杏吧原创

EACH summer, a team from the University of Oslo in Norway go hunting for monsters on the island of Spitsbergen. They carry guns in case they get menaced by the world鈥檚 largest living land carnivore, the polar bear. But it is not bears they are after. They are searching for much bigger quarry, the most formidable predators that ever lived.

Step back 150 million years and Spitsbergen was covered by a cool, shallow sea swarming with marine reptiles. The creatures died out and their fossils became part of an island stuffed full of bones. Nowhere else in the world are so many marine reptiles found in one place.

For a few short weeks the sun never sets and temperatures soar to just above freezing. Knowing that before long the ground will be frozen solid, the researchers dig like crazy. 鈥淚t鈥檚 like a gold rush, there are so many fossils waiting to be found,鈥� says team leader J酶rn Hurum. 鈥淭he site is densely packed with skeletons. As we speak there are probably more than 1000 skeletons weathering out.鈥�

Hurum鈥檚 Arctic discoveries are part of a remarkable renaissance in interest in the marine reptiles of the Mesozoic era, 251 to 65 million years ago 鈥� including this week鈥檚 . We now know more about this group of creatures than ever before.

Marine reptiles were among the first vertebrate fossils known to science and were key to the development of the theory of evolution. In the late 18th century the massive jaws of a lizard-like beast were found in a mine in Maastricht in the Netherlands. Later named Mosasaurus, the creature helped convince scientists that animals could become extinct, a radical concept in its day. In the early 19th century, ichthyosaurs and plesiosaurs discovered by legendary fossil hunter Mary Anning around Lyme Bay in south-west England helped establish the science of palaeontology. Marine reptiles were among the best-understood extinct creatures of the first half of the 19th century and played a major role in the intellectual debate nurturing Darwin鈥檚 theory of evolution.

Yet they faded from view as their terrestrial relatives moved to centre stage. It took nearly a century for marine reptile research to emerge from the shadow cast by the dinosaurs. 鈥溞影稍�创s thought they knew all there was to know,鈥� says plesiosaur expert Leslie No猫 of the Thinktank museum in Birmingham, UK. 鈥淭he idea was that they weren鈥檛 worth studying. Nobody would say that now. Our understanding of marine reptiles is phenomenally greater now than it was even 10 years ago.鈥�

In the modern world, marine reptiles are few and far between: saltwater crocodiles, turtles and sea snakes are rarities of coastal waters. However, in the ice-free greenhouse of the Mesozoic, reptiles cruised the oceans from pole to pole, occupying the ecological roles now largely filled by whales, dolphins, porpoises, seals and even sharks.

Much like today鈥檚 marine mammals, marine reptiles evolved from land-living ancestors and were air-breathing. For them, it was a true return to the water. Reptiles evolved around 300 million years ago from amphibian-like ancestors that needed to lay their eggs in water. Reptiles, in contrast, thrive in hot, dry environments.

Among the first to go back were the mesosaurs around 280 million years ago. They were fully aquatic with long, thin bodies, webbed feet and jaws bristling with teeth. They disappeared just a few million years later leaving no known descendants.

Only after the Permian mass extinction 251 million years ago did a full-scale reptilian invasion begin. The extinction was the greatest clear-out of life the world has ever seen and marine life was hit particularly hard: 19 out of every 20 marine species became extinct.

Air-breathers

The empty seas were ripe for colonisation and reptiles were well placed to take advantage. Temperatures were several degrees warmer than today, which suited cold-blooded reptiles very well. Being air-breathing meant they could thrive in the low-oxygen waters of the post-Permian world where fish struggled to survive. Large predatory fish were also few and far between.

Many types of marine reptile evolved during the Mesozoic, but four stand out owing to their abundance, dominance and global distribution: ichthyosaurs, plesiosaurs, pliosaurs and mosasaurs. All four groups were predatory and included the top marine predators of their time. Some species reached truly enormous sizes.

One key to their success was the evolution of live birth, or vivipary. It has been known for decades that ichthyosaurs reproduced in this way, thanks to well-preserved fossils found at Holzmaden quarry in Germany. One exquisite specimen, now in the , captures an ichthyosaur in the process of giving birth, also seen spectacularly in the Chinese fossil pictured below.

Vivipary was probably seen in all large marine reptiles. In 2001, Mike Caldwell of the University of Alberta, Canada, was examining a mosasaur fossil in the Museum of Natural History in Trieste, Italy. 鈥淎s soon as I popped open that drawer I knew we had an important discovery. In front of me was a mosasaur with embryos 鈥� it had tiny little versions of the adults lined up in its belly,鈥� he says.

