Paleontology news

The unusually intact fossilized skull of a giant, bony-toothed seabird that lived up to 10 million years ago was found on Peru's arid southern coast, researchers said Friday.

The fossil is the best-preserved cranium ever found of a pelagornithid, a family of large seabirds believed to have gone extinct some 3 million years ago, said Rodolfo Salas, head of vertebrate paleontology at Peru's National History Museum.

The museum said in a statement that the birds had wingspans of up to 20 feet (6 meters) and may have used the toothlike projections on their beaks to prey on slippery fish and squid. But studying members of the Pelagornithidae family has been difficult because their extremely thin bones — while helpful for keeping the avian giants aloft — tended not to survive as fossils.

"Its fossils are very strange, very rare and very hard to find," Salas told The Associated Press.

The cranium discovered in Peru is 16 inches (40 centimeters) long and is believed to be 8 million to 10 million years old, based on the age of the rock bed in which it was found.

"Rarely are any bones of these gigantic, marine birds found fossilized uncrushed, and to find an uncrushed skull of this size is very significant," said Ken Campbell, curator of vertebrate zoology at the Natural History Museum of Los Angeles.

Campbell, who examined photos of the find but was not involved in the dig, said he knows of "no specimen of comparable quality."

Dan Kepska, a paleontology researcher at North Carolina State University who also was not part of the project, agreed that the skull is the most complete ever reported.

He called the birds "one of the great enigmas of avian paleontology."

With fossils discovered in North America, North Africa and even Antarctica, Kepska said, the birds were ubiquitous only a few million years before humans evolved and scientists puzzle over why they died out. Some believe they are related to gannets and pelicans, while other say they are related to ducks.

Campbell said the Peru find "will undoubtedly be of great importance to our understanding of these gigantic birds, and it will help clarify the relationships of the other fossil pelagornithids found in the Pisco Formation."

The formation, a coastal rock bed south of the capital, Lima, is known for yielding fossils of whales, dolphins, turtles and other marine life dating as far back as 14 million years.

credited to news.yahoo.com

Paleontologists working in Peru have found a fossil from a bird that lived 10 million years ago, scientists said on Friday after returning from the dig site on the country's desert coast.

The species of bird had a wingspan of 19.7 feet and fed mostly on fish from the Pacific Ocean. It first appeared 50 million years ago and was extinct about 2.5 million years ago because of climate change, paleontologist Mario Urbina of Peru's Natural History Museum said.

Scientists discovered a rare fossil of the bird's head in Ocucaje, in the Ica region of Peru's southern coast, where an arid climate has preserved many fossils.

"The cranium of the bird, from the Pelagornithidae family, is the most complete find of its kind in the world. Its fossil remains are hard to find," Urbina said.

Old ocean seabeds in the area have been a treasure trove for fossil hunters.

"This site had marine sediments. The fossil was found with other remains from whales, sharks and turtles," Urbina said.

At the time of the bird's death, Peru's coast was hot and rainy, but millions of years later, it turned cool and dry, he said.

The fossil is 15.7 inches long and will go on display on Saturday at the museum.

The bird had some peculiar characteristics, including teeth at the tip of its beak and large wings that were less efficient than those of contemporary birds.

"The teeth helped capture its prey. This was an animal that perhaps trapped its prey and chewed while it flew. It had a hard time taking off from the ground, and needed an elevated point to take off from," Urbina said.

credited to msnbc.msn.com

The findings at a Northern Kenya site represent the oldest evidence of modern-human foot anatomy. They also help tell an ancestral story of humans who had fully transitioned from tree-dwellers to land walkers.

"In a sense, it's like putting flesh on the bones," said John Harris, an anthropologist with the Koobi Fora Field School of Rutgers University. "The prints are so well preserved ."

Harris and other colleagues report in the Feb. 27 issue of the journal Science on finding several footprint trails within two sedimentary rock layers. An upper sedimentary layer included two trails of two prints each, one group of seven prints, and a variety of isolated prints. The lower layer had a trail of two prints and a single isolated print likely from a smaller, juvenile human.

The researchers identified the footprints as probably belonging to a member of Homo ergaster, an early form of Homo erectus. Such prints include modern foot features such as a rounded heel, a human-like arch and a big toe that sits parallel to other toes.

