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Ichthyosaur's Turtle Supper Causes Extinction Debate

John Pickrell in England
for National Geographic News
August 5, 2003
 
Wide-eyed and fast moving, ichthyosaurs were the giant carnivores of Jurassic and Cretaceous seas. These marine reptiles are thought to have been powerful pursuit predators, evolving to chase small and equally speedy prey.

They were so specialized according to some experts, that a temporary plunge in squid-like belemnite numbers in the middle of the Cretaceous period (which started 144 million years ago and ended with the extinction of the dinosaurs 65 million years ago) was considered one potential cause for the extinction of the ichthyosaur group.


Now however, an astonishing new 110-million-year-old skeleton, with preserved hatchling turtle and ancient bird fragments in its gut, is causing some experts to question that possible explanation for the group's demise.

The reptile, known as Platypterigius longmani, was dug up from deposits in Queensland, Australia and is described in an upcoming print edition of the journal Biology Letters.

"You don't often see gut contents," commented paleontologist Chris McGowan of the Royal Ontario Museum in Toronto. "Finding this sort of preservation in a Cretaceous [Ichthyosaur] specimen is probably a first."

Superficially Tuna-Like

Ichthyosaurs were a successful group of reptiles. Their first awkward-looking reptilian ancestors took to the seas 230-odd million years ago, and never looked back. They patrolled the oceans for 135 million years, almost as long as dinosaurs rampaged through terrestrial environments.

Some ichthyosaurs—meaning "fish-lizard"—evolved a tuna, or porpoise-like body shape, and the group is thought to have played out the role that many dolphins fill in the oceans today. From obscure origins "they developed the most superbly adapted body form for life in water," said study lead author Benjamin Kear, paleontologist at the South Australia Museum in Adelaide.

Many of the 100-plus known species were a hefty 5 to 6 meters (16 to 20 feet) in length. However, one enormous fossil, recently unearthed in the wilderness of British Columbia, could have been as long as 25 meters (more than 80 feet) in life.

Most evidence of ichthyosaur diet comes from a handful of Jurassic-period fossil specimens with well-preserved gut contents. The Jurassic period started 210 million years ago and ended 144 million years ago.

Fossil workers first noticed odd hooked structures in ichthyosaur guts 150 years ago. These turned out to be remains from the tentacles of extinct squid-like belemnites. These animals have been the most commonly found gut contents of ichthyosaurs examined since then, along with fish and more rarely smaller ichthyosaurs and other animals.

Some experts believe that many of the last Cretaceous-era species—due to competition from other marine reptiles and predatory fish perhaps—fed on squid-like animals near exclusively. A theory suggested in the mid-1990s pins the sudden demise of ichthyosaurs on an invertebrate extinction event in the mid-Cretaceous period linked to changing sea level.

Dietary Evidence

However, the new fossil, described by Kear and colleagues at The University of New South Wales and the Australian Museum, both in Sydney, hints that this extinction event might not explain the demise of all ichthyosaurs. Dissolving the ancient skeleton's limestone substrate with acid revealed several unexpected menu items.

The pregnant female Platypterigius specimen (unlike most reptiles, ichthyosaurs bore live young) was found not with squid remains in its gut, but a mass of vertebrate bone fragments.

These included the bones of fish, hatchling turtles—probably snapped up while they bobbed along at the water's surface-—and an extinct bird. Some modern bird groups appear during the Cretaceous period. The bird may have been a floating carcass scavenged after it washed into the sea from woodland, said Kear.

"It is exciting to have a turtle and bird in an ichthyosaur stomach," but many don't agree that the reptiles fed exclusively on squid anyway, commented paleontologist Ryosuke Motani at the University of Oregon in Eugene. "All we know is that the disappearance of ichthyosaurs roughly corresponded with the [squid-like animal] extinction event," said Motani. We just don't have any better guess about the cause of the demise, he said.

"The problem is we have so little data," added McGowan, "Most of our theories are pure guesswork," he said.

Nevertheless, the new fossil hints that Cretaceous ichthyosaurs may have been far more opportunistic feeders than believed, says the study, which argues that the decline of ichthyosaurs may therefore be linked to competition with other types of speedy pursuit predators.

"Competitors might include bony fish," said Kear. Many of the modern types of fish had evolved by then and some of these would have been direct rivals. Another competitor may have been a different marine reptile, which returned to the sea quite independently of ichthyosaurs.

Long-necked, paddle-footed plesiosaurs may have caused ichthyosaur declines. The earliest members of one group, known as polycotylid plesiosaurs, are found in many of the same deposits as ichthyosaurs, said Kear, and like those animals, have physical characteristics ideal for the swift pursuit of small prey.

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Paul Sereno: National Geographic Explorer-in-Residence and Dinosaur Hunter
Dinorama
Wanted: Albertosaurus
Dinosaur Eggs
Pterosaurs
Destinations: Dinosaur National Monument
 

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