Caught In The Act: Butterfly Mate Preference Shows How One Species Can Become Two

Breaking up may actually not be hard to do, say scientists who've found a population of tropical butterflies that may be on its way to a split into two distinct species.

The cause of this particular break-up? A shift in wing color and mate preference.

In a paper published this week in the journal Science, the researchers describe the relationship between diverging color patterns in Heliconius butterflies and the long-term divergence of populations into new and distinct species.

"Our paper provides a unique glimpse into the earliest stage of ecological speciation, where natural selection to fit the environment causes the same trait in the same population to be pushed in two different directions," says Marcus Kronforst, a Bauer Fellow in the Center for Systems Biology at Harvard University who received his doctor's degree at The University of Texas at Austin. "If this trait is also involved in reproduction, this process can have a side effect of causing the divergent subpopulations to no longer interbreed. This appears to be the process that is just beginning among Heliconius butterflies in Ecuador."

Heliconius butterflies display incredible color pattern variation across Central and South America, with closely related species usually sporting different colors. In Costa Rica, for example, the two most closely related species differ in color: One species is white and the other is yellow. In addition, both species display a marked preference to mate with butter-flies of the same color.

The Ecuadorian population examined by Kronforst and his colleagues shows the same white and yellow variation found in Costa Rica but has not yet reached a level of strong reproductive isolation. The entire population lives in close proximity and individuals of both colors come in contact with -- and mate with -- each other.

But, by studying the Ecuadorian population in captivity, the scientists found the two colors do not mate randomly. Despite the genetic similarity between the groups -- white and yellow varieties differ only at the color-determining gene -- yellow Ecuadorian individuals show a preference for those of the same color. White male butterflies, most of which are heterozygous at the gene that controls color, show no color preference.

"This subtle difference in mate preference between the color forms in Ecuador may be the first step in a process that could eventually result in two species, as we see in Costa Rica," says Kronforst, who began studies of Heliconius color pattern and behavioral genetics in the laboratory of Professor Lawrence Gilbert at The University of Texas at Austin.

Previous studies of species formation have focused on the characteristics of well-differentiated species, and the health and viability of their hybrids in particular, in an effort to identify how the species may have emerged and how they stay distinct.

Heliconius provides a model for a different kind of study. The researchers considered species emergence from the opposite end, studying populations that have yet to diverge into separate species in order to identify the role of mate choice in the potential emergence of new species.

Having identified color-based mate preference in Heliconius, the researchers used a battery of genetic markers to compare the genomes of the white and yellow varieties, showing that they are genetically identical except for their different colors and preferences.

Their work suggests that the genes for color and preference are very close to one another in the genome; the two traits could even be caused by the same gene. Their next step is to identify the gene (or genes) responsible for the differences in color and mate preference.

"If we can identify this gene or genes, we can say conclusively how they influence both color and mate choice," says Kronforst. "Subsequent work could elucidate exactly how changes in individual genes can, over long periods of time, lead to novel species."

"This study shows the great potential of the genus Heliconius as a model system for integrating genetics, development, behavior, ecology and evolution," says Gilbert, professor in the Section of Integrative Biology. "It is the culmination of diverse contributions of the co-authors involving insectary, field and laboratory research over more than a decade."

Co-authors on the Science paper with Kronforst are Nicola L. Chamberlain and Ryan I. Hill, both of Harvard; Durrell D. Kapan of the University of Hawaii; and Lawrence E. Gilbert of The University of Texas at Austin. Their work was funded by the National Science Foundation and the National Institutes of Health.

Newly Discovered Ankylosaur Dinosaur Is 'Biological Version Of An Army Tank'

A husband and wife team of American paleontologists has discovered a new species of dinosaur that lived 112 million years ago during the early Cretaceous of central Montana.

The new dinosaur, a species of ankylosaur, is documented in the October issue of the Canadian Journal of Earth Sciences. Ankylosaurs are the biological version of an army tank. They are protected by a plate-like armour with two sets of sharp spikes on each side of the head, and a skull so thick that even 'raptors' such as Deinonychus could leave barely more than a scratch.