By contrast, apes have more curved fingers and toes made for grasping tree branches. The earliest human ancestors, such as Australopithecus afarensis, still possessed many ape-like features more than 2 million years ago — the well-known "Lucy" specimen represents one such example.

These latest footprints at Ileret, Kenya, appear intriguingly close by to another early human site. Researchers in 1984 found a nearly complete Homo erectus skeleton, dubbed the "Turkana Boy" specimen, just across the waters of Lake Turkana.

Turkana Boy's delicate foot bones were not well preserved. But the new footprints could represent the shoe that fits the wearer, so to speak.

"Many people have predicted this, but now we have the solid evidence for modern foot anatomy," Harris told LiveScience. "It's like a giant jigsaw puzzle, and those footprints complement the skeleton from the other end of the lake."

Modern feet mark just one of several dramatic shifts in early humans, specifically regarding the appearance of Homo erectus around 2 million years ago. Homo erectus is the first hominid to have the same body proportions as modern Homo sapiens.

"We're seeing a very different hominid at this stage," Harris said, pointing to both an increase in size and change in stride during the relatively short time between Australopithecus (the first in this genus lived about 4 million years ago and the last died out between 3 million and 2 million years ago) and Homo erectus. The latter hominids would have been able to travel more quickly and efficiently over larger areas.

This matches a pattern of more widely-distributed sites containing artifacts such as tools from 1.5 million to 1 million years ago, which may also point to wider-ranging early humans.

Climate changing and shifting physical landscapes would have also forced the likes of Homo erectus to wander farther in search of food, Harris said. But increased walking and running abilities may have allowed them to start seriously hunting big game

"You might even think in terms of dietary quality here, because maybe they're incorporating more meat into their diet," Harris said. "They would have competed with quite a large carnivore guild; lions, leopards, and all the cats that eat meat."

The tracks lead on

The Homo erectus footprints now lead further into the past of human evolution, as researchers may shift their focus to earlier examples of physical changes in human ancestor species.

"It's going to bring up controversy again about the Laetoli prints," Harris noted, referring to footprints preserved in volcanic ash roughly 3.6 million years ago in Tanzania. Anthropologists continue to debate whether these older footprints from an earlier "Lucy" type hominid show that Australopithecus walked about easily or awkwardly on two legs.

Other findings may yet be revealed with the latest footprints at the Ileret site. The prehistoric landscape near various water sources was likely a muddy surface that preserved a whole range of animal tracks, Harris hinted — perhaps fodder for additional studies in the future.

credited to msnbc.msn.com

Roman artefacts which are nearly two thousand years old with similarities to ancient remains found at Pompeii in Italy have been examined at the Science and Technology Facilities Council's ISIS neutron source (21-22 February). Researchers are hoping to learn more about our heritage by discovering whether the items were imported from southern Italy, or manufactured using similar techniques in Britain. The bronze artefacts, which include a wine-mixing vessel, jugs and ceremonial pan-shaped objects, were discovered in Kent in two high status Roman pit-burials that are among the best examples ever seen in Britain. Previous excavation in an area close to the A2 where the items were found - by construction group Skanska Civil Engineering during a Highways Agency road improvement scheme - had predicted archaeological discoveries, but they were bigger than expected, with settlements ranging from the Bronze Age to the late medieval period.

Archaeological scientists have been comparing the 1st Century AD artefacts from Kent with those from Pompeii in Italy. The neutron beams at the world-leading ISIS facility allow for detailed crystal structure analysis of intact delicate objects without cutting out a sample of the material.

Dana Goodburn-Brown, a conservator and ancient metals specialist commissioned by Oxford Archaeology, has been analysing the artefacts along with archaeological scientist Dr. Evelyne Godfrey at ISIS to see how they were made. It is hoped results from the experiments will answer many questions about how the items were made to give more insight into their origin: for example, the metals used in manufacturing, how they were cast and finished, and how metal pieces were joined together.

''Our experiments will hopefully aid us in characterising different Roman metalworking practices and perhaps recognising the distinction between imported south Italian goods and high standard copies produced by skilled local craftsman. These artefacts represent a time of great change in Britain - they appear shortly after the Romans arrived in this country, and may represent locals taking on cultural practices of these 'newcomers," Dana Goodburn-Brown said.

