2009-03-02 – L’origine della “camminata moderna” (birth of human walking)

Paleontologia: scoperte le più antiche orme umane che rivelano camminata ‘modernà

MILANO – Scoperte in Kenya orme fossili umane risalenti a 1,5 milioni di anni fa: sono le seconde più antiche, ma le prime a rivelare una camminata ‘modernà. Le orme infatti appartengono a un Homo ergaster e rivelano un piede anatomicamente simile al nostro e, appunto, anche una camminata identica a quella dell’uomo moderno. La scoperta è di ricercatori della Rutgers State University of New Jersey e della Bournemouth University in Gran Bretagna, e si è conquistata la copertina di Science. (Agr)

fonte: http://www.instablog.org/ultime/41778.html – 27 Febbraio 2009, 12:22

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Researchers Uncover 1.5-Million-Year-Old Footprints

Discoverers glean clues about human predecessors from tracks left on an ancient river shore in Kenya

By Katherine Harmon

Freshly discovered trails of ancient footprints, left on what was once the muddy shores of a river near Ileret, Kenya, indicate that some 1.5 million years ago human ancestors walked in a manner similar to that of people today. The international team of researchers who analyzed the prints say that those who left them had feet that looked a lot like ours.

The prints were probably left by Homo ergaster, an earlier, larger version of the widespread Homo erectus, says David Braun, a lecturer in archeology at the University of Cape Town in South Africa and co-author of the study set to be published tomorrow in Science. This discovery “lets us know that they were probably just as efficient at walking upright as we are,” he says.

Previous research has shown that human ancestors were perfectly capable of getting around on their hind legs 3.5 million years ago—and perhaps even earlier. But Braun says these prints reveal, for the first time, a very modern foot with a parallel big toe—unlike an ape’s opposable digit that’s easily curled for grasping tree branches. Homo sapiens proper are said to have emerged about 200,000 years ago.

Footprints can tell scientists a lot about creatures that a skeleton cannot. From them, scientists can learn about the gait, weight distribution and even the approximate size of those who made them.  Braun says these prints were apparently made by pedestrians who stood just under five feet (1.5 meters) tall. A modern upright stride can indicate a lot about behavior, as well, says David Raichlen, an assistant professor of anthropology at the University of Arizona in Tucson, who cites long-distance walking and running as possible benefits of this adaptation.

“It really is a snapshot of time,” Braun says. The preserved area also shows a wealth of animal prints, which gives more precise information about what creatures shared the space and time. Exhumed fossils can yield info on general environments; footprints can provide a glimpse into life over days rather than millennia. “With the footprints,” Braun says, “we can almost certainly say these things lived in the same time as each other, which is unique.”

It is much rarer to find footprints than bones, because conditions must be perfect for tracks to be preserved, according to Braun. In this case, the tracks were made during a rainy season near an ancient river just before that river changed course and swept a protective layer of sand over them.

The last major set of footprints, discovered in 1978 in Laetoli, Tanzania, were dated to about 3.6 million years ago. But those revealed a more ancient foot and gait, and it is still debatable whether those who made them had a stride more akin to humans or to chimpanzees, says Raichlen, who has studied the Laetoli prints.

The Ileret tracks were digitally scanned using a laser technique developed by lead study author, Matthew Bennett, a geoarchaeologist at Bournemouth University in Poole, England. Raichlen says the find gives people a rare view of those that have trod before. “It’s important to think about what you’re really getting: a glimpse of behavior in the fossil record that you wouldn’t really get in any other way,” he says. The research reveals “a moment in time when individuals are walking around the landscape. It sort of fleshes out and brings them back to life, in a way.”

source: http://www.sciam.com/article.cfm?id=15-million-footprints-uncover

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Fossil Footprints Pick up Ancient Man’s Trail in Africa

ScienceMode – ‎1 ora fa‎

The anthropology world is all abuzz with a discovery in Africa that’s knocking scientists off their feet. It’s the finding of 1.5 million-year-old …