Bill and Kris Parsons, Research associates of the Buffalo Museum of Science, found much of the skull of the newly described Tatankacephalus cooneyorum resting on the surface of a hillside in 1997. Because the skull was 90% complete, it was possible to justify this fossil as a new species.

"This is the first member of Ankylosauridae to be found within the Early Cretaceous Cloverly Geologic Formation," said Bill Parsons, who characterized the fossil as a transitional evolutionary form between the earlier Jurassic ankylosaurs and the better known Late Cretaceous ankylosaurs.

The skull is heavily protected by two sets of lateral horns, two thick domes at the back, and smaller thickenings around the nasal region. "Heavy ornamentation and horn-like plates would have covered most of the dorsal surface of this dinosaur" said Bill Parsons.

"For years, Bill and Kris have been collecting fossils from a critical time in Earth's history, and their hard work has paid off," said Lawrence Witmer, professor of paleontology at Ohio University who was not involved with this study. "This is a really important find and gives us a clearer view of the evolution of armored dinosaurs. But this is just the first; I'm sure, of what will be a series of important discoveries from this team."

Parsons also illustrated the dermal armour of this new species based on the theory by Museum of the Rockies paleontologist John R. Horner that there was an outer keratinous sheathing on it as found in modern turtle shells and bird beaks. In his new reconstruction, Parsons suggests that Tatankacephalus exhibited complex and colorful patterns rather than the dull appearance suggested in earlier ankylosaur portraits. "According to Horner's theory, many other dinosaurs also had this kind of sheathing and also may have been diversely colored," said Parsons.

As to its name, the broad, short horns on the back of its skull resemble the horns found on a modern buffalo skull and Tatankacephalus loosely translates as 'Buffalo head.' Parsons also noted, "of course any further allusions to the city of Buffalo are completely intentional as well."

New Analyses Of Dinosaur Growth May Wipe Out One-third Of Species

Paleontologists from the University of California, Berkeley, and the Museum of the Rockies have wiped out two species of dome-headed dinosaur, one of them named three years ago -- with great fanfare -- after Hogwarts, the school attended by Harry Potter.

Their demise comes after a three-horned dinosaur, Torosaurus, was assigned to the dustbin of history last month at the Society of Vertebrate Paleontology meeting in the United Kingdom, the loss in recent years of quite a few duck-billed hadrosaurs and the probable disappearance of Nanotyrannus, a supposedly miniature Tyrannosaurus rex.

These dinosaurs were not separate species, as some paleontologists claim, but different growth stages of previously named dinosaurs, according to a new study. The confusion is traced to their bizarre head ornaments, ranging from shields and domes to horns and spikes, which changed dramatically with age and sexual maturity, making the heads of youngsters look very different from those of adults.

"Juveniles and adults of these dinosaurs look very, very different from adults, and literally may resemble a different species," said dinosaur expert Mark B. Goodwin, assistant director of UC Berkeley's Museum of Paleontology. "But some scientists are confusing morphological differences at different growth stages with characteristics that are taxonomically important. The result is an inflated number of dinosaurs in the late Cretaceous."

Goodwin and John "Jack" Horner of the Museum of the Rockies at Montana State University in Bozeman, are the authors of a new paper analyzing North American dome-headed dinosaurs that appeared this week in the public access online journal PLoS One.

Unlike the original dinosaur die-off at the end of the Cretaceous period 65 million years ago, this loss of species is the result of a sustained effort by paleontologists to collect a full range of dinosaur fossils -- not just the big ones. Their work has provided dinosaur specimens of various ages, allowing computed tomography (CT) scans and tissue study of the growth stages of dinosaurs.

In fact, Horner suggests that one-third of all named dinosaur species may never have existed, but are merely different stages in the growth of other known dinosaurs.

