Mostrando entradas con la etiqueta Paleobiología. Mostrar todas las entradas
Mostrando entradas con la etiqueta Paleobiología. Mostrar todas las entradas

viernes, 19 de julio de 2013

Amber Time Machine

ORIGINAL: PBS
By George Poinar, Jr.
02.14.06

I lifted to the window a nugget of golden Dominican amber entombing a small stingless bee. The sunlight infused it and illuminated the bee caught forever in flight—gossamer wings outstretched and perfectly preserved down to the last hair. Stark eyes appeared to be gazing at me. I contemplated this lustrous burial chamber and thought how wondrous it would be if we could see what this insect had beheld in its lifetime. Would the vistas of just one day be sufficient to reveal the wonders of life millions of years ago? What was that last fateful day like? And what events had taken place in the eras before this specimen arrived in my hand?

This stingless bee had already collected resin from the resin tree and secured it on its hind legs before it somehow got entombed, beginning an epochal journey. Photo credit: Courtesy George Poinar, Jr.

A bee's life

One surmises that the bee was active in the dim light of early morning. She and her sisters gathered in the busy colony before beginning their various tasks. The young workers left for the nursery to attend the developing larvae. Older members flew out into the forest to collect pollen and nectar. The chore reserved for the aged bees was collecting the sticky resin utilized in nest construction from algarrobo trees. Our bee was among the resin gatherers. Off she skimmed with her companions, winding through the shadowy, towering amber forest, dodging the vines and lianas, avoiding tree trunks where hungry lizards lurked, and finally landing on the bark of an algarrobo near a large, yellow, viscous resin flow.

She scanned the surroundings, always on the lookout for hungry, sinister creatures that lurked in ambush—especially one well-adapted predator, the resin bug, a large, hairy-legged creature endowed with a huge beak that could easily penetrate the body and suck out the blood of an unwary bee. The resin bug's habit of coating the front legs and body parts with resin was repulsive, though effective in ensnaring prey. Only one swipe of the powerful front legs could pin the hapless victim long enough for the hypodermic-like beak to rip through the body wall.
Almost instantly, waves of thick fluid enveloped her.

The bee's compound eyes registered a kaleidoscopic image of the resin flow. Trapped within the vitelline pool lay other small arthropods, plant debris, and detritus. Not discerning any dreaded enemies, the bee began the painstaking job of removing small samples of resin with her mouth, coating them with saliva, and then attaching them to the hind legs in the form of little round balls. This exercise involved concentration and diligent work to prevent getting entrapped in the adhesive deposit. Finished at last, she was ready to return to the colony.

Like a creature out of a horror film, a long-legged resin bug looms over its intended victim, a stingless bee. In the end, both succumbed to the sticky resin. Photo credit: Courtesy George Poinar, Jr.

Into a golden tomb

The attack came with lightning speed. Only a hazy brown blur was detected as the resin bug thrust its front legs toward her. Acting on impulse, she retreated from the lunging bug, but in her frantic attempt to avoid the predator she flew directly into the sticky trap. Almost instantly, waves of thick fluid enveloped her.

A few feeble attempts were all that could be mustered to extricate herself from that tenacious snare. In spite of valiant efforts, death came in seconds as viscous liquid seeped over the breathing pores, wrapping a mantle of gold around its victim. The sun shimmered on the silent insect cradled in the glistening tomb on the algarrobo trunk. As a gentle breeze wafted through the leaves, a fine layer of dust drifted over the surface of the elixir. Suspended in a motionless world, the entombed bee endured, the balls destined for the hive still attached to her outstretched legs.

Almost immediately, chemical changes began to take place. Sugars in the sap pulled the moisture from the insect's tissues while other chemicals infiltrated its cells. Bacteria carried in the gut initiated spore development in response to the adverse conditions. The next day, the xanthous domain of the bee was covered by a series of subsequent flows. Changes were also occurring in the resin itself as exposure to sunlight and oxygen caused the bonds between molecules to strengthen.

A petal from the algorrobo, an extinct species of leguminous tree that produced the resin which, over millions of years, transformed into Dominican amber Photo credit: Courtesy George Poinar, Jr.
A long journey begins

Days grew into weeks and weeks into months. The tacky mass became harder but still clung to the bark of the tree. Finally, perhaps during a storm, the fossilized resin broke loose and came crashing down to the ground, lodging in a small crevice at the base of the tree. The material was now in the copal stage and no longer sticky. The chemical processes of polymerization and molecular cross-bonding would continue over the next few million years or so until the hardened resin, or copal, acquired the properties of amber.

