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The Cambrian Explosion’s Strange-Looking Poster Child
New York Times, July 2, 2015

The animal kingdom got off to a slow start. Studies on DNA indicate that the first animals evolved more than 750 million years ago, but for well over 200 million years, they left a meager mark on the fossil record. As best as paleontologists can tell, the animal kingdom during that time consisted of little more than sponges and other creatures rooted to the ocean floor.

But then, about 520 million years ago during the Cambrian Period, animal evolution shifted into high gear. Fast-moving predators, scavengers and burrowers evolved. Many of the major living groups of animals left their first fossils during this so-called Cambrian explosion, including our own ancestors. But the Cambrian explosion also brought many bizarre species that have long puzzled paleontologists.

For almost 40 years, the poster child for the Cambrian explosion’s strangeness has been a hand-size armored worm with a name to suit its bizarre appearance: Hallucigenia.

But recently, Hallucigenia has lost much of its mystery. Scientists have worked out the creature’s anatomy, and they have figured out a lot about how Hallucigenia and its relatives thrived in the Cambrian oceans. And despite its odd appearance, Hallucigenia isn’t an incomprehensible zoological experiment. Paleontologists have been able to place it comfortably on the evolutionary branch that led to a group of invertebrates alive today called velvet worms.

“What we now know is that these bizarre creatures were not so bizarre at all,” said Jakob Vinther, a paleontologist at the University of Bristol.

In 1977, a University of Cambridge paleontologist named Simon Conway Morris published the first account of Hallucigenia. The 508-million-year old fossil was part of a cache found in Canada in the early 1900s. Dr. Conway Morris struggled to make sense of the creature, and envisioned it with a worm-shaped body. Along one side was a row of tentacles, and on the other side were seven pairs of stilts. At one end of its body was a strange bulb.

When a colleague saw the sketch, Dr. Conway Morris later recalled, he burst out laughing.

In his report, Dr. Conway Morris presented the animal with its stilts resting on the ocean floor and its tentacles waving, perhaps sucking in food. It was so bizarre that he could offer no firm idea of how it was related to other animals, living or extinct. He named it Hallucigenia after its dreamlike appearance.

In 1989, the paleontologist Stephen Jay Gould wrote about Hallucigenia in an influential book on the Cambrian explosion called “Wonderful Life.” Dr. Gould argued that early animals burst into a vast diversity of forms, most of which have disappeared. He singled out Hallucigenia as the epitome of this process. When considering Hallucigenia, Dr. Gould wrote, “We are forced to enter a truly lost world.”

But a few years later, the paleontologists Hou Xianguang and Lars Ramskold began to dispel the mystery around the animal.

Studying Cambrian fossils in China, they discovered what were clearly relatives of Hallucigenia. The fossils were better preserved than those Dr. Conway Morris had studied, and they revealed that he had turned the animals upside down. In reality, Hallucigenia’s supposed tentacles were seven pairs of clawed legs. And its stilts were actually long spikes that rose from its back.

But fundamental questions remained. Which end, for example, was Hallucigenia’s head?

In recent years, Martin Smith of the University of Cambridge and Jean-Bernard Caron of the Royal Ontario Museum have been studying new fossils of Hallucigenia and revisiting old ones with better microscopes. They have published a series of papers, the latest of which appeared last week in Nature.

Their studies now give us a much clearer picture of the animal. “Hallucigenia is not a weird wonder — it’s something we can understand,” Dr. Caron said.

Among other things, the scientists have discovered that Hallucigenia’s bulb is its head. It carried two eyes, a mouth with a ring of stiffening spines around its edge, and teeth farther down its throat.

Dr. Smith and Dr. Caron also found that Hallucigenia’s front three pairs of limbs had no claws. Instead, they were flexible tentacles that could have reached the animal’s mouth.

“We think they were living on the sea floor, probably living on food particles from decaying matter,” Dr. Caron said.

While Hallucigenia kept its mouth pointed down on its food, its eyes would have scanned the water above. “Despite its slender spines, predators swimming overhead may have been an ever-present concern,” Dr. Smith suggested.

Now, a study published Monday in Proceedings of the National Academy of Sciences suggests that Hallucigenia had Cambrian cousins that evolved a very different way of life.

For more than two decades, scientists have been finding bits and pieces of Hallucigenia relatives. In recent years, though, paleontologists at Yunnan University in China have excavated a wealth of 517-million-year-old Chinese fossils belonging to a single species.

Last year, the Yunnan researchers emailed a photograph of one of the fossils to their collaborator, a University of Cambridge paleontologist, Javier Ortega-Hernández. “I thought, ‘This is damn weird,’ ” he said, “which is usually a good way to start.”
30, 2015. Watch in Times Video »

Dr. Ortega-Hernández soon went to China to examine the fossils. The species, which he and his colleagues have named Collinsium ciliosum, looks “like Hallucigenia on steroids,” he said

Instead of 14 spikes on its back like Hallucigenia, Collinsium had 72. And instead of 14 limbs, it had 30. What’s more, its limbs were different from those of Hallucigenia. Its front 12 limbs were adorned with fans of fine hairs.

Dr. Ortega-Hernández hypothesized that Collinsium would have climbed up seaweed or sponges, using those front limbs as a net to catch particles floating by.

The formidable armor on Collinsium may explain why, unlike Hallucigenia, it appears to have not had eyes. Predators may have passed it by, allowing it to blindly filter its food.

The recent research on Hallucigenia and its relatives has also enabled scientists to find a place for the animals on the tree of life. Last year, Dr. Ortega-Hernández and Dr. Smith reported that the animals had distinctive layers of tissue in their claws and spikes. That same layering can be found today in a group of animals called velvet worms, which live on the floor of tropical forests.

The scientists concluded that Hallucigenia and its relatives belonged to a lineage that split from other invertebrates before the Cambrian Period and ultimately produced today’s velvet worms.

While Hallucigenia may not be as exotic as it once seemed, the fact remains that its lineage was a lot more exuberant back in the Cambrian.

“To the untrained eye, any one modern velvet worm looks much the same as another,” said Dr. Smith. But no one would confuse Hallucigenia and Collinsium.

Dr. Vinther, who was not involved in the new studies, speculated that Hallucigenia and its relatives could not compete with other species in the oceans and became extinct. But some of their cousins managed to escape the sea and endure in more modest form.

“It might be that early on they colonized the land,” he said, “and that was a refuge for them.”

Copyright 2015 The New York Times Company. Reproduced with permission.
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