominant forms of non-human life on Earth.
Open a child’s dinosaur book and chances are you’ll be presented with three world views, corresponding to the three great divisions of the dinosaur age. First comes the Triassic, with primitive monsters such as Plateosaurus and sail-backed, snaggle-toothed Dimetrodon (not actually a dinosaur, but a mammal-like reptile). Then there’s the Jurassic and its familiar beasts: giant, long-necked sauropods like Diplodocus, Apatosaurus (formerly Brontosaurus) and Brachiosaurus, plate-backed plant eaters like Stegosaurus and the 12-metre predator Allosaurus. Last of all the Cretaceous, the climax of the era and home to the icons of the age: Tyrannosaurus rex and Triceratops, sickle-clawed Velociraptor, snorkel-crested Parasaurolophus, club-tailed Ankylosaurus and duck-billed Hadrosaurus.
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But these old friends may soon have to step aside. According to researchers like Sereno, the standard view of the Cretaceous is too regionalist, being based entirely on dinosaurs discovered in North America, Mongolia and a few other parts of the northern hemisphere. During the past ten years, he and a group of like-minded researchers have-literally-broken new ground all over the southern continents in search of the hemisphere’s lost dinosaurs. Thanks to their efforts, palaeontologists can begin making generalisations about life in these less explored regions. And some of their conclusions are startling.
For one thing, the southern hemisphere seems to have been a land of giants. Africa, for example, was home to a predator larger than T. rex. Argentina had two or three others that were even bigger. Their prey was truly gargantuan: one sauropod from Argentina was the largest land animal ever to walk the Earth.
In other respects, the south was an evolutionary backwater. While life in the northern hemisphere exploded into myriad forms, the southern continents plodded on as normal. “Something extraordinary happened in the north at the beginning of the Cretaceous,” says Philip Currie, director of the Royal Tyrrell Museum of Palaeontology in Drumheller, Canada, who has spent time hunting for dinosaurs in Patagonia. “Things happened much more gradually in the southern hemisphere. The dinosaurs there got more sophisticated, but basically we have an extension of the Jurassic.”
“It’s a divided world,” adds Sereno. “There are clear differences between north and south.” He should know. The dinosaurs he discovered in Niger in 1997 were so unusual they took more than two years to sort out. First there was Suchomimus, a crocodile-jawed, sail-backed monster that fished the rivers of West Africa 100 million years ago. Then there was Jobaria, a primitive, long-necked giant that was already a “living fossil” when it roamed the swamps 135 million years ago, since it looked 40 million years older. And last of all Nigersaurus, an enigmatic spade-headed herbivore with hundreds of teeth that’s so odd-looking even Sereno describes it as “marvellously bizarre”. What’s more, the region seems devoid of the archetypal Cretaceous animals. It’s as if Sereno had discovered a lost world in the middle of the Sahara.
And in a sense it is a lost world, at least when viewed from the northern hemisphere. For most of the dinosaur era, the world’s landmasses were cemented together in a massive supercontinent called Pangaea. Animals that evolved in one part of the world could-and did-spread quickly to others. Bones of the Jurassic predator Allosaurus, for example, have been found all over the world, from the US to Portugal, Australia and Tanzania.
Jurassic shift
But during the late Jurassic, the world started to change. Continental drift split Pangaea in two and by the early Cretaceous, 140 million years ago, the world’s dry land was divided into two continents. To the north lay Laurasia (which is now North America and Eurasia) and to the south, Gondwana (South America, Africa, Madagascar, India, Australia and Antarctica). Rifts continued to develop throughout the Cretaceous, dividing Laurasia in two and splitting the Gondwanan landmass into smaller and smaller chunks (see Diagram).
This fragmentation, of course, stopped dinosaurs from wandering freely all over the world. And that ought to mean that different types of animals evolved in different regions. The most pronounced division should be between Laurasia and Gondwana. Palaeontologists have long acknowledged that this should be the case, but had little evidence to back it up. “There was a feeling that the southern hemisphere was different,” says Dale Russell, the senior curator of palaeontology at the North Carolina State Museum of Natural Sciences in Raleigh, and another Niger veteran. “But the record was very scrappy.” Most fossil hunters were content to focus on the rich Cretaceous deposits in Asiamerica, today’s western North America and East Asia. Few bothered with the southern hemisphere. As recently as 10 years ago, the five best-known dinosaur faunas all came from Laurasian countries (the US, Mongolia, China, Canada and Britain).