Year after year, more debris, leaves, and twigs collected on top of the hardening copal. Wind, rain, and microorganisms degraded the accumulated plant litter but had no effect on our entombed bee. Eventually the mighty algarrobo crashed to the forest floor in a storm and added still another layer to the detritus covering the fossil.

For millennia all was tranquil. Ultimately a tempest fueled by a hurricane drove torrential rains across the bee's graveyard, washed away the accumulated organic matter, and exposed the fossil once again. Along with other fossilized pieces and plant debris, the specimen was washed by rivulets of floodwaters into raging streams, transported into a low-lying delta and shrouded with silt.

Another predator that lay in wait on algorrobo trunks for unsuspecting prey was the ant bug. Little did it expect that it would itself fall prey—to the engulfing resin. Photo credit: Courtesy George Poinar, Jr.

A bee in the sea

The sea level began to change and salt water slowly inundated the area, submerging our entombed bee. Ocean waves dislodged the silt and exposed the amber. Around the fossil now lived an array of sea creatures. Crabs scampered over the resin graveyard, fish swam up and nudged amber nuggets, barnacles selected larger pieces for resting sites, and in heavy storms the fossilized resin was tumbled repeatedly by the currents. Among the waves dwelled a myriad of microscopic, shelled animals called foraminifera and coccoliths. When they died, their minute shells settled to the bottom and, together with bits of sediment, slowly covered the precious fossil on the seafloor. No longer would a diffuse light illuminate our bee, not for millions of years.

Suddenly, the sarcophagus was jarred as a hammer cracked open the rock in which it was inhumed.

Meanwhile, the mass of earth containing the amber alluvium drifted farther into the Carribbean Sea, having already moved from its original position between North and South America. The advance of this island mass was gradual, and those that once lived in that extinct silva were indifferent to the earthquakes that periodically shook the terrain.

A miner gazes into the inky depths of a Dominican amber mine, where the lighted candles of two fellow miners are the only things visible. Photo credit: Courtesy George Poinar, Jr.

Brought to light

Eons passed and the sediment covering the amber now hardened into rock. Ultimately the strata containing the entombed creatures were subjected to still another natural force, that of mountain formation. Ponderously, those rocks that had been forged in the sea were elevated above the water. In some areas, the layers shifted and fractured, sometimes crazing the piece within. The bee was fortunate to be in a section of rock that was uplifted in its entirety. Towering skyward, the strata were folded into mountains. The upper boundaries, whose history could be told by the marine microorganisms in their matrices, were worn by the wind, washed by the rain, and eventually transformed into soil. Ultimately another forest became established, sending its roots down into the new loam that covered the bejeweled rocks, one quite dissimilar from the original forests.

One day, the silent graveyard, now lodged in a layer of marine rocks in the mountains of the northern Dominican Republic, was disturbed by some minor shock waves. These jolts continued for several days, eventually becoming more intense. The sediment containing the amber had been discovered and was being laboriously removed. Suddenly, the sarcophagus was jarred as a hammer cracked open the rock in which it was inhumed. Together with other pieces that had been dislodged from the rock, the bee and its tomb toppled to the floor of the mine. In the flickering candlelight, she was picked up and placed into a bag. Later, the day's take was sorted on a rickety wooden table where dark eyes scrutinized each piece intently, selecting those with fossils to be polished and sold and thereby continuing the process that brought this bee to my collection.

This feature originally appeared on the site for the NOVA program Jewel of the Earth.

George Poinar, Jr., is a research fellow at Oregon State University. Together with his wife Roberta, he has coauthored several books on amber, including The Amber Forest: A Reconstruction of a Vanished World (Princeton University Press, 1999), from which this article was excerpted with kind permission of the author and publisher.

lunes, 20 de agosto de 2012

Building blocks of life came from deep Earth

ORIGINAL: Phys.Org
by Tom Marshall


The rise of the first complex life depended on vital metals brought up to the Earth's surface from far below in vast granite deposits, a new study argues.
Metals like copper, zinc and molybdenum are essential ingredients for certain enzymes and proteins. These are needed for life forms with a complex internal structure, known as eukaryotes, to evolve. Without these metals the history of life could have been very different; plants and animals made of many cells could have taken hundreds of millions more years to develop, if they appeared at all.