"What we are seeing in the Hell Creek Formation in Montana suggests that we may be overextended by a third," Horner said, a "wild guess" that may hold true for the various horned dinosaurs recently discovered in Asia from the Cretaceous. "A lot of the dinosaurs that have been named recently fall into that category."

The new paper, published online Oct. 27, contains a thorough analysis of three of the four named dome-headed dinosaurs from North America, including Pachycephalosaurus wyomingensis, the first "thick-headed" dinosaur discovered. After that dinosaur's description in 1943, many speculated that male pachycephalosaurs used their bowling ball-like domes to head-butt one another like big-horn sheep, though Goodwin and Horner disproved that notion in 2004 after a thorough study of the tissue structure of the dome.

Many paleontologists now realize that the elaborate head ornaments of dinosaurs, from the huge bony shield and three horns of Triceratops to the coxcomb-like head gear of some hadrosaurs, were not for combat, but served the same purpose as feathers in birds: to distinguish between species and indicate sexual maturity.

"Dinosaurs, like birds and many mammals, retain neoteny, that is, they retain their juvenile characteristics for a long period of growth," Horner said, "which is a strong indicator that they were very social animals, grouping in flocks or herds with long periods of parental care."

These head ornaments, which probably had horny coverings of keratin that may have been brightly-colored as they are in many birds, started growing when these dinosaurs reached about half their adult size, and were remodeled as these dinosaurs matured, continuing to change shape even into adulthood and old age, according to the researchers.

In the new paper, Horner and Goodwin compared the bone structures of Pachycephalosaurus with that of a domeheaded dinosaur, Stygimoloch spinifer, discovered in Montana by UC Berkeley paleontologists in 1973, and a dragon-like skull discovered in South Dakota and named in 2006 as a new species, Dracorex hogwartsia.

With the help of CT scans and microscopic analysis of slices through the bones of Pachycephalosaurus and Stygimoloch, the team concluded that Stygimoloch, with its high, narrow dome, growing tissue and unfused skull bones, was probably a pachycephalosaur subadult, in a stage just before sexual maturity.

Dracorex is one of a kind, and thus unavailable for dissection, but morphological analysis indicates it is a juvenile that hasn't yet formed a dome, although the top of its skull shows thickening suggestive of an emerging dome.

"Dracorex's flat skull, nodules on the front end and small spikes on back, and thickened but undomed frontoparietal bone all confirm that, ontogenetically, it is a juvenile Pachycephalosaurus," Goodwin said.

Comparison of these skulls to other fossils in the hands of private collectors confirm the conclusions, they said. In all, they looked at 21 dome-headed dinosaur skulls and cranial elements from North America.

The key to this analysis, Horner said, was years of field work in Montana by his team and Goodwin's in search of fossils of all sizes.

"We have gone out in the Hell Creek Formation for 11 years doing nothing but collecting absolutely everything we could find, which is the kind of collecting that is required," he said. "If you think about Triceratops, people had collected for 100 years and still hadn't found any juveniles. And we went out and spent 11 years collecting everything, and we found all kinds of them."

"Early paleontologists recognized the distinction between adults and juveniles, but people have lost track of looking at ontogeny -- how the individual develops -- when they discover a new fossil," Goodwin said. "Dinosaurs are not mammals, and they don't grow like mammals."

In fact, the so-called metaplastic bone on the heads of horned dinosaurs grows and dissolves, or resorbs, throughout life like no other bone, Horner said, and is reminiscent of the growth and loss of horns today in elk and deer. In earlier studies, Horner and Goodwin found dramatic remodeling of metaplastic bone in Triceratops, which led to their subsequent focus on dome-headed dinosaurs.

"Metaplastic bones get long and shorten, as in Triceratops, where the horn orientation is backwards in juveniles and forward in adults," Horner said. Even in older specimens, such as the fossil previously named Torosaurus, bone in the face shield resorbs to create holes along the margin. John Scannella, Horner's student at Montana State, presented a paper reclassifying Torosaurus as an old Triceratops at the Society for Vertebrate Paleontology meeting in Bristol, U.K., on Sept. 25.