The revision of this world view began in earnest in 1985 when Argentinian palaeontologist José Bonaparte unearthed two new flesh-eating dinosaurs in Patagonia. One, Carnotaurus, was a superb specimen, an entire skeleton so well preserved that its skin had left impressions in the surrounding rock. It came from the middle Cretaceous, around 100 million years ago. Yet it was unlike anything ever discovered from that time. For one thing, it had horns above its eyes-hence the name, which means “meat-eating bull”. The shape of its brain case-one of the prime diagnostic features of predatory dinosaurs-was unfamiliar. Overall, it looked remarkably primitive. The other species, called Abelisaurus, was incomplete, but it had clear similarities to Carnotaurus. Taken together, the discoveries seemed to suggest that there was an unknown group of bipedal predators roaming Argentina around the time T. rex dominated the northern hemisphere. In fact, Carnotaurus and Abelisaurus were so unusual that palaeontologists assigned them to a wholly new group of dinosaurs, the abelisaurids, part of a lineage that diverged from the main branch of carnivorous dinosaurs around 230 million years ago. More abelisaurids later turned up in India and Madagascar.
Southern titans
Patagonia, meanwhile, continued to yield enigmatic monsters. In 1991, Bonaparte dug up a titanosaur, a long-necked sauropod herbivore around 24 metres in length. Two years later, he found another species. Though fragmentary, its remains indicated that it was 45 metres long and weighed 100 tonnes, making it the largest animal ever to have walked the Earth. He and his colleague, Rodolfo Coria of the Carmen Funes Museum in Neuquén, called it Argentinosaurus.
Coria himself was also finding spectacular new species. In 1995, this time working with Leonardo Salgado of the Museum of Natural Sciences in Neuquén, he discovered a meat eater that brought Patagonia’s dinosaurs to the world’s attention. It looked a lot like T. rex, but it was bigger. Giganotosaurus, as they called it, was around 14 metres long and weighed 8 tonnes, making it bigger than even the biggest T. rex, which up to that point had been thought of as the largest predator the world had ever seen (see Diagram, p 26). But Giganotosaurus was no match for T. rex in one respect: its brain was about half the size. In March of this year, Coria and Philip Currie announced the discovery of another meat eater that was bigger still. Currie says there are remains of a third Giganotosaurus-like species that dwarfs the lot.
The discoveries add up to an inescapable conclusion: in South America, Cretaceous dinosaurs tended to be bigger, dumber, and more primitive than their northern contemporaries. The most abundant herbivores were long-necked sauropods, a group that died out over most of Laurasia around the beginning of the Cretaceous. The most common predators, meanwhile, were the abelisaurids, dinosaurs that seem to have flourished in isolation on the southern continent and are absent further north. And right at the top of the food chain were monstrous, pea-brained beasts like Giganotosaurus-throwbacks to the allosaurs of an earlier age. “Most South American dinosaur fauna are oversized forms,” says Coria. “They represent primitive dinosaurs that were widely distributed around the world during the Jurassic period, but survived another 50 million years into the Cretaceous in South America. This was their last bastion before they went extinct.”
Many of the patterns found in Patagonia are repeated elsewhere. Abelisaurids, for example, have turned up in India, Madagascar and Africa. And titanosaurs have now been found all across the southern hemisphere. “Everywhere other than Asiamerica, sauropods are easily the most common dinosaurs,” says Thomas Holtz, a palaeontologist at the University of Maryland. That’s especially true in Africa. Apart from the titanosaurs, the continent has at least two other groups of sauropod, both discovered in Niger by Sereno. One is Jobaria, the herbivorous “living fossil”. The other is Nigersaurus, the spade-headed weirdo. Though Nigersaurus is descended from Diplodocus, the gangly Jurassic sauropod whose skeleton graces the foyer of London’s Natural History Museum, it looks very different. It’s one of the smallest sauropods on record, reaching only 15 metres in length. And its mouth is stuffed with rows and rows of teeth-as many as 600 per individual. Nigersaurus may have filled the same ecological niche as the duck-billed dinosaurs of Asiamerica. And, according to Sereno, there are hints of a similar species in South America.