The new study's authors realised that eukaryotes started appearing soon after a period of unusual geological activity, and wonder if it could have provided the raw materials they needed.

"Biologists have been saying for a long time that these three metals are essential for complex life to develop," says Professor John Parnell, a geologist at the University of Aberdeen and lead author of the paper, which appears in Geology. "And geologists have been aware that there was a period of unusual geological activity around the same time that would have brought an extraordinary amount of these metals to the surface. But I think we're the first to put the two together and suggest that the geological changes actually enabled the biological advances."

In particular, eukaryotic life is needed for sex differences to emerge. Until living things have both males and females, rapid evolution is impossible; sexual reproduction allows the mixing of genes from both parents, so that a population can contain much more variation for natural selection to work on. Before sexual reproduction, variations in populations of living things could stem only from occasional random mutations, so evolution moved much more slowly. 

The explosion of new life took place during a period known as the Mesoproterozoic, around 1.6 billion years ago. This followed the birth of a new supercontinent known as Nuna or Columbia around 1.9 billion years ago, which triggered major changes in the activity of the Earth's mantle beneath.

Because of the thicker crust below the supercontinent, heat flow at the base of the crust was unusually high, leading to rising magma plumes that brought up metals that had previously been locked deeper in the Earth. After emerging through volcanic activity, this material cooled into vast new fields of granite, with deposits of metal sulphides disseminated throughout. As weathering slowly uncovered these, they turned to sulphates and were washed into rivers, lakes and shallow coastal waters, where they became available for use by living things. With these nutrients in place, the stage was set for the appearance of eukaryotes.

"Metals do come to the surface through normal volcanic processes, but we think this episode of high heat flow greatly accelerated the process. So it's possible that eukaryotes would still have developed if this hadn't happened, but it might have taken a lot longer," says Parnell.

Until recently, scientists thought these metals came instead from changes in the chemistry of the oceans. The discovery is part of a wider move towards theories that complex life got its start on land or in shallow waters rather than the deep ocean, as previously believed.

"I suspect that this increases the focus on the terrestrial origins of eukaryotes, as opposed to the deep marine ones," Parnell comments. "The onus is now on the palaeobiologists to go out and see what traces of early life they can find in the terrestrial record."

Journal reference: Geology

Provided by PlanetEarth Online

This story is republished courtesy of Planet Earth online, a free, companion website to the award-winning magazine Planet Earth published and funded by the Natural Environment Research Council (NERC).

martes, 3 de julio de 2012

Rise of the feathered dinosaurs

ORIGINAL: Nature
02 July 2012

An exceptionally well-preserved fossil hints that feathers might have been a feature of all dinosaurs.

The fossil of an ancient theropod found in Germany indicates that feathers were widespread in dinosaurs. HELMUT TISCHLINGER
Feathers didn’t start with birds. Plumage of various sorts — from simple fuzz to the complex structures used for flight — adorned dinosaurs first, and was only later inherited by birds. And if a beautifully preserved juvenile dinosaur unearthed in the Jurassic strata of Germany is any indication, it is possible that all dinosaurs bore patches of filamentous feathers.

Since the discovery of the fluffy theropod Sinosauropteryx in 1996, palaeontologists have recognized more than 30 types of feathered dinosaur. Most of these are coelurosaurs — a disparate group of theropods that includes not only the fearsome tyrannosaurs, sickle-clawed deinonychosaurs and bizarre therizinosaurs, but also birds.

However, there are exceptions to this rule. Palaeontologists have also discovered simple, feather-like structures along the vertebral columns of the dinosaurs Psittacosaurus and Tianyulong. These ornithischians are about as distantly related to birds as dinosaurs can be, and the discoveries hinted that swaths of simplified protofeathers were a common feature among dinosaurs. The discovery of the most recent fossil indicates that feathery body coverings were widespread in these creatures.
Ancient adornment
The 150-million-year-old theropod Sciurumimus albersdoerferi unearthed in Germany and described today 1 was coated in a form of plumage. The filamentous structures have been seen before in other dinosaurs, but what makes Sciurumimus so noteworthy is that this dinosaur was a megalosauroid.

Megalosauroids were a group of archaic sharp-toothed dinosaurs near the base of the theropod family tree, and greatly removed from the various types of feathered dinosaur and early birds recognized so far. According to first author Oliver Rauhut, a palaeontologist at the Ludwig Maximilian University in Munich, Germany, this means that simple feathers were a very ancient dinosaur trait. The filaments of Sciurumimus, Rauhut says, are very similar to the simple structures seen in Psittacosaurus, Tianyulong and even pterosaurs — extinct flying reptiles that were the closest cousins that dinosaurs had. The wide evolutionary spread of this characteristic means that protofeathers are probably as old as the Dinosauria itself.

