Near-complete remains of tiny creature support early origin for lineage that led to humans.
Sid Perkins - 05 June 2013
[Video] 3D reconstruction of the left foot of the ancient primate fossil PAUL TAFFOREAU/XIJUN NI .. 7 sec embedded ..
The near-complete fossil of a tiny creature unearthed in China in 2002 has bolstered the idea that the anthropoid group of primates — whose modern-day members include monkeys, apes and humans — had appeared by at least 55 million years ago. The fossil primate does not belong to that lineage, however: it is thought to be the earliest-discovered ancestor of small tree-dwelling primates called tarsiers, showing that even at this early time, the tarsier and anthropoid groups had split apart.
How the ancient primate might have looked, in its natural habitat of trees (artistic reconstruction). - XIJUN NI
The slender-limbed, long-tailed primate, described today in Nature, was about the size of today’s pygmy mouse lemur and would have weighed between 20 and 30 grams, the researchers estimate. The mammal sports an odd blend of features, with its skull, teeth and limb bones having proportions resembling those of tarsiers, but its heel and foot bones more like anthropoids. “This mosaic of features hasn’t been seen before in any living or fossil primate,” says study author Christopher Beard, a palaeontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
By analysing almost 1,200 morphological aspects of the fossil and comparing them to those of 156 other extant and extinct mammals, the team put the ancient primate near the base of the tarsier family tree. Dubbed Archicebus achilles, the creature’s genus name roughly translates as 'original long-tailed monkey', whereas the species name is a wry nod to the creature’s anthropoid-like heel bone.
'Impressive work'
The number of anatomical characteristics scrutinized by Beard and his colleagues is much higher than is typically covered by a single study, largely thanks to high-resolution scans of the well-preserved fossil and the team’s detailed, decade-long analysis. “This is really fantastic, impressive work,” says Zhe-Xi Luo, a vertebrate palaeontologist at the University of Chicago in Illinois. “Every detail that can be extracted from these fossils has been extracted.”
The primate's remains were recovered from a layer of shale formed from sediments deposited in a lake in what is now eastern China between 54.8 million and 55.8 million years ago, says paper co-author Xijun Ni, a palaeontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Fragmentary fossils of primates had already been unearthed from around this era, says Beard, but those usually consisted of just teeth and bits of jawbones; the oldest well-preserved primate skeletons until the new find came from around 48 million years ago.
Reconstruction of the fossil of Archicebus achilles, based on microtomography scanning. - PAUL TAFFOREAU/XIJUN NI
Because the fossil indicates that the tarsier and anthropoid primate groups split before that era, the anthropoid lineage is also at least that old, says Beard. He says that he and others had suggested a tarsier/anthropoid split of about this time previously, but based on thinner evidence.
Its long hindlimbs and grasping feet suggest that A. achilles lived in trees, with a tail thought to be at least twice its body length helping it to maintain balance as it leapt from branch to branch. The moderate size of the primate’s eye sockets hint that it was active in the day.
The size and shape of the primate's teeth — particularly the sharply pointed premolars, which were well adapted for shearing prey — strongly suggest that the tiny mammal fed mostly on insects. Such prey would have been abundant, because A. achilles evolved during an era when global temperatures were exceptionally high and jungles stretched as far north as the Arctic. “It was a great time to be a primate,” says Beard.
Because A. achilles sits near the base of the tarsier family tree, scientists say it probably resembles the yet-to-be-discovered creatures that lie at the base of most primate groups — including the anthropoid lineage that ultimately gave rise to humans. “If you retrace primate evolution to its beginning, [A. achilles] is what our ancestors most likely looked like,” says Luo.
John Pickrell in England for National Geographic News December 15, 2003
Amateur fossil hunters have helped to uncover the oldest known ancestor to kangaroos, koalas, possums, and wombats. A near complete skeletal fossil of the chipmunk-size, marsupial ancestor Sinodelphys szalayi has been dug up from 125-million-year-old shales in China's northeastern Liaoning Province.