In 2004 came evidence that a group ancestral to the plesiosaurs, the keichousaurs, also gave birth to live young. As a result, researchers now think that plesiosaurs must also have been viviparous ().鈥滾ive birth allows you to get much bigger because you don鈥檛 need to come into shallow water or make your way onto land to lay eggs,鈥� says Caldwell. 鈥淚f you can give birth in water then you can colonise the oceans of the planet鈥� ().

The first big success story was the ichthyosaurs, which appear in the fossil record around 245 million years ago. Early ichthyosaurs were eel-like creatures that stayed close to shore, but over the next 40 million years they evolved into streamlined, dolphin-shaped cruisers that raced through the open oceans, according to ichthyosaur expert Ryosuke Motani of the University of California, Davis ().

Some lineages evolved into the biggest marine reptiles that ever lived. In 2004, a team led by Elizabeth Nicholls of the Royal Tyrrell Museum in Drumheller, Canada, excavated a monstrous ichthyosaur from 210-million-year-old rocks in British Columbia. At 21 metres long, Shonisaurus was as big as a fin whale, the world鈥檚 second-largest living animal. 鈥淚f you blow up a dolphin and make it skinnier then that is probably what Shonisaurus looked like,鈥� says Motani, who was part of the excavation team (). Fragmentary remains suggest that even bigger ichthyosaurs existed around that time.

By the start of the Jurassic period of 200 million years ago, the behemoths were joined by smaller, faster cruisers. In 2002, Motani estimated that Stenopterygius, a 180-million-year-old ichthyosaur from Europe, had a cruising speed comparable with tuna, which are among the fastest of all living fish ().

The Jurassic was the ichthyosaurs鈥� golden age. They were more abundant than any other marine reptile and were the first group to conquer the deep oceans, as Motani demonstrated through research into the optical properties of their eyes.

In general, eye size and body size are closely correlated in vertebrates: blue whales are the largest living vertebrates and have the biggest eyes, 15 centimetres in diameter.

Many ichthyosaurs bucked that trend. 鈥淚chthyosaur eyes were the biggest of any vertebrate,鈥� says Motani. The 4-metre-long Ophthalmosaurus, for example, had eyes 23 centimetres across, the size of frisbees, while the eyes of the 9-metre Temnodontosaurus were 26 centimetres. Among living creatures, only deep-sea giant squid have eyes of comparable size. Motani argues that giant eyes were an adaptation for diving down 500 metres or more to hunt for squid and other cephalopods, such as the now extinct belemnites.

Montani estimated the visual acuity of ichthyosaur eyes by calculating their light-gathering capacity based on size and focal length. He concluded that they were more sensitive than a typical nocturnal mammal. 鈥淎t 500 metres down a human would not be able to see a thing but an ichthyosaur would have been able to see moving objects,鈥� he says (Nature, vol 402, p 747).

At the start of the Jurassic the ichthyosaurs were joined by the plesiosaurs and pliosaurs, which thrived right through until the end of the Cretaceous some 65 million years ago. They were closely related, though they didn鈥檛 look it: plesiosaurs had long necks, small heads and graceful bodies, while the pliosaurs had massive bodies, short necks and large heads. Both swam using two large pairs of paddles.

Central to the plesiosaurs鈥� biology were their long necks, which in extreme cases could be longer than the rest of the body and tail combined. The neck of Elasmosaurus has 72 vertebrae, more than any other animal that we know of. 鈥淟ong-necked marine animals disappear with the extinction of the plesiosaurs. That way of living just doesn鈥檛 exist any more,鈥� says No猫. Yet long necks were integral to the plesiosaur success story.

Perhaps they were using their long necks to sneak up under schools of fish silhouetted against the sky, suggests marine reptile expert Mike Everhart of the Sternberg Museum of Natural History in Hays, Kansas. 鈥淭he plesiosaur would have approached from a blind spot as fish can鈥檛 see well underneath or behind. Then it grabs what it can before the school is alerted.鈥� With plesiosaur stomach contents showing fish were a main prey item, this explanation is widely accepted.