Fossil footprints reflect modern humans Newsday

Another ancient fossil found in Kenya Capital FM

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tutte le notizie (245) »

2008-12-23 – I dinosauri e le cure parentali (Dinosaurs and parental care)

 

DINOSAURI I PRIMI BABYSITTER

2008-12-19 19:06

 

Fare il babysitter è un lavoro antichissimo, preistorico addirittura, come mostrano le uova di dinosauro che 75 milioni di anni fa erano covate da maschi. Secondo uno studio americano pubblicato questa settimana su Science, i maschi dei terribili velociraptor, i dinosauri carnivori onnipresenti nel romanzo e nel film “Jurassic Park”, erano degli ottimi babysitter.

Come accade oggi in moltissime specie di uccelli, erano i maschi ad occuparsi del nido, a sorvegliare, difendere e accudire le uova. “Si ritiene comunemente che le femmine si concentrassero nel deporre le uova e nel procacciare il cibo e stanno diventando sempre più numerose le evidenze che additano i maschi come coloro che effettivamente si prendevano cura delle uova”, osserva il coordinatore della ricerca, Jason Moore, dell’università del Texas. In collaborazione con le università del Montana e della Florida e con il Museo Americano di Storia Naturale di New York, Moore ha scoperto e studiato sei nidi di dinosauro nello Stato americano del Montana e in Mongolia. I nidi risalgono a a circa 75 milioni di anni fa, sono in buono stato di conservazione e ognuno di essi contiene da 22 a 30 uova.

Sui nidi e attorno ad essi si trovano numerosa ossa i individui adulti. I ricercatori hanno scoperto che uova e ossa appartengono almeno a tre specie di dinosauri: troodonti, oviraptor e citipati, parenti dei velociraptor resi celebri da “Jurassic Park”. Nelle tre le specie, tutte fortemente imparentate con i moderni uccelli, le ossa trovate in prossimità dei nidi appartenevano a maschi. Sono infatti prive delle cavità che si formavano all’interno delle ossa delle femmine quando deponevano le uova. “Le ossa che abbiamo trovato più vicine alle uova – rileva Moore – non mostrano alcuna caratteristiche che indichi l’appartenenza a femmine di dinosauro”.

Di qui la conclusione dei ricercatori che in alcune specie di dinosauri erano i maschi a prendersi cura delle uova e a sorvegliarle, proprio come oggi accade negli uccelli, nei quali i maschi partecipano alle cure parentali per il 90% del tempo. Nei mammiferi, invece, i maschi si prendono cura della prole solo nel 5% delle specie. Per i ricercatori la scoperta dei dinosauri-babysitter è un punto di partenza per studiare comportamenti analoghi in altre specie animali.

 

fonte: http://www.ansa.it/opencms/export/site/notizie/rubriche/inbreve/visualizza_new.html_846323092.html

 

ANSA DINOSAURI I PRIMI BABYSITTER ANSA – 2 ore fa Secondo uno studio americano pubblicato questa settimana su Science, i maschi dei terribili velociraptor, i dinosauri carnivori onnipresenti nel romanzo e nel film “Jurassic Park”, erano degli ottimi babysitter. Come accade oggi in moltissime specie di … dinosauri i primi babysitter L’Unione Sarda e altri 6 articoli simili » —————————————————————————————————————————-

 

Florida State researcher Erickson ties rare bird behavior to dinosaur ancestors

Sure, they’re polygamous, but male emus and several other ground-dwelling birds also are devoted dads, serving as the sole incubators and caregivers to oversized broods from multiple mothers. It is rare behavior, but research described in the Dec. 19 Science found that it runs in this avian family, all the way back to its dinosaur ancestors. 

Gregory Erickson

 Scientists had long wondered about the origins of polygamy and paternal care patterns among modern-day Paleognathes — an ancient avian lineage that branched off soon after birds evolved from dinosaurs and includes ostriches, emus and tinamous. No such reproductive behavior exists among the vast majority of other vertebrates. Males contribute to parental care in less than 5 percent of mammal and non-avian reptile species, and while more than 90 percent of bird species co-parent to some degree, it is only among the Paleognathes that both polygamy and paternal care rule.