"In order for that huge amount of bone to move, there has to be a lot of deposition and resorption," Horner said.

Horner and Goodwin continue to search for dinosaur fossils in the Hell Creek Formation, which is rich in Triceratops, dome-headed dinosaurs, hadrosaurs and tyrannosaurs. Analysis of growth stages in these taxa will have implications for other horned dinosaurs that are being uncovered in Asia and elsewhere.

"There are other horned dinosaurs I think may be over split," that is, split into too many new species rather than being lumped together as one species, Goodwin said.

The work was supported by grants from the UC Museum of Paleontology and the Museum of the Rockies.

Polarized Peepers: Crustacean's Eyes Surpass Man-Made Optical Devices in Manipulating Light

A fierce crustacean known as the peacock mantis shrimp has eyes so refined they can perceive polarized light, including information that is invisible to nearly every other member of the animal kingdom. Not only can the ocean dweller extract polarization information from light, it can do so when the light is circularly polarized—an ability unknown outside a few species of the order of stomatopods to which the peacock mantis belongs.

Unlike linearly polarized light, in which the electric field oscillates along a plane, circularly polarized light's field twists like a spiral spring as the ray propagates. Such light is not commonly reflected from animal bodies and so was long dismissed as a virtual nonfactor in physiology, but research last year showed that some stomatopods have the ability to discriminate circular polarization. A paper published online October 25 in Nature Photonics unpacks the mechanism behind the mantis shrimp's ability and concludes that its eyes handle circularly polarized light more effectively than man-made optical devices do. (Scientific American is part of the Nature Publishing Group.)

The peacock mantis, or Odontodactylus scyllarus, packs a surprisingly powerful punch for its size. The crustacean, which ranges from three to 18 centimeters in length, is capable of shattering the glass of an aquarium with a blow from its forelimb, says Roy Caldwell, a University of California, Berkeley, biologist who did not participate in the new research. "We have had a couple cases where animals have hit a pane of glass dead in the center and there was a massive explosion," Caldwell says.

But the creature is physiologically remarkable in at least one other way: The compound eye of the peacock mantis, the new study's authors found, harbors a natural quarter-wave retarder, a sort of filter that converts circularly polarized light to linearly polarized light, which then activates receptors below. "Biologically, this is unique," says study co-author Thomas Cronin, a professor of biological sciences at the University of Maryland, Baltimore County. "There is nothing else known anywhere in biology" that enables detection of circularly polarized light, he adds.

The stomatopods reflect circularly polarized light from their bodies, so their ability to detect such light—and to parse clockwise from counterclockwise polarization—likely plays a role in signaling or identification. In some stomatopod species, reflection of circularly polarized light is sex-specific, which could play a role in sexual signaling or mate selection.

Wave retarders work by refracting light differently depending on the angle of its polarization, delaying one wave component of a light wave relative to the other. "If it's just the right degree of delay, which is one-quarter wave or 90 degrees phase, that converts circularly polarized light to linearly polarized," Cronin explains. But unlike wave retarders available commercially, which are tuned for specific wavelengths (and hence colors) of visible light, the wave plate in the O. scyllarus eye performs almost identically across the visible spectrum.

The mantis shrimp's eye, Cronin explains, "works on a principle that is not used currently but could be used in manufacturing systems"—balancing the optical properties of the eye structure with those of the lipid molecules that fill the structure. "The two have different wavelength functions—they have different curves of changing retardance with wavelength—and so the animal trades them off," Cronin says. "It trades off structure against material to cancel out the two variations."

Sonja Kleinlogel, a biologist at the Max Planck Institute of Biophysics in Frankfurt am Main, Germany, points out that she and a colleague published a similar analysis last year in the journal PLoS ONE—an article that she was surprised to see omitted from the references section of the new paper. Nevertheless, she is pleased to see the subject advanced, noting that the research "is the first to look at the detailed structure" of the cells that act as quarter-wave retarders and to compare their efficacy with man-made analogues. U.C. Berkeley's Caldwell concurs, noting that the unique capability of the stomatopod eye had been described but "how that actually was done was pretty much a mystery."