Dr Andrew Taylor, ISIS Director said: "For these rare and highly-valued objects, analysis with neutrons can give fantastic insight. Neutrons are a very powerful way to look at matter at the molecular level and they give unique results that you can't easily get with any other technique. The measurements are extremely delicate and non-destructive, so the objects are unharmed by the analysis and can be returned to the museums unscathed.

The neutron beams we have at ISIS are a very versatile research tool and we're always keen to help researchers answer a broad range of questions. Here we realised that we could take the same analysis methods we developed to look at parts of aircraft and power plants and use them to help archaeologists understand how ancient objects were traded and manufactured."

credited to esciencenews.com

Karl Bates and his colleagues in the palaeontology and biomechanics research group have reconstructed the bodies of five dinosaurs, two T. rex (Stan at the Manchester Museum and the Museum of the Rockies cast MOR555), an Acrocanthosaurus atokensis, a Strutiomimum sedens and an Edmontosaurus annectens.

Acrocanthosaurus atokensis was a large predatory dinosaur that looked like T. rex but with large spines on its back and roamed the earth much earlier in the mid Cretaceous period, around 110M years ago. The team suggest Acrocanthosaurus probably weighed in at a similar mass to MOR555 and other medium sized adult T. rex at about 6 tonnes.

The Strutiomimum sedens, whose name means “ostrich mimic”, lived alongside T. rex in the late Cretaceous period and probably weighed somewhere between 0.4 – 0.6 tonnes

The reconstruction of Edmontosaurus annectens, a plant-eating hadrosaur was based on a juvenile specimen, but still weighed in at between 0.8 – 0.95 tonnes. As adults, some hadrosaurs grew as big as T. Rex, again living in the late Cretaceous period.

The team used laser scanning (LiDAR) and computer modelling methods to create a range of 3D models of the specimens, attempting to reconstruct their body sizes and shape as in life. The laser scanner images the full mounted skeleton, resulting in a detailed 3D model in which each bone retains its spatial position and articulation. This provides a high resolution skeletal framework around which the body cavity and internal organs such as stomach, lungs and air sacs can be reconstructed. This has allowed calculation of body segment masses, centres of mass and moments of inertia for each animal – all the information that is needed to analyse body movements.

Having created their ‘best-guess’ reconstruction of each animal, they then varied the volumes of body segments and respiratory organs to find the maximum plausible range of mass for the animals. Even scientists cannot be sure exactly how fat or thin animals like T. rex were in life, and the team were interested in exactly how broad the range of possible values were for body mass. They believe that the lower weight estimates are most likely to be correct as there is no good reason for the dinosaurs to weigh more than they need to as this would affect their speed, energy use and demands on the respiratory system.

The team also measured the body mass of an ostrich, as an existing subject that would show how accurate their technique was, and found the results to be correct.

They will now use the results to further investigate the locomotion of dinosaurs, specifically how they ran.

Karl said: “Our technique allows people to see and decide for themselves how fat or thin the dinosaurs might have been in life. You can see the skeleton with a belly. Anyone from a five-year-old to a Professor can see it and say, ‘I think this reconstruction is too fat or too thin’.

He added: “This study will help us in our research on how dinosaurs ran in 3-D rather than 2-D as in previous studies.

“Reconstructing more dinosaurs in such detail will allow us to examine changes in body mass and particularly centre of mass as they evolved. As we know, dinosaurs evolved into birds. As they did so, the centre of mass moved forward and different walking styles evolved. Although the dinosaurs we have reconstructed are not very close relatives of the birds, we can nevertheless see a small forwards movement in the position of the centre of mass from Acrocanthosaurus atokensis to the T. rex, which lies closer to modern birds on the evolutionary lines.”

credited to sciencedaily.com

A missing link in the evolution of the front claw of living scorpions and horseshoe crabs was identified with the discovery of a 390 million-year-old fossil by researchers at Yale and the University of Bonn, Germany. The specimen, named Schinderhannes bartelsi, was found fossilized in slate from a quarry near Bundenbach in Germany, a site that yields spectacularly durable pyrite-preserved fossils — findings collectively known as the Hunsrück Slate. The Hunsrück Slate has previously produced some of the most valuable clues to understanding the evolution of arthropods – including early shrimp-like forms, a scorpion and sea spiders as well as the ancient arthropods trilobites.

