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2014

A Long-Ago Ancestor: A Little Fish, With Jaws to Come
New York Times, June 11, 2014
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Half a billion years ago, a new study suggests, your ancestors may have looked like this:



This two-inch, 505-million-year-old creature belonged to the lineage that would later produce sharks, eels and other fish — along with birds, reptiles and mammals like us. This early vertebrate, known as Metaspriggina, was something of a mystery for years, known only from a pair of ambiguous fossils. But recently, scientists unearthed a trove of much more complete Metaspriggina fossils.

As they report today in the journal Nature, the new fossils offer a remarkably detailed understanding of the first vertebrates, helping scientists understand how major parts of our own anatomy — from eyes to jaws to our muscles — evolved.

“It’s clearly a benchmark early vertebrate, which we haven’t had before,” said Thurston Lacalli of the University of Victoria in British Columbia, who was not involved in the research.

Discovering the origins of vertebrates has occupied biologists for decades. A few living invertebrates, such as worm-like animals called lancelets, are closely related to vertebrates, but our ancestors split off from theirs more than 600 million years ago.

To discover how vertebrates emerged, paleontologists have hunted for fossils. But the earliest vertebrates lacked hard bones, and their soft, cartilage-based bodies rarely fossilized well. In China, paleontologists have found several species of early vertebrates dating back about 515 million years, yet their rarity and poor condition has left many unanswered questions.

In 2012, Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto, and his colleagues discovered an astonishingly dense collection of fossils in the Canadian Rockies. In two weeks, they found 3,000 fossils from 55 species, all dating back 505 million years ago.

This extraordinary cache included 40 fossils of Metaspriggina. To study them, Dr. Caron invited the top expert on the creature, Simon Conway Morris of the University of Cambridge, to join him in Toronto.

Dr. Conway Morris first laid eyes on the two known Metaspriggina fossils in the 1970s. In 2008, he published a study suggesting the animal was either an early vertebrate or a close relative. But the fossils were too incomplete for him to be more certain than that.

Last April, he flew to Toronto, where he and Dr. Caron inspected the new fossils, which were still partly embedded in rock. Day after day, they carefully chipped away at the rocks to uncover more anatomical details.

“The material is fiendishly difficult to prepare,” said Dr. Conway Morris.

Their patience paid off. Dr. Caron and Dr. Conway Morris discovered that some of the fossils were in extraordinary shape. For the first time, the scientists could see well-preserved parts of Metaspriggina’s body, including its muscles, its liver, its gills and even its eyes.

Now that Dr. Caron could see Metaspriggina in its entirety, he realized that he had seen the animal before. Over the years, he had examined unidentified fossils in various museum collections that were actually parts of Metaspriggina.

“The brain suddenly connects the dots,” he said.

Later, Dr. Caron inspected those mysterious fossils and confirmed that they were indeed left by Metaspriggina. All told, he and Dr. Conway Morris have now examined about 100 specimens. Together, the fossils add up to an extraordinarily detailed picture.

The scientists could see, for example, that Metaspriggina had a series of W-shaped blocks of muscle running the length of its body, the kind fish today contract when swimming.

“It’s what you’d see in your favorite salmon steak,” said Dr. Caron.

At 505 million years old, Metaspriggina offers the oldest, clear evidence of these vertebrate muscles, which allowed fish to move quickly.

While Metaspriggina didn’t have any hard bones in its skeleton, it did grow a rod of cartilage from head to tail, called a notochord, to keep its body stiff. Human embryos develop a notochord, too, but it later turns into the disks of cartilage between the vertebrae in our spine.

Dr. Caron and Dr. Conway Morris also discovered exquisitely well-preserved eyes on Metaspriggina. The eyes appear to have had lenses, meaning that Metaspriggina could see complex images. Its eyes sit atop its head, bulging upward, suggesting that Metaspriggina kept watch on other animals overhead.

Vision was not the only sense that Metaspriggina had. Along with powerful eyes, the scientists found what appears to be a nose at the front of its head.

A living shark uses its nose and eyes to detect prey, and then uses its muscle blocks to chase it. But Dr. Caron doubts that the first fish were predators. He notes that Metaspriggina’s mouth was a just tiny opening, without jaws or teeth.

“We think it was filtering particles at the bottom of the sea,” he said.

Early vertebrates may have evolved powerful vision and smell, along with fast swimming muscles, for another purpose. Instead of chasing prey, Metaspriggina was the prey.

Paleontologists have found fossils of large predators from the same age, which were distantly related to today’s lobsters and insects. Once Metaspriggina sensed these predators nearby, it may have used its powerful muscle blocks to swim away.

“It probably looked juicy and nice to eat with all that muscle,” said Dr. Caron.

Jon M. Mallatt of Washington State University, who was not part of the study, agreed with Dr. Caron’s interpretation. “These were the super-escape artists,” he said.

Metaspriggina fueled those escapes with gills, which extracted oxygen from the water. Dr. Caron and Dr. Conway Morris found that it grew six pairs of gills, each supported by bars of cartilage. Metaspriggina probably also used its gills to trap the particles it sucked into its mouth.

But Dr. Caron and Dr. Conway Morris also found a surprise. In front of its gills, Metaspriggina grew an extra pair of cartilage bars that were bigger than the others but did not support any gills. Dr. Caron speculates that they may have been the precursors to our jaws.

Millions of years later, the bars may have become even bigger and more flexible. Fish could then use them to grab prey, rather than just suck up bits of food.

“Then they became rulers of the seas,” Dr. Caron suggested.

Philip Donoghue, a paleontologist at the University of Bristol, is cautious about drawing too many conclusions at this point.

“The fossils are really nice,” he said, but noted that it was hard to make out the body wall on them. As a result, the precise anatomy of its head is still ambiguous, making it hard to determine if it fed on particles or on other types of food. “I can’t personally interpret its ecology.”

Dr. Morris said that he still expected to learn much more about Metaspriggina from fossils. After all, it only took two weeks for Dr. Caron and his colleagues to find dozens of them, and they’re planning to dig more at the site this summer.

“One’s fingers tingle at the prospect,” Dr. Conway Morris said.

Copyright 2014 The New York Times Company. Reproduced with permission.
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