Now, in a groundbreaking paper (“Avian Paternal Care Had Dinosaur Origin”), paleobiologist Gregory M. Erickson of The Florida State University and researchers from three other institutions connect the evolutionary dots linking the polygamous, paternal reproductive patterns of extant (living) birds to the behavior of their extinct dinosaur kin.

“In those cases where adult dinosaurs have been found on top of nests, we found that the volume or mass of the egg clutch (total number of eggs in the nest) is very large relative to the size of the nesting animals,” Erickson said. “This suggests multiple females contributed the eggs and the male guarded them. Notably, the ratio of egg volumes to the nesting animal’s size is consistent with those in living birds where the male is the sole or primary nest attendant.”

The researchers now had their link from the theropod dinosaurs (omnivores and carnivores that walked on two hind legs with bird-like feet) to the polygamy and nesting scenarios exhibited by their avian descendants, according to David Varricchio of Montana State, the study’s principal investigator.

But to test the theory, Varricchio needed to determine the sex of the brooding dinosaurs whose bones have been found atop those communal nests.

For that, he turned to Erickson at Florida State, a renowned expert in dinosaur paleobiology.

Erickson examined the bone microstructure of tibiae (shin bones), femora (thigh bones) and metatarsus (ankle bones) from oviraptorids and deinonychosaurs (Jurrasic Park “raptors”) — small theropod dinosaurs whose adult skeletons have been repeatedly discovered in brooding postures atop nests containing dozens of large eggs.

The key was what he didn’t find in the bones: They showed no signs whatsoever of the maternal and reproductively associated microscopic features common to living non-Paleognath bird groups, extinct non-avian dinosaurs or living reptiles.

“I found no evidence of medullary bone (the extra bone laid down by breeding female birds and dinosaurs to make eggs) or extensive bone resorbtion (the means by which female reptiles such as crocodiles acquire mineral salts to make eggs),” Erickson said. “This is consistent with the brooding dinosaurs being males.”

Thus, the researchers had confirmation that the dinosaurs found in nests with large egg clutches were polygamistic males and the source of the peculiar avian behavior. Moreover, those brooding dinosaurs were fathers — and today’s emus, rheas and tinamous owe their paternal care model to them.

Co-authors of “Avian Paternal Care Had Dinosaur Origin” — accessible via the journal Science Web site at www.sciencemag.org/current.dtl — are Florida State’s Erickson, associate professor of anatomy and vertebrate paleobiology in the Department of Biological Science; Varricchio, Frankie D. Johnson and John J. Borkowski of Montana State University; Jason R. Moore of Texas A&M University; and Mark A. Norell of New York City’s American Museum of Natural History.

By Libby Fairhurst

source: http://www.fsu.edu/news/2008/12/18/rare.bird/

Canada.com Dinosaur day care dads Science News – 3 ore fa New analyses of fossilized dinosaur eggs and bones suggest that male dinosaurs likely sat on nests and cared for their young, similar to the parental division of labor seen in some modern birds. Daddy day-care: dinosaur fathers guarded the eggs Reuters Dinosaur Dads Played “Mr. Mom”? National Geographic eFluxMedia – Discover Magazine – FOXNews – eFluxMedia e altri 56 articoli simili » Scientist Live Polygamy, Paternal Care In Birds Linked To Dinosaur Ancestors Science Daily (press release) – 18 dic 2008 19 Science found that it runs in this avian family, all the way back to its dinosaur ancestors. Scientists had long wondered about the origins of polygamy and paternal care patterns among modern-day Paleognathes — an ancient avian lineage that … Odd bird fathering styles may come from dinos World Science Avian Paternal Care Had Dinosaur Origin Science Magazine (subscription) ScienceBlogs – United Press International e altri 19 articoli simili »

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Scientific article:

Avian Paternal Care Had Dinosaur Origin

David J. Varricchio, Jason R. Moore, Gregory M. Erickson, Mark A. Norell, Frankie D. Jackson, and John J. Borkowski
Science 19 December 2008: 1826-1828.
The large egg clutches of troodontid and oviraptor dinosaurs and evidence that fossils of brooding dinosaurs were males shows that paternal care was ancestral to birds.