"With a head like the giant Cambrian aquatic predator Anomalocaris and a body like a modern arthropod, the specimen is the only known example of this unusual creature," said Derek Briggs, director of Yale's Peabody Museum of Natural History and an author of the paper appearing in the journal Science.

Scientists have puzzled over the origins of the paired grasping appendages found on the heads of scorpions and horseshoe crabs. The researchers suggest that Schinderhannes gives a hint. Their appendages may be an equivalent to those found in the ancient predatory ancestor, Anomalocaris — even though creatures with those head structures were thought to have become extinct by the middle of the Cambrian Period, 100 million years before Schinderhannes lived.

The fossil's head section has large bulbous eyes, a circular mouth opening and a pair of segmented, opposable appendages with spines projecting inward along their length. The trunk section is made up of 12 segments, each with small appendages, and a long tail spine. Between the head and trunk, there is a pair of large triangular wing-like limbs — that likely propelled the creature like a swimming penguin, according to Briggs. Unlike its ancestors from the Cambrian period, which reached three feet in length, Schinderhannes is only about 4 inches long.

This finding caps almost 20 years of study by Briggs on the Hunsrück Slate. "Sadly, the quarry from which this fabulous material comes has closed for economic reasons, so the only additional specimens that are going to appear now are items that are already in collectors' hands and that may not have been fully prepared or realized for what they are," said Briggs.

credited to esciencenews.com

Some 60 million years ago, well after the demise of the dinosaurs, a giant relative of today’s boa constrictors, weighing more than a ton and measuring 42 feet long, hunted crocodiles in rain-washed tropical forests in northern South America, according to a new fossil discovery.

But the existence of such a large snake may also help clarify how hot the tropics became during an era when the planet, as a whole, was far warmer than it is now, and also how well moist tropical ecosystems can tolerate a much warmer global climate.

That last question is important in assessments of how human-driven global warming might affect the tropics. Some scientists foresee the Amazon’s drying up, for instance, although other work cuts against that conclusion.

The discovery and its climatic implications are described in Thursday’s edition of the journal Nature.

An independent critique of the work by Matthew Huber, an earth and climate scientist at Purdue, also published in Nature, said the findings provided a hint that the tropics could get a lot warmer than they are now, but also “attest to the resiliency of tropical ecosystems in the face of extreme warming.”

With scant precise evidence of past temperature changes on land in the tropics, there is still substantial debate about whether these regions have gotten much warmer than typical steamy tropical conditions today — with an annual average temperature of 75 to 79 degrees Fahrenheit.

The team examined how warm it had to be for a snake species to be that large by considering conditions favoring the largest living similar tropical snake, the green anaconda, said Jason J. Head, the lead author of the paper and a paleontologist at the University of Toronto. They concluded that Titanoboa could have thrived only if temperatures ranged from 86 to 93 degrees.

credited to nytimes.com

June 30, 1908, 7:14 a.m., central Siberia—Semen Semenov, a local farmer, saw “the sky split in two. Fire appeared high and wide over the forest.... From ... where the fire was, came strong heat.... Then the sky shut closed, and a strong thump sounded, and I was thrown a few yards.... After that such noise came, as if . . . cannons were firing, the earth shook ...”

Such is the harrowing testimony of one of the closest eyewitnesses to what scientists call the Tunguska event, the largest impact of a cosmic body to occur on the earth during modern human history. Semenov experienced a raging conflagration some 65 kilometers (40 miles) from ground zero, but the effects of the blast rippled out far into northern Europe and Central Asia as well. Some people saw massive, silvery clouds and brilliant, colored sunsets on the horizon, whereas others witnessed luminescent skies at night—Londoners, for instance, could plainly read newsprint at midnight without artificial lights. Geophysical observatories placed the source of the anomalous seismic and pressure waves they had recorded in a remote section of Siberia. The epicenter lay close to the river Podkamennaya Tunguska, an uninhabited area of swampy taiga forest that stays frozen for eight or nine months of the year.

Ever since the Tunguska event, scientists and lay enthusiasts alike have wondered what caused it. Although most observers generally accept that some kind of cosmic body, either an asteroid or a comet, exploded in the sky above Siberia, no one has yet found fragments of the object or any impact craters in the affected region. The mystery remains unsolved, but our research team, only the latest of a steady stream of investigators who have scoured the area, may be closing in on a discovery that will change our understanding of what happened that fateful morning.