"This mammal could be the … great-grandparent of all marsupial mammals," said Zhe-Xi Luo, one of the paleontologists behind the find, based at the Carnegie Museum in Pittsburgh. "This new fossil provides precious new information, and sheds light on the evolution of all marsupial mammals," he said.
Sinodelphys, which shares more wrist, ankle, and dental features with living marsupials than other mammals, will help researchers understand what the ancestor to all marsupials may have been like. The fossil will also help scientists piece together the early history of all placental and marsupial mammals, according to the Chinese and American paleontologists who detail their analysis of the fossil in the current edition of the research journal Science.
"Beautifully Preserved"
The near-complete skeleton of Sinodelphys is strikingly well preserved, with tufts of fur and even soft tissues imprinted on the slate slab in which it was found. In life, the slight, rodent-like marsupial would have measured six inches (15 centimeters) in length and weighed around an ounce (30 grams). The animal was a proficient climber and likely spent its days scampering across the low branches of trees and bushes, and feeding on insects, worms, and other invertebrates.
"To find such a beautifully preserved mammal of such antiquity is extraordinary … and this one will likely form the basis of debate for decades," commented marsupial fossil expert Steven Wroe at the University of Sydney in Australia. "The authors present a very convincing case for their conclusion that this new species is closely related to living marsupials," he said.
Most archaic marsupial and marsupial-like fossils are known from North America, commented Mike Archer, vertebrate palaeontologist and director of the Australian Museum in Sydney. This discovery now shifts the limelight of the group's origins to Asia. "China is now regularly producing stunning fossils such as the world's oldest-known flower, Archaeofructus, [see below]from the same deposit as Sinodelphys," he said.
"[Sinodelphys] increases our appreciation for the antiquity of this great and fascinating group of mammals, and makes those of us who have obsessions about pouches proud to be so afflicted," said Archer.
Though most marsupials are restricted to Australia today, all early fossil ancestors of the mammalian group are known from Asia and North America. The previous oldest known marsupial skeleton was unearthed from 75-million-year-old Mongolian deposits (though jaw fragments and teeth up to 105 million years of age have also been documented). The discovery of Sinodelphys follows the uncovering by the same research team, of the world's oldest placental mammal Eomaia scansoria last year. The discovery of both fossils was part funded by grants from the National Geographic Society.
To Pouch or Not to Pouch
Marsupial and placental mammals all share the habit of giving birth to live young. However the more than 4,300 placentals (such as humans, horses, elephants, and whales) have relatively long pregnancies and give birth to well-developed young. Marsupials on the other hand, have short gestation times, but further complete their maturation in their mother's pouch. The world's 270 marsupials mostly live in Australia, neighboring islands, and South America, with the exception of North America's Virginia opossum, a relatively recent immigrant.
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Placental and marsupial mammals are more closely related to one another than to the third living group of mammals, the monotremes. These exotic egg-laying animals are represented by just three species: the duck-billed platypus and two echidnas.
"Looking across the world, 99.9 percent of modern mammals are placentals and marsupials, so finding how they came about is an important question," said Luo. "Establishing the origin of these two groups depends on determining the ancestral condition of the earliest fossil." The discovery of Sinodelphys brings that goal one step closer to reality.
Chinese co-author Qiang Ji, a paleontologist of the Chinese Academy of Geological Sciences in Beijing and discoverer of many important Liaoning fossils, acquired the fossil from Chinese peasants in 2000. Ji was on a field trip to fossil-bearing sites of Liaoning Province, 200 kilometers (120 miles) northeast of Beijing. These sites have yielded the remains of birds, frogs, some feathered dinosaurs and a few mammals including Eomaia, also 125 million years old.
Digging for fossils is more lucrative for locals than traditional subsistence farming. Luckily most of the important specimens have ended up in the collections of Chinese research institutions rather than dealers or private collectors, said Luo, adding that fossils from these sites should be better protected.
Because of its age, and because it lacks sepals and petals and its reproductive organs, interpreted as carpels and stamens, are produced on an elongate stem rather than condensed into a flower as in modern angiosperms,
Archaefructus liaoningensis - an artist's impression
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