Abstract »|   Full Text »|   PDF »|   Supporting Online Material »|  

2008-11-15 – Etiopia – Homo erectus: ossa di bacino femminile (female pelvis)

Scoperto in Etiopia un bacino fossile appartenente a una femmina di Homo Erectus.

Il ritrovamento ha consentito ai ricercatori che hanno pubblicato l’articolo su Science di fare interessanti considerazioni di carattere antropologico (ad esempio sulle dimensioni del bambino alla nascita, o sul grado di socializzazzione della specie).

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Wide-hipped fossil changes picture of Homo erectus

Thu 13 Nov 2008, 20:09 GMT

By Maggie Fox, Health and Science Editor

WASHINGTON, Nov 13 (Reuters) – The fossil of a wide-hipped Homo erectus found in Ethiopia suggests females of the pre-human species swayed their hips as they walked and gave birth to relatively developed babies with big heads, researchers said on Thursday.

The finding transforms thinking about some early human ancestors and evolution and suggests that helpless babies came along relatively late in the human lineage.

“We could look at this pelvis and then, using a series of measurements, we can calculate … how big the baby’s head could be at birth,” said Scott Simpson, a paleontologist at Case Western Reserve University who worked on the study.

Writing in the journal Science, Simpson and colleagues said the size and shape of the 1.2 million-year-old pelvis indicates that H. erectus females had hips wider than those of modern human females and their infants were born with heads about 30 percent larger than previously calculated.

“What this means is the offspring were not as helpless as a modern human,” he said in a telephone interview.

“It is not coming out walking and talking. But it was probably capable of more advanced behavior at a younger age like grasping, like sitting up … than we would see in a modern human.”

An extended childhood is a particularly human characteristic. Helpless babies require intensive care, not only from the mothers but from an extended group, which may have spurred the development of human society and culture.

Homo erectus, Latin for “upright man,” arose in Africa 1.8 to 2 million years ago, migrating to Asia and Europe before becoming extinct about half a million years ago. Experts agree it was likely a direct ancestor of modern humans.

Scientists did not know much about what its body would have looked like until the discovery of “Turkana Boy,” an adolescent H. erectus whose bones were discovered in 1984.

His slim-hipped build led researchers to believe that H. erectus gave birth to small-headed babies that would have required a great deal of care in early life, much like modern human infants.

But Simpson said Turkana boy’s pelvis was damaged and the restoration of a near-complete female pelvis from Gona, Ethiopia, changes this picture.

“This H. erectus would have even wider hips (than modern women),” Simpson said.

One main difference between human males and females is hip width, which makes women sway as they walk and which allows men to run and walk more efficiently.

“The reason women do have that sway is their hips are a little further apart,” Simpson said. “She would have had a good one.” (Editing by Alan Elsner)

source: http://africa.reuters.com/wire/news/usnN13527961.html

afrol News Discovery of 1.5 Million Year Old Fossil of Female Pelvis in … MarketWatch – 20 ore fa GOSPORT, Ind., Nov 14, 2008 /PRNewswire-USNewswire via COMTEX/ — The Stone Age Institute announces an important new fossil of a Homo erectus female pelvis from approximately 1.3 million years ago. This fossil reveals important new information about … Could fossil adds key piece to human evolution? Salt Lake Tribune 1.3 million year old human fossil found in Ethiopia afrol News New Scientist (subscription) – Reuters South Africa – RedOrbit – Deseret News e altri 101 articoli simili »

2008-10-10 – L’assenza di masticamento é il segreto dei grandi Sauropodi (Dinosaurs, Sauropod, not chewing)

Biggest Dinosaurs Grew Huge by Not Chewing Their Food

Kate Ravilious
for National Geographic News

October 9, 2008

Dinosaurs known as sauropods—the largest land animals that ever lived—grew huge and were an evolutionary success in part because they didn’t bother to chew their food, new research suggests.