"We didn't know anything about the operating principle of the retarder," Cronin says. "It wasn't like anything we had seen in the lab."

Tool-making Human Ancestors Inhabited Grassland Environments Two Million Years Ago

In an article published in the open-access, peer-reviewed journal PLoS ONE on October 21, 2009, Dr Thomas Plummer of Queens College at the City University of New York, Dr Richard Potts of the Smithsonian Institution National Museum of Natural History and colleagues report the oldest archeological evidence of early human activities in a grassland environment, dating to 2 million years ago. The article highlights new research and its implications concerning the environments in which human ancestors evolved.

Scientists as far back as Charles Darwin have thought that adaptation to grassland environments profoundly influenced the course of human evolution. This idea has remained well-entrenched, even with recent recognition that hominin origins took place in a woodland environment and that the adaptive landscape in Africa fluctuated dramatically in response to short-term climatic shifts.

During the critical time period between 3 and 1.5 million years ago, the origin of lithic technology and archeological sites, the evolution of Homo and Paranthropus, selection for endurance running, and novel thermoregulatory adaptations to hot, dry environments in H. erectus have all been linked to increasingly open environments in Africa.

However, ecosystems in which grassland prevails have not been documented in the geological record of Pliocene hominin evolution, so it has been unclear whether open habitats were even available to hominins, and, if so, whether hominins utilized them. In their new study, Plummer and colleagues provide the first documentation of both at the 2-million-year-old Oldowan archeological site of Kanjera South, Kenya, which has yielded both Oldowan artifacts and well-preserved faunal remains, allowing researchers to reconstruct past ecosystems.

The researchers report chemical analyses of ancient soils and mammalian teeth, as well as other faunal data, from the ~2.0-million-year-old archeological sites at Kanjera South, located in western Kenya. The principal collaborating institutions of the Kanjera project are QueensCollege of the City University of New York, the Smithsonian Institution's Human Origins Program, and the NationalMuseums of Kenya. The findings demonstrate that the recently excavated archeological sites that preserve Oldowan tools, the oldest-known type of stone technology, were located in a grassland-dominated ecosystem during the crucial time period.

The study documents what was previously speculated based on indirect evidence -- that grassland-dominated ecosystems did, in fact, exist during the Plio-Pleistocene (ca. 2.5-1.5 million years ago) and that early human tool-makers were active in open settings. Other recent research shows that the Kanjera hominins obtained meat and bone marrow from a variety of animals and that they carried stone raw materials over surprisingly long distances in this grassland setting. A comparison with other Oldowan sites shows that by 2.0 million years ago, hominins, almost certainly of the genus Homo, lived in a wide range of habitats in East Africa, from open grassland to woodland and dry forest.

Plummer and colleagues conclude that early Homo was flexible in its habitat use and that the ability to find resources in both open and wooded habitats was a key part of its adaptation. This strongly contrasts with the habitat usage of older species of Australopithecus and appears to signify an important shift in early humans' use of the landscape.

Funding from the L. S. B. Leakey Foundation, the Leverhulme Trust, the National Geographic Society, the National Science Foundation, the Professional Staff Congress-City University of New York Research Award Program, and the Wenner-Gren Foundation for Kanjera field and laboratory research is gratefully acknowledged. Logistical support was provided by the Human Origins Program of the Smithsonian Institution. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

New Mesozoic Mammal: Discovery Illuminates Mammalian Ear Evolution While Dinosaurs Ruled

An international team of paleontologists has discovered a new species of mammal that lived 123 million years ago in what is now the Liaoning Province in northeastern China. The newly discovered animal, Maotherium asiaticus, comes from famous fossil-rich beds of the Yixian Formation.