Palaeontologist Paul Barrett of London’s Natural History Museum agrees that the structures on Sciurumimusare probably protofeathers. Although additional geochemical work is needed to study the features' details, Barrett says, the fossilized wisps are very similar to the fuzz seen on other dinosaurs. But he notes that the presence of these filaments among all dinosaurs is “speculation”. Feathery structures might be a common feature of dinosaurs, but it’s also possible that they evolved multiple times. “We need more examples in both non-coelurosaurian theropods, and particularly in the other big dinosaur groups, before we can really speculate that these features are a character of dinosaurs as a whole,” Barrett says.

Rauhut is sure there are many more feathery dinosaurs yet to be found. The trick will be finding specimens in the sorts of fine-grained sediments capable of preserving feather fossil. But on the basis of findings from Sciurumimus and other feathery dinosaurs, Rauhut notes, “all dinosaurs had at least this kind of simple, hair-like feathery integument”. Instead of being a novelty that evolved among particularly bird-like dinosaurs, “feathers seem to be an ancestral dinosaurian trait”, Rauhut says. If so, we will have to start thinking about what kind of feathery covering these creatures display when we depict them in art and film. Nature doi:10.1038/nature.2012.10933

Rauhut, O. W. M., Foth, C., Tischlinger, H. & Norell, M. A. Proc. Natl Acad. Sci. USA 


From nature.com

08 February 2012

13 October 2011

15 September 2011


27 July 2011

08 September 2010

18 March 2009





lunes, 11 de junio de 2012

Jewel Of The Earth (NOVA Documentary)

ORIGINAL: Nova

Broadcast (2007) Forty million years ago a diverse community of insects living at the bottom of a tree in a temperate forest chanced into a sticky pool of pine resin. Then a mere 67 years ago a young boy named David Attenborough was given the amber stone containing the entombed bugs. "Jewel of the Earth" explores the remarkable time capsule of ancient life preserved in this and countless other samples of fossilized tree resin, or amber.

Sir David Attenborough, now grown up and a celebrated naturalist and TV personality, hosts the program. As he makes abundantly clear in the show, he is still entranced with the amber specimen from his youth and the seemingly magical quality of the material to serve as a crystal-clear window to an age before humans walked the Earth. Coincidentally, David's brother Richard starred in the movie that made amber famous: Jurassic Park, in which Richard plays a billionaire entrepreneur who extracts DNA from amber-entombed mosquitoes in order to clone living replicas of their prey—dinosaurs. While such a scenario is probably unlikely, amber can resurrect prehistoric life in a quite different way, as NOVA demonstrates by probing the amber-encased clues that paint a fascinating picture of ancient biomes.


For example, most of the world's amber comes from the Baltic region of northern Europe, where, on the ample evidence of insects, plant fragments, and other trapped material, a vast temperate forest flourished about 40 million years ago. Sir Attenborough's boyhood keepsake is a piece of Baltic amber, which he investigates through a microscope with the help of biologist Elzbieta Sontag of the University of Gdansk, finding a long-legged fly, a fungus gnat, an aphid, an ant, and a mite—all denizens of the lower forest floor. By contrast, much of the amber found in the Dominican Republic—the second most significant source studied so far—is about 20 million years old and hails from an ancient rain forest. George and Roberta Poinar of Oregon State University have reconstructed this vanished ecosystem in spectacular detail, based on such clues as a tadpole that probably resided in a water-filled tropical bromeliad before being upended, along with a marsh beetle, into a patch of tree resin that eventually turned into amber.

An even more ancient Dominican sample, from 150 million years ago, contains a honeypot ant. Since this ant is now found only in Australia, the specimen is evidence for a conjectured super-continent that once comprised most of Earth's landmasses. High-tech medical scanners have shed light on many other amber inclusions, diagnosing a broken back on a gecko, for instance, which suggests the lizard was a bird's prey before being accidentally dropped into resin. The most controversial research on amber, however, is the effort to extract DNA from trapped creatures, just like in Jurassic Park. So far, two teams, including the Poinars, have announced success. However, follow-up studies indicate the DNA found by both groups was a contaminant, not the real, ancient stuff. Setbacks aside, scientists have only just begun to reveal the secrets to be discovered in the warm, glowing, glassy world of amber.