Sauropods weighed up to 88 tons (80 metric tons)—ten times more than an African elephant—and measured as high as 23 feet (7 meters).

The group of dinosaurs, which included the brachiosaurus and diplodocus, loomed over the animal kingdom for more than 140 million years until the late Cretaceous period, 65 million years ago. (See a brachiosaurus herd.)

Scientists think the animals evolved to be so large to discourage big predators, like Tyrannosaurus rex, from eating them. But how they maintained such massive body sizes has remained mysterious.

The herbivores, or plant-eaters, had hardly any teeth and are thought to have swallowed their food whole—an entire bush in one gulp, for example. They browsed large areas, barely moving and consuming vast quantities in short periods of time.

So they needed long necks to reach food high in trees and a huge gut to process and break down their unchewed meals, said Martin Sander, a palaeontologist at the University of Bonn in Germany and co-author of the study, published tomorrow in the journal Science.

“You can only have this long neck if you don’t chew your food, otherwise your head would be full of teeth and too heavy to support,” he said.

Paul Upchurch, an expert on sauropods from University College London, said that “most palaeontologists agree that feeding is the key to understanding sauropod gigantism.”

(Related: “Bizarre Dinosaur Grazed Like a Cow, Study Says” [November 15, 2007].)

Survival Strategies

To outgrow their predators, sauropods didn’t just need lots of food. They also needed to develop fast, so they could attain their full size before being eaten, experts said.

Sauropod bones show that they did indeed grow swiftly. A 22-pound (10-kilogram) hatchling could become a 220,000-pound (100,000-kilogram) grown-up in about 20 to 30 years—quick by dinosaur time.

“This tells us that they must have been warm-blooded and had a high metabolic rate compared to cold-blooded creatures,” said the University of Bonn’s Sander.

Like all dinosaurs, sauropods laid nestfuls of eggs. By producing so many young at a time, “a population could recover quickly, even after a big catastrophe,” Sander said.

Large modern mammals, such as elephants, give birth to far fewer offspring, raising their chances of extinction should a disaster occur.

So why don’t we see gigantic elephants and crocodiles roaming around today?

Experts think that reptiles, such as crocodiles, still maintain the egg-laying advantage, but their cold blood prevents them from growing fast enough to reach a great size.

Mammals have warm blood, but can’t grow as big as sauropods due to their slow reproductive strategy and the need to chew their food.

source: http://news.nationalgeographic.com/news/2008/10/081009-dinosaur-big.html

SOURCES AND RELATED WEB SITES

• University College London: Paul Upchurch
• University of Bonn

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Other links:

A mystery to chew on
Globe and Mail – 3 ore fa
It is a colossal mystery, one an international team of researchers has been trying to solve for four years. Why did some dinosaurs get so big? …

The Press Association – 9 ore fa
An inability to chew may be one reason why giant long-necked dinosaurs such as Brachiosaurus and Diplodocus got so big, scientists believe. …

and in German:

Paläontologie: Wachstum ohne Grenzen
Die Presse – 13 ore fa
Größe garantiert Überleben, das zeigen die Sauropoden, die mächtigsten aller Landtiere. Ihr Geheimnis: Hinunterschlingen, was geht. …

Der Gigantismus der Dinosaurier
FOCUS Online – 14 ore fa
Sauropoden waren die größten Landtiere, die je die Erde bevölkerten. Jetzt erklären Wissenschaftler, wie die Tiere so riesig werden konnten. …

Dinosaur giants ‘could not chew’

     