However, No猫 recently suggested that they were bottom feeders. According to this scenario, the plesiosaur鈥檚 peg-toothed head rummaged for prey on the sea floor while its body floated above (). Support for this idea came from a 2005 discovery in Queensland, Australia, where Colin McHenry of the University of Newcastle in New South Wales found plesiosaur stomachs full of sea-floor invertebrates (). McHenry believes that both explanations are correct. 鈥淎 long neck is a fantastic general-purpose feeding mechanism. It allows you to drift along the bottom and pick out bits that interest you but also gives you the agility to catch fish and squid,鈥� he says.

Although plesiosaurs could reach 14 metres, much of their length was taken up by their necks. Overall they were dwarfed by their relatives the pliosaurs, the unquestioned top predators of the Mesozoic seas.

There is some dispute over the identity of the very largest pliosaur, but Pliosaurus must come close. It is known from a 3-metre jaw found in Oxfordshire, UK, and No猫 estimates that it was up to 18 metres long. 鈥淵ou could put your arm inside its tooth sockets, they are so huge,鈥� says No猫, who described the specimen in 2004 (). He estimates that it weighed as much as 30 tonnes. In comparison, a fully grown T. rex was a puny 7 tonnes.

Hurum has found fragments of pliosaurs of similar size in Spitsbergen, two of which 鈥� nicknamed 鈥減redator X鈥� and 鈥渢he monster鈥� 鈥� could have been as much as 15 metres long. The huge British pliosaur announced this week had a jaw around 2.4 metres long, putting it in the same ballpark as predator X. But it was unlikely to have been as big as Pliosaurus itself.

Not only were they huge, they were also formidable. The stomach contents of an 11-metre Australian pliosaur, Kronosaurus, which lived 100 million years ago, reveal it ate plesiosaurs, according to as-yet unpublished research by McHenry. Comparisons with living crocodiles suggest Kronosaurus had a much more powerful bite than would be expected for an animal with such a long snout.

For unknown reasons ichthyosaurs and large pliosaurs had died out by 90 million years ago, but it didn鈥檛 take long for their ecological roles to be refilled.

Mosasaurs were a new breed of marine reptile that branched off from the monitor lizard lineage. Knowledge of the mosasaurs goes back to the discovery of Mosasaurus, and their fossil record is more complete than for other marine reptiles. Uniquely, we also know of semi-aquatic transitional forms at the base of the family tree.

Perhaps the best of these 鈥渕issing links鈥� is the 98-million-year-old Haasiasaurus, discovered near Ramallah in the Palestinian West Bank. 鈥�Haasiasaurus could get around on land just as easily as in the water,鈥� says Mike Polcyn of the Southern Methodist University in Dallas, Texas, who described the species in 1999 ().

These early mosasaurs went on to evolve into fully marine forms up to 15 metres long. The final evolutionary radiation of sea monsters had begun and competition was fierce. 鈥淢osasaurs were getting into vicious fights with one another,鈥� says Everhart. 鈥淚鈥檝e seen broken bones, crushed skulls and huge bite marks.鈥� A 5-metre tylosaur from Kansas that he studied in 2008 was killed by a massive bite to its head. The only animal capable of delivering such an injury was a larger mosasaur, says Everhart ().

鈥淭hey were getting into vicious fights. I鈥檝e seen broken bones, crushed skulls and huge bite marks鈥�

The very latest mosasaurs showed an interesting evolutionary trend. 鈥淧rimitive mosasaurs were slender creatures that undulated their bodies like eels,鈥� says Johan Lindgren of Lund University in Sweden. 鈥淥ver time they stiffened their bodies and eventually only swam with their tails, like sharks.鈥� This process peaked with Plotosaurus, the most advanced mosasaur we know of. In a stunning example of convergent evolution, Plotosaurus had evolved a body shape approaching that of the ichthyosaurs ().

Known only from the latest Cretaceous, the 8-metre-long Plotosaurus hints at the way mosasaurs would have evolved 鈥� had they not gone extinct.

At the end of the Cretaceous the mosasaurs, plesiosaurs and pliosaurs joined the dinosaurs in the roll call of another mass extinction. 鈥淭he great marine reptiles were at the top of a long food chain that collapsed 65 million years ago. There was no longer enough food to keep them alive,鈥� says No猫.

The sea monsters had had their day. But a vacuum was waiting to be filled, and 10 million years later Pakicetus, a carnivorous mammal that looked a bit like a wolf, took a tentative dip in the water. The invasion of the sea had begun again. But that鈥檚 another story.

Editorial: Sea monsters鈥� irresistible attraction