China Ash Yields Salamander Evolution Secrets
for National Geographic News
|March 27, 2003|
A cataclysmic volcanic eruption 160 million years ago was a dark day for thousands of salamanders scurrying about a series of lakes in northern China. The hot ash smothered and buried the amphibians in their tracks.
For scientists who study salamanders, however, that inundation of hot ash was manna from heaven: It allowed for the exquisite preservation of the salamanders, giving scientists a trove of fossils that are revealing the secrets of evolution.
"We have whole bodies, evidence of the soft tissues, numerous individuals, and several species," said Neil Shubin, a biologist at the University of Chicago in Illinois who has been collecting the fossils for three years with his colleague Ke-Qin Gao, a professor of Earth and space science at Beijing's Peking University.
To date the scientists have discovered five new salamander species. They report in the March 27 issue of Nature on a new species that is oldest-known relative to living salamanders.
The fossils of the new species are of tadpole-like larvae and young adults, with soft tissues including eyes, gills, and stomach contents all perfectly preserved. In one of the fossils shown in the paper, the salamander's last dinner is preservedclam shrimp.
The researchers named the species Chunerpeton tianyiensis. It closely resembles the North American hellbender, a salamander with large, flat head found in the Allegheny Mountains near Pittsburgh, Pennsylvania, that can grow more than 2 feet (61 centimeters) long. It is also similar to the endangered Asian giant salamander, which can grow more than 5 feet (152 centimeters) long.
"This discovery greatly extends the known history of the modern salamander groups, with species specifically recognizable as belonging to two living families," said Robert Carroll, a zoologist at McGill University in Montreal, Canada, who studies the origins and interrelationships of modern amphibians.
The similarities between the Chunerpeton tianyiensis fossils and living salamanders include specific features of the ribs, back, and back of the skull. Parts of the front of the skull and fingers are different, according to Shubin.
Shubin and Gao conducted their fieldwork with funding from the National Geographic Society's Committee for Research and Exploration. Lab analysis of these fossils is supported by a grant from the United States National Science Foundation.
Prior to this latest discovery, the oldest known salamander fossils dated back to the Tertiary period, which began 65 million years ago. Despite the 100-million-year gap, however, there are only minor differences between the fossils and their modern equivalents.
"These finds point to Asia as a crucible for the early evolution of salamanders," said Shubin. "It shows that much of the major evolutionary changes in salamanders likely happened very early in their known evolutionary history. It also shows that remarkably little anatomical change has happened over the past 160 million years or so."
Chunerpeton tianyiensis and its modern equivalents like the North American hellbender (Crytobranchus) and the Asian giant salamander (Andrias) belong the salamander family known as the Cryptobranchidae, one of the two most primitive groups of salamanders.
The other primitive group of salamanders is known as the Hynobiidae. They are small- to medium-sized salamanders found throughout Asia.
"Cryptobranchids and hynobiids have long been suggested as the most primitive of the living salamander groups," said Carroll. "Their divergence, which must pre-date these particular fossils from China, must have preceded the appearance of all the other living salamander families."
Putting this in perspective, Carroll says modern salamanders essentially appeared early in the history of the dinosaurs and that some of the more primitive families of salamanders have changed very little since that time.
Shubin says that salamanders have been able to survive for such a long time period with relatively little evolutionary changes because they are generalists. "They have apparently hit on a very stable ecological and evolutionary strategy very early in their evolutionary history," he said.
Scientists like Shubin, Gao, and Carroll say they are attracted to the study of salamanders because the amphibians give them a window to see how evolutionary mechanisms work.
Salamanders are more than 160 million years old, they have learned to live in a variety of environments, they have one of the largest genomes of any known animal, and researchers know quite a bit about their variation, said Shubin.
"Put all this together and it means we can understand how evolutionary changes to genes and development produce changes in anatomical features such as heads, limbs, tails," he said.
Of practical interest to humans, said Carroll, is the salamander's ability to regenerate limbs. This characteristic is unique among vertebrates. "It suggests the possibility that we may learn something of this capacity from salamanders that could be applied in the case of severe limb damage," he said.
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