The Oldest Tree in the World


Photo: James Neeley

As the harsh wind whistles around the ancient, branched giants, visitors can’t help experience a sense of awe, faced with the trees' magnificence. This humbling feeling is well justified: these trees are older than any other single living organism known to man. And, as the visitors come and go, the wise old trees simply stand, impassive, as if gently guarding the world.

In this beautiful night shot, a bristlecone tree trunk sits against a backdrop of star trails in the Ancient Bristlecone Pine Forest in eastern California's White Mountains. This particular trunk can be found in the park’s Methuselah Grove, which was named after the oldest of its inhabitants, “Methuselah” (in turn named after the oldest person in the Bible) – an unmarked bristlecone pine that is, by some accounts, 4,844 years old! Yup, that’s right, we didn’t add an extra digit at the end by mistake; the tree really is just 156 years short of living for five millennia, making it 1,000 years older than any other tree on Earth!
With the concentric star trails crowning this bristlecone pine, the image looks almost out of this world.
Unless otherwise noted, most of the incredible photos seen here were taken in the Ancient Bristlecone Pine Forest. The Great Basin bristlecone pine (Pinus longaeva) grows at a high elevation of between 9,800 and 11,000 feet (3,000 and 3,400 m) and is protected within the White Mountains by the Inyo National Forest. To guard these ancient specimens further – from trophy seekers, vandals and the overly curious – the trees are unmarked, meaning that only experts and those who are really clued-in know where the oldest can be found.
Written by: Simone Preuss

Written by: Simone Preuss

See more pictures

martes, 8 de mayo de 2012

Killer Canadian coelacanth discovered

ORIGINAL: CosmosMagazine

The fast-swimming coelacanth Rebellatrix chasing smaller species of fishes in the Early Triassic ocean west of Pangaea. Credit: Artwork by Michael Skrepnick.
LONDON: Canadian scientists have discovered a fossilised ‘rebel’ coelacanth fish with an unusual body shape that suggests it was a fast-swimming predator.

The finding suggests that contrary to popular belief, coelacanths, the so-called ‘living fossils’, have undergone significant morphological change during their history.

“Rebellatrix divaricerca, most importantly, shatters the commonly held notion that coelacanths were an evolutionarily stagnant group,” said Andrew Wendruff from University of Alberta, in Edmonton, one of the researchers who discovered the fossil and lead author of the paper published in the Journal of Vertebrate Palaeontology.

Dramatically different to other coelacanth

The largest specimen of Rebellatrix, though partial, shows that it grew upwards of 1 meter in length. Credit: Photograph and drawing by A. Wendruff
“Rebellatrix is dramatically different from any coelacanth previously known, and thus had undergone significant evolutionary change in its ancestry,” he said.

Until the 1930s coelacanths were thought to have been extinct since the Late Cretaceous period, around 70 million years ago, when dinosaurs were still alive on Earth.

However, in 1938 a fisherman in Africa caught a living coelacanth. The most striking thing about the newly discovered fish was that their body plan differed very little from ancient fossil specimens resulting in the fish being referred to as ‘living fossils’.

Rebel without a cause

The modern coelacanth, Latimeria chalumnae, and most of the fossil specimens discovered to date have broad, flexible tails suitable for moving at slow speeds and lying in wait for prey.

In contrast, “the forked tail of Rebellatrix indicated that it was a fast-moving, aggressive predator,” said Wendruff.

“Rebellatrix was able to search actively for the fishes that it preyed upon and catch them at high speed,” added Mark Wilson, co-author of the study and fellow University of Alberta researcher.

Wendruff and Wilson found the R. divaricerca fossil in the Lower Triassic Sulphur Mountain Formation near Tumbler Ridge in the Rocky Mountains in British Columbia, Canada.

It is thought to be from the Early Triassic period, approximately 250 million years ago. The closest coelacanth fossil in age is known as Alenypterus. It dates from the Mississippian period and is at least 70 million years older. It also has a very different body plan that is more similar to that of living coelacanths than that of Rebellatrix.

Prior to findings such as Rebellatrix, scientists believed that coelacanths have changed little over time, but this now seems unlikely.

“The similar body forms are not a result of close relationships,” said Wendruff. “Nature is constantly recycling ideas, body forms and structures. This is referred to as convergent evolution. Many times there is one particular form that is most efficient for a particular function.”