The study of the Tunguska event is important because past collisions with extraterrestrial bodies have had major effects on the evolution of the earth. Some 4.4 billion years ago, for example, a Mars-size planetoid seems to have struck our young planet, throwing out enough debris to create our moon. And a large impact may have caused the extinction of the dinosaurs 65 million years ago. Even today cosmic impacts are evident. In July 1994 several astronomical observatories recorded the spectacular crash of a comet on Jupiter. And only last September, Peruvian villagers watched in awe and fright as a heavenly object streaked across the sky and landed not too far away with a loud boom, leaving a gaping pit 4.5 meters deep and 13 meters wide.

Using satellite observations of meteoric “flares” in the atmosphere (“shooting stars”) and acoustical data that record cosmic impacts on the surface of the earth, Peter Brown and his co-workers at the University of Western Ontario and Los Alamos National Laboratory estimated the rate of smaller impacts. The researchers have also extrapolated their findings to larger but rarer incidents such as the Tunguska event. The average frequency of Tunguska-like asteroidal collisions ranges from one in 200 years to one in 1,000 years. Thus, it is not unlikely that a similar strike could occur during our lifetimes. Luckily, the Tunguska impact took place in an unpopulated corner of the globe. Should something like it explode above New York City, the entire metropolitan area would be razed. Understanding the Tunguska event could help us prepare for such an eventuality and maybe even take steps to avoid its occurrence altogether.

The first step in preparing ourselves would be to decide whether the cosmic object that affected Siberia was an asteroid or a comet. Although the consequences are roughly comparable in either case, an important difference is that objects in the solar system that circle far away from the sun on long-period orbits before returning, such as comets, would hit the earth at much greater velocities than close-orbiting (short-period) bodies, such as asteroids. A comet that is significantly smaller than an asteroid thus could release the same kinetic energy in such a collision. And observers have much more difficulty detecting long-period objects before they enter the inner solar system. In addition, the probability that such objects will cross the earth’s orbit is low relative to the probability that asteroids will. For these reasons, confirmed comet impacts on the earth are so far unknown. Therefore, if the Tunguska event was in fact caused by a comet, it would be a unique occurrence rather than an important case study of a known class of phenomena. On the other hand, if an asteroid did explode in the Siberian skies that June morning, why has no one yet found fragments?

Part of the enduring mystery of the Tunguska event harks back to the stark physical isolation of central Siberia and the political turmoil that raged in Russia during the early 20th century, a time when the czarist empire fell and the Soviet Union emerged. These two factors delayed scientific field studies for nearly 20 years. Only in 1927 did an expedition led by Leonid Kulik, a meteorite specialist from the Russian Academy of Sciences, reach the Tunguska site. When Kulik got to the site, he was confronted with some almost unbelievable scenery. Amazingly, the blast had flattened millions of trees in a broad, butterfly-shaped swath covering more than 2,000 square kilometers (775 square miles). Furthermore, the tree trunks had fallen in a radial pattern extending out for kilometers from a central area where “telegraph poles,” a lone stand of partially burned tree stumps, still remained. Kulik interpreted this ravaged landscape as the aftermath of an impact of an iron meteorite. He then began to search for the resulting crater or meteorite fragments.

Kulik led three additional expeditions to the Tunguska region in the late 1920s and 1930s, and several others followed, but no one found clear-cut impact craters or pieces of whatever had hit the area. The dearth of evidence on-site gave rise to various explanatory hypotheses. In 1946, for instance, science-fiction writer Alexander Kazantsev explained the puzzling scene by positing a scenario in which an alien spacecraft had exploded in the atmosphere. Within a few years, the airburst theory gained scientific support and thereafter limited further speculation. Disintegration of a cosmic object in the atmosphere, between five and 10 kilometers above the surface, would explain most of the features investigators observed on the ground. Seismic observatory records, together with the dimensions of the devastation, allowed researchers to estimate the energy and altitude of the blast.

The lack of an impact crater also suggested that the object could not have been a sturdy iron meteorite but a more fragile object, such as a relatively rare, stony asteroid or a small comet. Russian scientists favored the latter hypothesis because a comet is composed of dust particles and ice, which would fail to produce an impact crater. Another explanation for the tumult in the Tunguska region claimed that the de­struc­tion resulted from the rapid combustion of methane gas released from the swampy ground into the air.