Dinosaur giants ‘could not chew’
The Press Association – 9 ore fa
An inability to chew may be one reason why giant long-necked dinosaurs such as Brachiosaurus and Diplodocus got so big, scientists believe. …

Super-sized dinos had super-sized stomachs
MSNBC – 10 ore fa
By Jeanna Bryner Brachiosaurs and other long-necked giants of the dinosaur world weighed as much as 10 African elephants. Researchers now think they know …

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Article on Science links:

 

Sauropod Gigantism

P. Martin Sander¹ and Marcus Clauss²

¹Division of Paleontology, Steinmann Institute, University of Bonn, D-53115 Bonn, Germany. E-mail: martin.sander@uni-bonn.de

Science 10 October 2008 322: 200-201 [DOI: 10.1126/science.1160904] (in Perspectives)

……Triassic (about 210 million years ago), sauropods diversified into about 120 known genera…current geological survival time. Thus, sauropods were not only gigantic but also, in evolutionary…their unusual biology (see the figure).Sauropods had an elephantine body supported by……

Summary »   Full Text »   PDF »  

 

 

2008-10-10 – China, Chengjiang Lagerstätte: Nuovi Artropodi “in catena” sorprendono i paleontologi (“Daisy-chain” shrimp-like fossils)

New arthropod species really stuck together

By Laura Sanders

Web edition : Thursday, October 9th, 2008

Fossilized chain formations reveal community behavior

A new fossil find reveals that in an ancient arthropod species, no animal was an island.

The discovery of 525-million-year-old fossils belonging to a new species of arthropod shows that these animals formed communal chains never before seen in fossilized invertebrates, scientists report in the Oct. 10 Science.

These arthropods, a genus that includes lobsters, beetles and tarantulas, were found in sturdy chain configurations in southern China’s Chengjiang Lagerstätte, a treasure trove of fossils often compared to Canada’s Burgess Shale. In the chain formations, each animal faced the same way, with its tail latched onto the carapace of the individual behind. This unique fossil configuration provides evidence that these arthropods did not act as solitary beings, the authors report.

Nigel Hughes, a paleobiologist from the University of California, Riverside comments that these types of finds provide snapshot scenes of “normal” life.

“Of the millions of fossils, the chances of getting an occurrence where we can determine collective behavior is quite rare,” says coauthor Derek Siveter of the University of Oxford in England. He and his colleagues found 22 complete or partial chains, but only one solitary specimen.

“As far as I know, the group behavior exhibited by these fossils is nearly unknown in modern invertebrates, and has never been demonstrated in fossil invertebrates,” says geobiologist Anthony Martin of Emory University in Atlanta.

Modern-day animals are known to display collective behavior. Siveter points to spiny lobsters in the Caribbean that form trains in which the antennae of one lobster grasp the carapace of the next individual in line. This formation is thought to aid in feeding, molting or migration following a storm. But unlike those lobster trains, these fossil arthropod chains, dated to the early Cambrian, appear to have been almost unbreakable. The animals collectively show signs of twisting, turning, bending and telescoping, all without coming apart. 

Based on the twists and turns of these fossilized chains, Siveter and his colleagues postulate that the chains may have floated in the early Cambrian ocean.

But the researchers still don’t know why these arthropods linked themselves together. Feeding behavior is an unlikely reason since each individual’s mouth is covered by the tail of the preceding arthropod. And it is also unlikely that being in a chain made it easier to reproduce. Because other types of fossilized arthropods were known to migrate, the scientists’ best guess is that these chains may have provided members protection against predators while on a long migration — in other words, strength in numbers.

“When you’re dealing with 525-million-year–old animals, it’s not like math where five plus five is ten. There are a lot of interesting discussions to have,” explains Siveter. 

The purpose of these chains is something scientists will puzzle over for a long time, but one thing is clear: These ancient animals did not act alone.