This new remarkably well preserved fossil, as reported in the October 9 issue of the journal Science, offers an important insight into how the mammalian middle ear evolved. The discoveries of such exquisite dinosaur-age mammals from China provide developmental biologists and paleontologists with evidence of how developmental mechanisms have impacted the morphological (body-structure) evolution of the earliest mammals and sheds light on how complex structures can arise in evolution because of changes in developmental pathways.

"What is most surprising, and thus scientifically interesting, is this animal's ear," says Dr. Zhe-Xi Luo, curator of vertebrate paleontology and associate director of science and research at Carnegie Museum of Natural History. "Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, and hearing is fundamental to the mammalian way of life. The mammalian ear evolution is important for understanding the origins of key mammalian adaptations."

Thanks to their intricate middle ear structure, mammals (including humans) have more sensitive hearing, discerning a wider range of sounds than other vertebrates. This sensitive hearing was a crucial adaptation, allowing mammals to be active in the darkness of the night and to survive in the dinosaur-dominated Mesozoic.

Mammalian hearing adaptation is made possible by a sophisticated middle ear of three tiny bones, known as the hammer (malleus), the anvil (incus), and the stirrup (stapes), plus a bony ring for the eardrum (tympanic membrane). These mammal middle ear bones evolved from the bones of the jaw hinge in their reptilian relatives. Paleontologists have long attempted to understand the evolutionary pathway via which these precursor jawbones became separated from the jaw and moved into the middle ear of modern mammals.

To evolutionary biologists, an understanding of how the sophisticated and highly sensitive mammalian ear evolved may illuminate how a new and complex structure transforms through evolution. According to the Chinese and American scientists who studied this new mammal, the middle ear bones of Maotherium are partly similar to those of modern mammals; but Maotherium's middle ear has an unusual connection to the lower jaw that is unlike that of adult modern mammals. This middle ear connection, also known as the ossified Meckel's cartilage, resembles the embryonic condition of living mammals and the primitive middle ear of pre-mammalian ancestors.

Because Maotherium asiaticus is preserved three-dimensionally, paleontologists were able to reconstruct how the middle ear attached to the jaw. This can be a new evolutionary feature. Or, it can be interpreted as having a "secondarily reversal to the ancestral condition," meaning that the adaptation is the caused by changes in development. (See graphics of mammalian ear evolution, as represented by Maotherium).

Modern developmental biology has shown that developmental genes and their gene network can trigger the development of unusual middle ear structures, such as "re-appearance" of the Meckel's cartilage in modern mice. The middle ear morphology in fossil mammal Maotherium of the Cretaceous (145-65 million years ago) is very similar to the mutant morphology in the middle ear of the mice with mutant genes. The scientific team studying the fossil suggests that the unusual middle ear structure, such as the ossified Meckel's cartilage, is actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution.

Maotherium asiaticus is a symmetrodont, meaning that it has teeth with symmetrically arranged cusps specialized for feeding on insects and worms. It lived on the ground and had a body 15 cm (5 inches) long and weighing approximately 70 to 80 grams (.15 to .17 lbs). By studying all features in this exquisitely preserved fossil, researchers believe Maotherium to be more closely related to marsupials and placentals than to monotremes—primitive egg-laying mammals of Australia and New Guinea such as the platypus.

The article in Science is authored by Dr. Qiang Ji of Chinese Academy of Geological Sciences (Beijing), Dr. Zhe-Xi Luo (Carnegie Museum of Natural History) and Mr. Xinliang Zhang (Henan Provincial Geological Museum), along with other collaborators.

The researchers received support from National Science Foundation (USA), National Natural Science Foundation (China), Ministry of Science and Technology (China), and National Geographic Society.

Ancient Earth's Magnetic Field Was Structured Like Today's Two-pole Model

Princeton University scientists have shown that, in ancient times, the Earth's magnetic field was structured like the two-pole model of today, suggesting that the methods geoscientists use to reconstruct the geography of early land masses on the globe are accurate. The findings may lead to a better understanding of historical continental movement, which relates to changes in climate.

