Dinosaur Bones Yield ‘Missing Link’

Guided by a black-market fossil dealer, researchers found an ancient killing field where as many as a million dinosaur bones from a previously unknown species were embedded in two acres of primordial mudstone, scientists in Utah said Wednesday.

The mass graveyard preserves a 130-million-year-old species caught in the act of evolving into a vegetarian, the researchers said.

Blending the runner’s stance and 5-inch claws of a predator with the leaf-shaped teeth and potbelly of a grazer, the agile, whip-tailed dinosaur was an intermediate form in a shift from a carnivorous to a vegetarian diet, the researchers believe.

Thousands of the curious animals died en masse at the site in a mysterious calamity during an epoch when flowers first bloomed in the spring of the world and the ancestor of all modern mammals was a furry dormouse that cowered in the shrubbery.

They are entombed today on a sun-scorched mesa in rocks as dense as fruitcake with cobbles of bone, teeth and claw.

The new species “is a missing link in the history of dinosaurs,” said Scott D. Sampson, curator of vertebrate paleontology at the Utah Museum of Natural History, who was part of the research team.

“It is half raptor and half herbivore,” Sampson said. “It gives us amazing documentation of an evolutionary shift.”

Scientists from the Utah Geological Survey, the Utah Museum of Natural History and the University of Utah reported the find in the current issue of the science journal Nature.

The team named the species Falcarius utahensis, which means the Utah sickle maker, in honor of its prominent claws. The fossil site is at the Crystal Geyser Quarry, near one of the largest geysers in the world, outside Green River, Utah.

Researchers sorted through 1,700 bones to reconstruct a typical skeleton. The creature walked on two stubby legs, crouched low to the ground. It stretched 13 feet long from nose to tail, and was about 4.5 feet high.

Its head was small for its size, but its brain -- comparable to that of a modern opossum -- likely made it one of the smartest dinosaurs of its day, said Utah state paleontologist James Kirkland, who analyzed the fossils. Its skull suggests it also had a keen sense of smell.

In most respects, the dinosaur was nothing notable. It lacked the threatening horns, spiked armor or thundering bulk that made its close cousins the beloved monsters of childhood play.

But in the history of appetite, Falcarius is a milestone, the scientists said.

Its shift in diet may have been spurred by the advent of flowering plants, which first appeared on Earth more than 140 million years ago.

Researchers have no direct evidence of the dinosaur’s diet. They don’t know whether it survived on meat, plants or both. But they believe that its distinctive teeth are an important clue to its eating habits.

The jaws of meat-eaters typically were lined with blades of serrated teeth, well-suited for slashing flesh. Like other herbivores, however, Falcarius had teeth shaped more like spoons, better suited for shredding plants. The shape of the pelvis suggests that its gut expanded to accommodate the larger stomach needed to digest tough shoots, leaves and tubers.

It still might have been able to swallow the occasional lizard as a meat appetizer, but its main course, no doubt, was a salad, the researchers reported.

“It seems to be the first step going from a meat-slicing tooth to a leaf-shredding tooth,” Kirkland said. “We are catching this guy just at the beginnings of an adaptation to being a plant-eating dinosaur.”

So densely packed are the remains of Falcarius that a cubic meter of rock contains up to 400 fossil bones that can take field workers from four to six weeks to properly document.

“The Crystal Geyser Quarry is really a gem in the study of ancient life,” said Lindsay Zanno, a University of Utah researcher who specializes in the study of the new species.

Almost all of the fossils belong to the one species, with skeletons from every stage of development, as well as males and females.

Ancient crocodiles gnawed on the remains, judging by the evidence of bite marks and broken reptilian teeth.

The researchers can only speculate about why so many dinosaurs died at once.

There is no geologic evidence of volcanic eruptions that might have smothered them in ash; nor are there traces of a large river that might have caught them in a flood.

Kirkland theorized that the creatures might have been drawn to an ancient spring and died from botulism poisoning or from asphyxiating fumes of carbon dioxide, methane or sulfur dioxide.

The scientists acknowledged Wednesday that they owed the discovery to the troubled conscience of a man named Lawrence Walker, who found the creature while poaching fossils from federal lands, digging at night hidden under a tarpaulin. He had sold several of the fossil fragments at mineral shows in 1999.

Convinced the fossils belonged to a new species, Walker revealed to Kirkland the latitude and longitude of the remote mesa where the bones had been found.

When the paleontologist twice failed to find his way to the bone beds, Walker escorted him to the site, knowing he courted serious legal trouble.

Walker already was nervous about being arrested. Kirkland was caught in a dilemma of his own conscience. The state paleontologist was indebted to Walker for coming forward, but he also believed that the commercial black market in fossils was wrong.

They compromised. Kirkland told Walker that he would not turn him in but warned that, if asked, he would reveal Walker’s role to federal authorities.

A week after Kirkland sought a federal permit to excavate the site on public land, he was asked to give a sworn deposition detailing how the fossils were found. He answered honestly.

“I believe this stuff should be in public hands, within the realm of science,” Kirkland said. Even so, he felt guilty.

“I was the only link to [Walker] and that site on federal land,” he said.

Walker served five months in prison for theft of government property and paid $15,000 in restitution as the price for bringing the bones to the attention of science.

In a sense, scientists had been standing on the fossil field all along.

“I had known this area had been worked by 30 volunteers from the Denver Museum of Natural History for two summers,” Kirkland said. The fossils were located in the formation that the researchers strolled as they scanned the rocks above for specimens.

“He had me look under the hill we were standing on,” Kirkland said. “There were bones everywhere. Claws. Toe bones.”