Scientists Find First Intact Fossil of One of Earth’s Earliest Animals : Science: They say a 550-million-year-old slug will shed light on a key period of evolution. The discovery reveals some ‘entirely unexpected’ features.
On a shale-covered hillside on the inaccessible northern shore of Greenland, British and Danish researchers have for the first time found intact fossils of one of the first complex animals to inhabit the Earth, a discovery that sheds new light on a crucial period of evolution that is still largely hidden by time.
The animal, a two-inch-long, armor-coated slug called a halkieriid, dates from the period 550 million years ago when the Earth’s inhabitants abruptly evolved from single-celled microorganisms to multi-celled species.
Previously, paleontologists had discovered millions of bits and pieces of shells of the halkieriids, which dominated the Earth’s oceans. But they were unsure how to put them together because the organism was unlike anything that is alive today.
The new discovery, reported today in the British journal Nature, indicates that some of the researchers’ speculations about the shape of the animal were accurate, but it reveals “entirely unexpected” features of the creature.
Their finding is “an extraordinary discovery . . . (that) answers the prayers of many paleontologists,” said paleontologist Stefan Bengtson of Uppsala University in Sweden. It is “truly remarkable,” added paleontologist Stephen Jay Gould of Harvard University.
The new specimen, furthermore, is the first from a treasure trove of new fossils that may finally bring order into the jumbled family relationships that now characterize this unusual period of time.
The Cambrian period, which began about 550 million years ago, has long intrigued evolutionary biologists because it was the focus of an unprecedented sweep of events that quite literally changed the face of nature.
For more than 3.5 billion years, the Earth’s oceans had been filled with bacteria, algae and a host of other single-celled microorganisms. Then suddenly, over the geologically minuscule period of a few million years, those swarms of microorganisms gave way to the complex plants and animals that were the ancestors of today’s life.
Scientists are still not sure why the changes occurred, but it was most likely because the concentration of oxygen in the atmosphere rose to a critical level, sparking the evolution of species that could take advantage of the higher concentrations.
“This event still dazzles us and stands as a major biological revolution on a par with” the development of reproduction, Bengtson wrote in a commentary in Nature. “The animal phyls emerged out of the Precambrian mists with most of the attributes of their modern descendants.”
The primitive creatures left behind many traces of their passage, primarily fossils of small parts of their anatomies scattered and intermingled.
“It’s almost as if someone took a thousand jigsaw puzzles and threw them out the window of a tall building,” said paleontologist S. Conway Morris of the University of Cambridge, a co-author of the new paper with paleontologist John S. Peel of the Geological Society of Greenland, which is headquartered in Copenhagen.
“Paleontologists are traditionally famous (or infamous) for reconstructing whole animals from the debris of death,” wrote Bengtson. “Mostly they cheat,” modeling the long-dead species after still-living organisms that seem to share similar characteristics.
But deducing the shape of the Cambrian species, most of which have no contemporary counterparts, is much more difficult. “For such work, we must employ a combination of vivid imagination, scrupulous pedantry and deep humility.”
Researchers have pieced together the shapes of many animals of the Cambrian period, but their uncertainty about their conclusions is reflected in the often whimsical species names chosen: Hallucigenia, Anomalocaris, . . . inexpectans. They think Hallucigenia, for example, stood on seven pairs of pointed spines and carried a pair of tentacles on its back. Compared to this, the halkieriids must have seemed rather prosaic, simple slug-like worms bearing a suit of articulated armor.
The new site is on the Arctic Ocean in the Peary Land area of Greenland. “It’s not the most accessible place in the world . . . and the weather is rather ghastly,” Morris said in an interview.
The first fossils from the site were brought back in 1988 by two geologists with no training in paleontology. They showed the fossils to Peel “and immediately we realized they were significant,” Morris said. Morris, Peel and two other researchers spent three weeks there last summer collecting specimens.
“You can sit on a hillside covered with loose shale and choose what you want,” he continued. “Nobody had ever collected there before. (The fossils) are just lying on the ground waiting.” Among the specimens they found were ancestors of crabs, spiders, worms and sponges.
But the “most spectacular” of the specimens they have studied so far are the halkieriid fossils, the first completely intact specimen of any organism from that era. “It’s an interesting beast,” he said.
Morris, an elder statesman among Cambrian period paleontologists, and others had predicted that the halkieriids were basically worms covered with hundreds of small shells that provided protection while allowing movement. But they were astounded to see that the organism had two large round shells, one at each end. That was “a complete surprise to us.”
The question now, Bengtson said, is “where do they come from?” He speculated that the halkieriids might have evolved from a clam-shaped organism, growing the smaller shells as its length expanded. Alternatively, it might have evolved the shield-like shells on each end to protect both openings of a U-shaped burrow.
Meanwhile, Morris and Peel are studying and classifying the other fossils obtained from the area and are confident that the work will shed much light on the Early Cambrian. “It turns out that the organization of species (in that period) is really much more complicated than we had thought,” Morris concluded, “but we should be able to work out many evolutionary relationships.”