When asked about why Rebellatrix evolved such a different body shape to other members of the coelacanth family, Wendruff suggested that as the mass extinction event that happened just before the beginning of the Triassic period wiped out 90% of marine life Rebellatrix could have evolved to fill a newly vacant predator ‘niche’ in the marine ecosystem.

Forked tail before it's time

“Large predators seem to be particularly vulnerable during such intervals, so this extinction might have 'opened the door' for coelacanths to explore new functional roles,” said palaeobiologist and coelacanth expert Matt Friedman from the University of Oxford in the UK.

The newly evolved body form was not successful, however, because “while other fork-tailed fishes appear later in the fossil record, Rebellatrix and its descendants are noticeably absent. This leads us to believe that Rebellatrix was a dead end in the evolution of cruising predation,” said Wendruff.

Friedman said: “this discovery adds to a growing view that coelacanth evolution was more nuanced than textbook portrayals would suggest.”

He added: “It is the latest in a series of finds over the past decade or so that have shown coelacanths didn't just have a static evolutionary history when it came to body form. Rebellatrix, with its unusual tail fin and body proportions, can be added to other anatomical 'experiments' in coelacanth history, including eel- and leaf-shaped forms.”

Wendruff believes that other unique forms of coelacanth have yet to be found and that “in the future they will have many more surprises in store.”

miércoles, 2 de mayo de 2012

Seawater key to early evolution

ORIGINAL: Cosmos Magazine
27 April 2012
by Anthony King

Evolutionary biologists have often pondered why life suddenly exploded into different forms 543 million years ago. Now, it seems seawater held the missing ingredient.

WHILE RIDING OVER THE Canadian Rockies in 1909, veteran geologist Charles Walcott discovered fossil gold among half-a-billion-year-old shale. These rocks contained evidence for all sorts of remarkable animals, and charted in great detail one of the enduring mysteries of evolution – the Cambrian explosion.

Trilobites were successful creatures during the Cambrian Explosion.
The Cambrian period began around 543 million years ago. Rocks from the time before that offer poor territory for any fossil hunter. There are few fossils and what is there is mostly microscopic or can be placed in the “could be a fossil” drawer. The Burgess Shale – around 505 million years old – belongs to a different era. Once you enter the Cambrian, you witness a sudden explosion of animals in the rocks. All sorts of exotic marine creatures appear, but also recognisable relatives of worms, arthropods and even vertebrates.

This sudden appearance of allied species troubled Charles Darwin, who could give no satisfactory answer to this “grave difficulty,” as he described it, in his book On the Origin of Species. Why would life suddenly proliferate? Where were their ancestors?

SCIENTISTS HAVE SINCE IDENTIFIED good candidates of early animal life before the Cambrian – from the Ediacaran Period. It gets its name for the Ediacara Hills in the Flinders Range of South Australia, where famous fossils from this time where found.

From that time, what we find consists mainly of evidence for soft-bodied organisms lying peacefully on algal mats; some it seems were grazers, while others were filter feeders, but many possibly just lived in symbiotic partnerships with the mats they were sitting on. Generally everything was quiet and peaceful, says palaeobiologist Peter van Roy of Ghent University in Belgium. So what happened to this tranquil, if dull world? What set off a period of innovation in shells, plates, spines and other skeletal elements?

Van Roy believes the entrance of a new profession into the food chains of these tranquil oceans – predation – had major repercussions. “If you introduce predators in such a system, these soft, docile creatures of course make an easy lunch. So animals need to protect themselves from predation.” Creatures are pushed into developing defensive behaviors and other ways to protect themselves – this is where hard mineralised shells, exoskeletons and armour come into play.

The predators can counter this move by developing sturdy, possibly mineralised implements. “Prey, in turn, has to respond to these new challenges, so you end up with a classical arms race between prey and predators,” says Van Roy.

It had been hotly contested whether a slow build of genetic traits set the ball rolling or whether a trigger from outside lit the fuse for the explosion of diversity. The idea that factors within ecosystems were responsible gained the upper hand over the last decade or so. Now, though, US scientists have come along and upended the apple cart.

US SCIENTISTS REPORT evidence of what ignited the Cambrian explosion of life. The chemistry of the seawater changed dramatically, they say, supplying a glut of raw material for shells, armour, exoskeletons, levers and mineralised parts. In support of their hypothesis, geologists Shanan Peters of the University of Wisconsin and Robert Gaines of Pomona College, California, reported in the journal Nature that what happened is evident in the “Great Unconformity,” a puzzling and very substantial gap in the sedimentary – and hence fossil – record in many locations. In effect, the approximately 525 million year old Cambrian rocks rest on much older rocks.