“This find provides extraordinary insights into the early evolution of group behaviors, possibly representing a combination of reproductive and anti-predatory strategies,” says Martin. “In short, this find should cause all who are interested in fossil behavior to reconsider what we know about group behavior.”

source: http://www.sciencenews.org/view/generic/id/37463/title/New_arthropod_species_really_stuck_together

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Other links:

Oxford University

Nature “Photo of the week”

Shrimps’ ancient chain gang mystifies scientists Times Online – 10 ore fa Mysterious shrimp-like creatures from 525 million years ago have been identified as the first to exhibit collective behaviour. All but one of dozens of the …

“Daisy Chains” of Fossil Creatures Found in China National Geographic – 13 ore fa “Daisy chains” of small fossil creatures recently discovered in southwest China reveal an extremely ancient and bizarre type of animal grouping, … with SOURCES AND RELATED WEB SITES Chengjiang Fossils University of Oxford: Department of Earth Sciences Science A fossil chain of shrimplike marine organisms that lived around 525 million years ago has been unearthed in China, researchers reported in October 2008. Photograph from National Geographic, courtesy Derek Siveter

Fossil daisy-chain
ScienceBlogs – 10 ore fa
Here’s a very strange fossil from the Chengjiang Lagerstätte, an early Cambrian fossil bed from 525 million years ago.

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Science Article links:

Collective Behavior in an Early Cambrian Arthropod

Xian-Guang Hou, Derek J. Siveter, Richard J. Aldridge, and David J. Siveter

To whom correspondence should be addressed. E-mail: xghou@ynu.edu.cn

Science 10 October 2008 322: 224 [DOI: 10.1126/science.1162794] (in Brevia)

Abstract »   Full Text »   PDF »  

Abstract:

Examples that indicate collective behavior in the fossil record are rare. A group association of specimens that belong to a previously unknown arthropod from the Chengjiang Lagerstätte, China, provides evidence that such behavior was present in the early Cambrian (about 525 million years ago), coincident with the earliest extensive diversification of the Metazoa, the so-called Cambrian explosion event. The chainlike form of these specimens is unique for any arthropod, fossil or living, and most likely represents behavior associated with migration.

2008-10-03 – Fluttuzioni del livello del mare nel Paleozoico (Paleozoic Sea-Level Changes)

A Chronology of Paleozoic Sea-Level Changes

Bilal U. Haq^1* and Stephen R. Schutter²

Science 3 October 2008:
Vol. 322. no. 5898, pp. 64 – 68
DOI: 10.1126/science.1161648

Abstract »|   Full Text »|   PDF »|   Supporting Online Material »|  

In the free access Pdf of “supporting online material” there is the Chart with sea level fluctuations

COVER Ordovician sedimentary rocks at Presqu

Abstract – Sea levels have been determined for most of the Paleozoic Era (542 to 251 million years ago), but an integrated history of sea levels has remained unrealized. We reconstructed a history of sea-level fluctuations for the entire Paleozoic by using stratigraphic sections from pericratonic and cratonic basins. Evaluation of the timing and amplitude of individual sea-level events reveals that the magnitude of change is the most problematic to estimate accurately. The long-term sea level shows a gradual rise through the Cambrian, reaching a zenith in the Late Ordovician, then a short-lived but prominent withdrawal in response to Hirnantian glaciation. Subsequent but decreasingly substantial eustatic highs occurred in the mid-Silurian, near the Middle/Late Devonian boundary, and in the latest Carboniferous. Eustatic lows are recorded in the early Devonian, near the Mississippian/Pennsylvanian boundary, and in the Late Permian. One hundred and seventy-two eustatic events are documented for the Paleozoic, varying in magnitude from a few tens of meters to 125 meters.