In 1975 Ari Ben-Menahem, a seismologist at the Weizmann Institute of Science in Rehovot, Israel, analyzed the seismic waves triggered by the Tunguska event and estimated that the energy released by the explosion was between 10 and 15 megatons in magnitude, the equivalent of 1,000 Hiroshima atomic bombs.

Astrophysicists have since created numerical simulations of the Tunguska event to try to decide among the competing hypotheses. The airburst of a stony asteroid is the leading interpretation. Models by Christopher F. Chyba, then at the NASA Ames Research Center, and his colleagues proposed in 1993 that the asteroid was a few tens of meters in diameter and that it exploded several kilometers above the ground. Comparison of the effects of nuclear test airbursts with the flattened pattern of the Tunguska forest seems to confirm this suggestion.

More recent simulations by N. A. Artemieva and V. V. Shuvalov, both at the Institute for Dynamics of Geospheres in Moscow, have envisioned an asteroid of similar size vaporizing five to 10 kilometers above Tunguska. In their model, the resulting fine debris and a downward-propagating gaseous jet then dispersed over wide areas in the atmosphere. These simulations do not, however, exclude the possibility that meter-size fragments may have survived the explosion and could have struck the ground not far from the blast.

Late last year Mark Boslough and his team at Sandia National Laboratories concluded that the Tunguska event may have been precipitated by a much smaller object than earlier estimates had suggested. Their supercomputer simulation showed that the mass of the falling cosmic body turned into an expanding jet of high-temperature gas traveling at supersonic speeds. The model also indicated that the impactor was first compressed by the increasing resistance of the earth’s atmosphere. As the descending body penetrated deeper, air resistance probably caused it to explode in an airburst with a strong flow of heated gas that was carried downward by its tremendous momentum. Because the fireball would have transported additional energy toward the surface, what scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons, according to Boslough. All this simulation work only strengthened (and continues to strengthen) our desire to conduct fieldwork at the Tunguska site.

Our involvement with the Tunguska event began in 1991, when one of us (Longo) took part in the first Italian expedition to the site, during which he searched for microparticles from the explosion that might have become trapped in tree resin. Later, we stumbled on two obscure papers by Russian scientists, V. A. Koshelev and K. P. Florensky, that reported their discovery of a small body of water, Lake Cheko, roughly eight kilometers from the suspected epicenter of the phenomenon. In 1960 Koshelev speculated that Lake Cheko might be an impact crater, but Florensky rejected that idea. Florensky instead believed the lake was older than the Tunguska event, based on having found loose sediments as thick as seven meters below the bottom of the lake.

Word that a lake sat close to ground zero piqued our interest in mounting a field trip there because lake-bottom sediments can store a detailed record of events that occurred in the surrounding region, the basis of paleolimnological studies. Although our team knew little of Lake Cheko, we thought that we could perhaps apply paleolimnological techniques and find in the lake’s sediments clues to unravel the Tunguska mystery, as if the lake were the black box from a crashed airliner.

After circling the lake’s dark waters warily, the helicopter hovered precariously above the swampy lakeside (which was too soft for a landing) as we jumped down amid a torrential rainstorm. With eight blades rotating furiously above our heads, the resulting hurricane of air and water seemed set to sweep us away when at last we managed to unload our heavy cargo. With a roar, the craft lifted upward, and we were left drenched and exhausted near the edge of the lake, suddenly immersed in the deep silence of the Siberian wilderness. Any small relief we felt when the rain stopped was immediately forgotten as clouds of voracious mosquitoes descended on us like massed squadrons of tiny dive-bombers.

We spent the next two days organizing the camp, assembling our survey boat (a catamaran) and testing our equipment. Our studies would require a range of technologies, such as acoustic echo sounders, a magnetometer, subbottom acoustic profilers, a ground-penetrating radar, devices to recover sediment cores, an underwater television camera and a set of GPS receivers to enable study teams to track their position with a resolution of less than a meter.