“It is better called the great non-conformity,” says Peters, “because it juxtaposes two different types of rocks.” The sequence was named by an explorer navigating the Grand Canyon for the first time. Here, near the base of the canyon, he noted a dramatic switch in rock type. Rocks below a line are crystalline and hard, formed within the Earth – igneous and metamorphic – while almost a mile of rocks above the line are layered sedimentary rocks deposited from the Cambrian on.

THE GREAT UNCONFORMITY records a transition from a world where the continental surfaces were being eroded and weathered over vast areas of the planet (below the line), to a world where the seas flooded back onto the continents and marine sediment once again began to accumulate. The pre-existing rocks broke down at the exposed surface over millennia, explains geologist Patrick Orr of University College Dublin, Ireland. It was primarily the residue that this produced that was flushed into the oceans as sea levels rose and flooded back onto the continents, he explains.

Peters and Gaines argue that the exposure and chemical weathering of these rocks released the materials which later fuelled the evolution of hard parts such as shells and armour.

Freshly exposed rock weathers chemically at rates more than three times faster than undisturbed soils, freeing up chloride, magnesium, iron, potassium, sodium, carbonate and calcium ions. Peters, and other geochemists, believe calcium levels may have been so high in the ocean that it posed a challenge for animal life, interfering with cellular functions. Converting ions of calcium into a mineral such as calcium carbonate, however, would put them out of harm’s way. Once you start precipitating a hard mineral in this scenario, evolution has something to work with.
“We argue that biomineralisation didn’t evolve for claws and things like that; it evolved in response to a change in ocean chemistry,” says Peters.

“Once there was an initial metabolic reason to make a biomineral, of course natural selection then could use it as a tool and that gets pushed rapidly by ecology. The functional capability afforded by biomineralisation gets picked up very quickly by natural selection as an advantage and that gives us the Cambrian explosion and the diversity and morphology that we see.”
It is possible, argues Peters, that before the weathering of the continents the supply of chemicals was too low and so it was expensive to make biominerals. Once you have biominerals, it becomes possible and advantageous to evolve shells, eyes and other parts made of calcite and ultimately bones.

TRILOBITES FOR INSTANCE were among the most successful of early animals. These arthropods – in the same group as insects and crustaceans – had hard exoskeletons made of calcite (calcium carbonate) minerals along with the protein chitin. An array of trilobites roamed the seas and crawled along the seafloor, watching their Cambrian world through remarkably capable calcite eyes.

What they were looking out for were perhaps hunters such as Anomalocaris, the bizarre apex predator of the Cambrian seas. This is believed to be the proud owner of the amazingly complex 515 million year old eyes reported last year from the Emu Bay Shale of South Australia. Each eye consisted of at least 16,000 individual lenses, rivalling the best eyes in modern arthropods in terms of sight. Improving eyes among prey and predator is a sign of one-upmanship in an evolutionary arms race.

Van Roy agrees that biomineralisation could have been one of the changes that set off the Cambrian explosion, arming predators and prey. However, Nick Butterflied of the University of Cambrige doubts that changes in ocean alkalinity could have acted as a trigger for the Cambrian explosion.

BIOMINERALISING ORGANISMS, HE ARGUES, represent a trivial percentage of marine diversity and especially in the Cambrian. “The Cambrian explosion would have happened even in the absence of biomineralisation” he believes. His views offer a foretaste of the battle to come.

“The explosive radiation of biomineralization organisms in the early Cambrian wasn’t just about calcification,” he says. “Skeletalisation had very little to do with ocean alkalinity and a lot to do with ecology.” Butterfield notes that carbonate biomineralisation was an innovation in the Ediacaran, well before the “great unconformity.” The trigger he believes is the evolutionary appearance of animals, which sets of a cascade of unprecedented shifts in ecosystem function and expression – including biomineralisation.

The pendulum had swung toward intrinsic ecological factors as driving Cambrian evolution, such as predator-prey relationships, but this paper marks a clear wakeup call that environmental factors must be considered too as drivers of evolution, says Orr. Moreover, the time before the Cambrian teemed with creatures too, but without durable minerals these animals were ill-quipped to survive in forms that could be collected by palaeontologists, he explains.