By taking a closer look at the 1.1 billion-year-old volcanic rocks on the north shore of Lake Superior, the researchers have found that Earth's ancient magnetic field was a geocentric axial dipole -- essentially a large bar magnet centered in the core and aligned with the Earth's spin axis.

Some earlier studies of these rocks had led other teams to conclude that the magnetic field of the ancient Earth had a far more complex structure -- some proposing the influence of four or even eight poles -- implying that present models of the supercontinents that relied on paleomagnetic data and an axial dipole assumption were wrong.

The report, which will appear in the October issue of Nature Geoscience, says that previous efforts to interpret the ancient geomagnetic field in rocks from North America were confused by the rapid migration of the continent toward the equator in the distant past.

The researchers "neatly lay to rest the long-standing controversy over the nature of Earth's magnetic field 1.1 billion years ago," writes geoscientist Joseph Meert of the University of Florida in an essay that accompanies the report.

"In this paper, we show that Earth's magnetic field has been more stable in the past than originally believed," said Adam Maloof, an assistant professor of geosciences at Princeton and one of the paper's authors.

The Earth's magnetic field wraps around the globe, shielding life from harmful cosmic rays. It is emanated by the Earth's iron core and is shaped by a multitude of factors, including the spinning of the Earth and circulatory motion influenced by the Earth's rotation and temperature differences between the inner core's outer layers and the lower mantle.

The researchers obtained magnetic measurements from a thick stack of lava flows in the Lake Superior region. The lavas erupted when geologic forces attempted to tear apart central North America forming the Keweenawan Rift. The researchers used the tiny magnetic minerals within the volcanic rocks to record the orientation of the geomagnetic field at the time the rocks erupted onto the Earth's surface. By knowing how those grains pointed to the magnetic field of that time, the scientists could deduce the latitude where they were located when the lava flows erupted and cooled. The grains pointed to where "paleo-north" was for each rock.

Studying layers of the basaltic lava flows, they used the information to track how the Earth's magnetic poles have "flipped" over the eons, with the North Magnetic Pole becoming the South Magnetic Pole and vice versa. The team studied three of these reversals that occurred over a few million years.

The scientists plan to use the data to better understand how continents moved in the distant past, massing to form supercontinents. "We needed to be able to have a working model of how the geomagnetic field behaved in the past if we are going to talk about where plates have moved, how fast they've moved and how ancient supercontinents were configured," said Nicholas Swanson-Hysell, a graduate student at Princeton and the first author on the paper.

Knowing the proper location of continents is key to understanding the climate of any era, Maloof said, because the shape and location of continents affect ocean currents, global average temperatures and wind patterns. And by understanding in detail what Earth's climate was like in ancient times, he noted, scientists can better comprehend the climate of today and make more accurate projections for the future.

According to scientific reconstructions, a supercontinent known as Rodinia existed between 1 billion and 800 million years ago. The extreme cooling of the global climate about 700 million years ago and the rapid evolution of primitive life during subsequent periods are often thought to have been triggered by the breaking up of Rodinia.

Rodinia predated a more recently created supercontinent called Pangaea, which came together about 300 million years ago. Scientists have pieced Rodinia together by comparing rocks with similar geological features that are now widely dispersed.

Knowing that they have confirmed the structure of the Earth's magnetic field at that time gives Maloof and Swanson-Hysell the confidence to learn more about the supercontinent and that epoch.

"For the past 30 years, scientists have feared that the geometry of Earth's field was complex and varied," Maloof said. "Such a complex field made it very hard for people to reconstruct the ancient geography of the planet because they could not rely on a predictable field. We show that these fears were unfounded -- at least for 1.1 billion years ago -- and that the evidence for a complex ancient field was an artifact of the way rocks had been sampled."

Other researchers on the paper included: Benjamin Weiss of the Massachusetts Institute of Technology; and David Evans of Yale University.

The research was funded by the National Science Foundation, Sigma Xi, Agouron Institute and Princeton University.