1 National Science Foundation, Arlington, VA 22230, USA.
2 Murphy Oil International, Houston, TX 77094, USA.
* To whom correspondence should be addressed. E-mail: bhaq@nsf.gov

Abstract »|   Full Text »|   PDF »|   Supporting Online Material »|  

 

E 7 – I dinosauri, dominatori per caso

L’imponderabile nell’evoluzione

I dinosauri, dominatori per caso

 
Nella competizione con i crurotarsi per il dominio della Terra, 200 milioni di anni fa vennero “baciati dalla fortuna”, così si legge in un articolo su “Science”

Solitamente si pensa che i dinosauri abbiano dominato la Terra per oltre 160 milioni di anni perché disponevano di caratteristiche che li rendevano superiori agli altri gruppi di animali dell’epoca. Ora un gruppo di ricercatori diretti da Steve Brusatte, della Columbia University e dell’American Museum of Natural History, confutano in un articolo su “Science” questa presunta superiorità dimostrando che che per i primi 30 milioni di anni della loro storia si trovarono a fronteggiare un gruppo, quello dei crurosauri, che aveva tutti i numeri per contendere loro lo scettro.

“Per lungo tempo si è pensato che i dinosauri avessero qualcosa di speciale che li aiutò a diventare dominanti nel corso del Triassico, ossia nei primi 30 milioni di anni della loro storia, ma questo non è vero” osserva Brusatte. “Se qualcuno di noi fosse vissuto nel Triassico e avesse dovuto dire quale gruppo di animali sarebbe stato quello dominante nei successivi 130 milioni di anni, avrebbe indicato i crurotarsi e non i dinosauri.”

Sia i dinosauri che i crurotarsi si sono diffusi occupando diverse nicchie ecologiche dopo un’estinzione di massa verificatasi alla fine del Permiano (250 milioni di anni fa). Attualmente gli unici discendenti viventi dei crurotarsi sono i coccodrilli, me nel Triassico si presentavano come un gruppo ricchissimo di diversità – dai rauisuchi ai fitosauri, fino agli erbivori aetosauri – contendendo le risorse ai dinosauri.

Entrambi i gruppi sopravvissero all’evento di estinzione di massa avvenuto 228 milioni di anni fa, ma solo pochissimi crurotarsi riuscirono a superare un successivo periodo di rapido riscaldamento globale avvenuto alla fine del Triassico, ossia 200 milioni di anni fa. E fu proprio per questo “colpo di fortuna”, come lo hanno definito i ricercatori, che i dinosauri – sopravvissuti fino a 65 milioni di anni fa – divennero dominanti.

Brusatte e colleghi hanno misurato l’evoluzione dei due gruppi in competizione valutando 437 caratteristiche degli scheletri di 64 specie di dinosauri e crurotarsi e tracciando un nuovo albero filogenetico, ricavando un indice di “disparità”, capace di rappresentare un indicatore dei differenti stili di vita, diete, e habitat dei due gruppi.

I ricercatori hanno così stabilito che i crurotarsi avevano un indice di disparità doppio di quello dei dinosauri. “In base a questa informazione è difficile affermare che i dinosauri fossero ‘superiori’ durante il Triassico. Sono stati piuttosto baciati dalla fortuna quando i crurotarsi sono stati messi a dura prova dall’estinzione alla fine del Trassico”, ha commentato Brusatte.

I ricercatori hanno anche valutato il tasso di evoluzione dei due gruppi per vedere se i dinosauri si stessero diversificando in nuove specie a una velocità superiore a quella dei crurotarsi, come ci si dovrebbe aspettare se avessero avuto capacità particolari tali da surclassare i loro competitori. Ma le misure eseguite hanno mostrato che i due gruppi avevano un tasso evolutivo simile nel corso dei 30 milioni di anni durante cui sono convissuti.

“Molti pensano che l’evoluzione sia progressiva: che i mammiferi siano migliori dei dinosauri perché sono venuti dopo”, ha commentato Michael Benton, coautore della ricerca. “Per questo può essere difficile accettare che i dinosauri abbiano raggiunto la loro posizione dominante sulla Terra quasi per caso, proprio come fecero i mammiferi quando i dinosauri vennero spazzati dalla collisione di un meteorite.”  (gg)

 (12 settembre 2008)

http://lescienze.espresso.repubblica.it/articolo/I_dinosauri,_dominatori_per_caso/1333190