To muddy the waters further, many experts doubt that a single event caused the Cambrian explosion. Suggestions as to what caused or at least contributed to the explosion include

  • an increase in the oxygen level in the atmosphere to a level that would sustain large and complex organisms, 
  • changes in oceanic microplankton, 
  • the development of visual organisms and 
  • the rapid continental movements leading to large methane releases.
In their recent Nature paper, the US researchers point out that an expansion of shallow sea areas coincided with the Cambrian explosion, offering bountiful conditions for life to thrive.

“It is probably a combination of some of the various suggested mechanisms,” says Jim Jago, Cambrian expert at the University of South Australia. “It should also be remembered that the Cambrian ‘explosion’ took place over a period of at least 30 million years in the bottom part of the Cambrian.”
It is likely that various biological and non-biological factors influenced each other, some possibly triggering others in some cascade, and possibly reinforcing each other, agrees Van Roy. “The Cambrian explosion is a complex event that cannot be explained by a single trigger. You have several factors – biotic and abiotic – influencing and potentially mutually reinforcing each other.” Such a complex set of pieces to an evolutionary puzzle, judged from a distance of 500 plus million years, offers fertile ground for lively debate for years to come.

lunes, 5 de marzo de 2012

Los orígenes del hombre (y los cordados) se remontan a un fósil de gusano en Canadá

ORIGINAL: Physorg (AP)
05 de marzo 2012

Los paleontólogos han rastreado los orígenes de los seres humanos y otros vertebrados a un gusano que nadaban en los océanos la mitad de mil millones de años, según un estudio publicado el lunes.

Pikaia gracilens, un cordado primitivo
 de la Burgess Shale
(ROM 61232). Royal Ontario Museum
Un nuevo análisis de los fósiles desenterrados en las Montañas Rocosas de Canadá determinó que el extinto Pikaia gracilens es el miembro conocido más primitivo de la familia cordados, que hoy incluye a los peces, anfibios, aves, reptiles y mamíferos.

La investigación publicada en la revista científica británica Biological Reviews identifica una notocorda o varilla que se convertiría en parte de la columna vertebral en los vertebrados, y el tejido muscular llamada miómeros en 114 especímenes fósiles de la criatura.

También encontraron un sistema vascular.

"El descubrimiento de miómeros es la prueba irrefutable que durante mucho tiempo hemos estado buscando", dijo el autor principal del estudio, Simon Conway Morris, de la Universidad de Cambridge.

"Ahora, con miómeros, un sistema nervioso, una notocorda y un sistema vascular, todos ya identificados, este estudio pone claramente a los Pikaia como los cordados más primitivos del planeta."

"Así que, la próxima vez que pongamos una fotografía familiar de una pieza, allá en el fondo estará Pikaia".

Las Montañas Rocosas canadienses se reflejan en el lago Louise en Alberta, Canadá. Los paleontólogos han rastreado los orígenes de los seres humanos y otros vertebrados a un gusano que nadaban en los océanos la mitad de mil millones de años, según un estudio. Un nuevo análisis de los fósiles desenterrados en las Montañas Rocosas de Canadá determinó que el extinto gracilens Pikaia es el miembro conocido más primitivo de la familia cordados.

Los primeros ejemplares de Pikaia fueron recogidos por los primeros exploradores de Burgess Shale en 1911. Sin embargo, los animales fueron pasados ​​por alto como un antepasado de las lombrices de tierra o anguilas.

No fue sino hasta la década de 1970 que Morris sugirió que los animales de cinco centímetros (dos pulgadas) de largo, de lados-aplanado, algo parecido a la anguila que probablemente nadaba moviendo su cuerpo en una serie de curvas de lado a lado, podrían ser el miembro más antiguo conocido de la familia cordado.

"En particular, fue el uso de un microscopio electrónico que nos permitió ver detalles muy finos de su anatomía", dijo a AFP Jean-Bernard Caron, profesor asistente de ecología y biología evolutiva en la Universidad de Toronto y coautor del estudio.

"Es una lección de humildad para saber que los cisnes, serpientes, osos, cebras e, increíblemente, los seres humanos comparten una historia profunda con esta pequeña criatura más corta que mi pulgar", dijo.

Más información: Pikaia gracilens Walcott, un cordado madre-grupo del Cámbrico Medio de la Columbia Británica, el artículo más reciente: 04 de marzo 2012. DOI: 10.1111/j.1469-185X.2012.00220.x

Abstract