For two weeks after that, our group surveyed the lake from the catamaran, tormented the entire time by hordes of mosquitoes and horseflies. These efforts focused on exploring the sedimentation and structure of the lake’s subbottom. Other team members, in the meantime, busied themselves with their own tasks. With his ground-penetrating radar, Michele Pipan, a geophysicist at the University of Trieste, gradually mapped the subsurface structures (some three to four meters deep) below the 500-meter shore perimeter. Eugene Kolesnikov, a geochemist at Moscow State, and his colleagues excavated trenches in peat deposits near the lake, a tough job given the resistance of the hard permafrost layer below the surface. Kolesnikov’s team searched the peat layers for chemical markers of the Tunguska event. At the same time, Romano Serra of Bologna University and Valery Nesvetailo of Tomsk State collected core samples from nearby tree trunks to study possible anomalies in the tree-ring patterns. Meanwhile, high above us, the aircraft that brought us to Krasnojarsk returned and circled the region to take aerial photographs so that we could compare them with those Kulik made some 60 years before.

We had assumed that the lake-bottom sediments might contain markers of the Tunguska event. After completing just a few runs across Lake Cheko with our high-resolution acoustic profiler, it became clear that the sediments blanketing the lake’s bottom were more than 10 meters thick. Some sediment particles had been transported to the lake by winds, but most of them came by way of the inflow of the little Kimchu River that fed Lake Cheko. We estimated that sediment deposition in a small body of water that stays frozen for most of the year would probably not exceed a few centimeters a year, so such a thick sediment layer might imply that the lake existed before 1908.

Soon our time in Tunguska was nearly over. The expedition members spent the last day frantically disassembling the boat, packing the equipment and dismantling the camp. When the helicopter arrived at noon the next day, we rushed to load all our stuff and ourselves into the hovering chopper amid the storm of human-made turbulence and finally began our return.

Back in our laboratories in Italy, the three of us completed processing our bathymetric data, which confirmed that the shape of Lake Cheko’s bottom differs significantly from those of other Siberian lakes, which typically feature flat bottoms. Most lakes in the region form when water fills the depressions left after the ubiquitous permafrost layer melts. The funnellike shape of Lake Cheko, in contrast, resembles those of known impact craters of similar size—for instance, the so-called Odessa crater, which was created 25,000 years ago by the impact of a small asteroid in what is now Odessa, Tex.

The idea that Lake Cheko might fill an impact crater became more attractive to us. But if the lake is indeed a crater excavated by a fragment of the Tunguska cosmic body, it cannot have been formed earlier than 1908. We sought evidence that the little lake existed before the event. Reliable, pre-1908 maps of this uninhabited region of Siberia are not easy to come by, but we found a czarist military map from 1883 that fails to show the lake. Testimony by local Evenk natives also asserts that a lake was produced by the 1908 explosion. But if the lake was not formed before 1908, how can one explain the thickness of the deposits carpeting its floor? Our seismic-reflection data revealed two distinct zones in the lake’s deposits: a thin, roughly meter-thick upper level of laminated, fine sediments typical of quiet deposition overlying a lower region of nonstratified, chaotic deposits.

Our survey team also observed the half-buried remains of tree trunks in the deeper part of the lake via underwater video. And high-frequency acoustic waves reflected back from the same zone showed a characteristic “hairy” pattern that could have resulted from the presence of the remains of trunks and branches. Perhaps these results are a trace of the forest obliterated by the impact.

To explain the lower chaotic deposits, we can imagine a cosmic body hitting soggy ground overlying a layer of permafrost several tens of meters thick. The impactor’s kinetic energy is transformed into heat, which melts the permafrost, releasing methane and water vapor and expanding the size of the resulting crater by as much as a quarter. At the same time, the impact would have plastered preexisting river and swamp deposits onto the flanks of the impact crater, where they would later be imaged as the chaotic deposits in our acoustic-echo profiles.

We are anxious to find out. Our team is now preparing to return later this year to attempt to drill the center of the lake to reach the dense seismic reflector. The year 2008 is the centennial of the Tunguska event. We hope it will also be the year the Tunguska mystery is solved.

Luca Gasperini, Enrico Bonatti and Giuseppe Longo have studied the Tunguska mystery for many years. Gasperini is a research scientist at the Institute of Marine Science in Bologna, Italy. Bonatti is professor of geodynamics at the University of Rome La Sapienza and special scientist at Columbia University's Lamont-Doherty Earth Observatory. Longo is professor of physics at the University of Bologna (www-th.bo.infn.it/tunguska).

credited to sciam.com