Nigersaurus isn’t the only odd African herbivore. In 1999, Russell published details of an eccentric creature from Niger called Lurdusaurus (“weighty lizard”). “It’s a funny thing,” he says. “It’s like a hippopotamus, low to the ground, barrel-chested and with a small head. And it’s got this powerful thumb claw.” Not, in other words, a lot like your regular northern plant-eater.
Africa also has big predators that fit the Gondwanan pattern. On their maiden expedition to Niger, in 1993, Sereno’s team unearthed the complete skeleton of a meat eater, the best specimen ever found in Africa. They called it Afrovenator (“African hunter”) and assigned it to a group called the torvosauroids. These evolved during the Jurassic but, until 1993, were unknown in the Cretaceous. Two years later the team unearthed two more predators, this time in the Kem Kem region of Morocco. One, Deltadromeus (“delta runner”), was a swift and graceful hunter that seems to have evolved in isolation in Africa. The other, Carcharodontosaurus (“shark-toothed lizard”), was a flesh-guzzling monster that is related to the giants of South America. Carcharodontosaurus could easily have gone toe-to-toe with Giganotosaurus: it was more than 14 metres long and weighed a gargantuan 8 tonnes (see Diagram).
Crocodile jaws
More finds continued to surface. In 1997, Sereno found another torvosauroid in Niger, the crocodile-jawed Suchomimus. This was the most common predator of its day. It was at least 11 metres long and had an elongated snout bristling with hook-shaped teeth for snaring fish. In 1998, Russell found traces of a similar creature in Morocco. South America, too, has a crocodile-jawed fish eater, the delightfully named Irritator challengeri. This got its name because the only known skull had been damaged by amateur palaeontologists.
The pictures that are emerging of Africa and South America, then, are strikingly similar. Both were home to giant sauropods and Allosaurus -like predators long after these groups had started to dwindle in Laurasia. And both had eccentric-looking species that evolved in isolation following the split of Pangaea. “You have two sorts of animal,” says Sereno. “There are animals that survived only on the southern continents, and there are animals that evolved on the southern continent and are not found elsewhere.”
Laurasian echoes
Within this broad pattern, however, there are some stray threads. The dinosaurs of Australia and Antarctica seem much more like those found in Laurasia. In 1998, for example, a tooth from a duck-billed dinosaur was found in Cretaceous deposits in the Antarctic. “It’s a bit of an enigma,” says Currie. “Some of the material down there is not like Gondwanan animals. There are some things that are more suggestive of the northern hemisphere.” Gondwanan animals have also started showing up in Laurasia. Suchomimus, for example, has a close European relative called Baryonyx, found in early Cretaceous rocks on the Isle of Wight, off England’s south coast. The two are so similar that that they may in fact be the same species. What’s more, titanosaurs have been uncovered in Western Europe, North America and Mongolia.
“It’s a complex picture,” says Sereno. “The break-up of Pangaea didn’t create insurmountable obstacles to these intrepid explorers.” One possible explanation is that land bridges formed from time to time as a result of changes in sea level, temporarily reuniting divided land masses and allowing migration and intermingling of species. Another possibility is that the fossil record simply isn’t good enough to give us a complete picture.
Whatever the real reason, Sereno and his fellow explorers are sure that the fossil beds of Gondwana will continue to yield surprises. “There’s lots more to come,” says Russell. They’re also convinced that what they find will continue to marginalise T. rex, Triceratops and the other Laurasian icons. “The dinosaurs of North America and Eurasia were an unusual endemic fauna, real weirdos that were generated in isolation,” says Holtz. “Southern hemisphere dinosaurs are the main strand of dinosaur history.”
