A 161 million-year-old Mongolian fossil not only reveals a new species of salamanders, but also provides proof that much of the evolution of salamanders occurred in Asia.
For more than three years, scientists from the University of Chicago and Peking University in Beijing have been collecting thousands of salamander fossils, many of which preserve the entire skeleton and impressions of soft tissues, from seven excavation sites in Mongolia and China. Prior to the discovery in 1996 of the Chinese sites, scientists had complete salamander fossils dating back only to the Tertiary period, which began 65 million years ago.
“It’s remarkable to have the earliest-known salamanders with so much diversity, so many specimens and such high-quality preservation,” said Neil Shubin, Ph.D., professor and chairman of organismal biology and anatomy at the University of Chicago and lead author in the study. “Usually when you find the earliest-known animal, you only have one representative. But we have thousands. It’s a real opportunity to look at how salamanders have evolved.”
To date, the scientists have discovered five new species of salamanders from the Asian sites – one of which, Chunerpeton tianyiensis, is described in the March 27, 2003, issue of the journal Nature.
According to the paper, the newly found species closely resembles the North American hellbender, a common salamander currently found in Asia, as well as in the Allegheny Mountains near Pittsburgh, Penn.
Most of the variations in the fossil animal are due to small changes in the shape of the bones in the front of the skull, in the features of the fingers and toes, and in variations of the ribs. One unique feature is that it bears “unicapitate” ribs, meaning the rib has only one facet, or head, where it connects to the vertebra. Most modern salamanders have two-headed ribs.
A volcanic eruption in northern China during the Middle Jurassic period (165 to 180 million years ago) provided key material for the origin of the salamander -- 100 million years earlier than the oldest known salamander fossil. The eruption wiped out whole communities of the earliest-known salamanders but left thousands of beautifully preserved fossils.
According to Shubin, the presence of Chunerpeton (a member of the Cryptobranchae family) in the Middle Jurassic period of China implies that the split between the two oldest families – hynobiids and cryptobranchids – occurred in Asia.
“The new cryptobranchid shows extraordinary morphological similarity to its living relatives,” noted the study authors. “Indeed, exant cryptobranchid salamanders can be regarded as living fossils whose structures have remained little changed for more than 160 million years.”
“What this tells us,” Shubin said, “is that the major families of salamanders are probably relatively ancient. The distribution of the families today is a relic of what happened in the distant past.
“The diversity of species in this find, combined with molecular data and study of characteristics from living salamanders, leads to the inescapable conclusion that almost all the major groups of salamanders evolved very early,” he said, “And not much has happened since.”
Soon after excavation began, it became apparent to Shubin and co-author Gao Ke-Qin, professor of earth and space science at Peking University, that these specimens provided compelling evidence that the salamander originated in Asia, which they detailed in a paper published two years ago in Nature.
Salamanders, one of the three major groups of modern amphibians, are important to understanding fundamental questions in evolution. Their wide geographic distribution, highly variable species (approximately 150) and ecological diversification have served as a model system for assessing developmental, anatomical and biogeographic evolution.
Complete fossils, some including rare soft tissue impressions, offer a wealth of new information of the salamander's origin, life cycle and evolutionary strategies.
“We were able to see all the stages of the life cycle, larvae and adults, as well as a range of different kinds of animals,” Shubin said. “The exquisite condition of the fossils offers clues to evolutionary strategies -- larval details such as gills in adult animals, for example.”
In the paper, the researchers feature one such well-preserved fossil that reveals the animal’s eye, folds in the tail and a stomach bulging with clams. “This is very unusual, to get that kind of detail,” Shubin said. The fossil also shows that the vertebral column and the limbs are not yet formed, and that the animal’s internal gills remain.
Salamanders are living fossils -- they have retained the same body plan for millions of years. “Whether you look at a salamander you find under a rock in the local forest preserve or in a rock in China dating back 165 million years, they look alike. In fact, they look alike in great detail – the bones in their wrists are the same, the way their skulls are formed – intricate details are the same,” he said.
At the same time, their limbs and heads have served as a model of how variation arises during evolution. One of the great puzzles of evolution is how different types of salamanders evolved the same features independently. This phenomenon is called parallel evolution. These fossils may provide answers to this old question.
Salamanders are disappearing worldwide today despite their longevity over evolutionary time. “Here is an animal that has been around for at least 165 million years,” Shubin said. “They made it through several major extinction events. They made it through the event that killed the dinosaurs. Yet today, along with other amphibians, salamanders are disappearing and we really don't know why.”
Fieldwork was supported by the National Geographic Society, and a recent grant from the National Science Foundation supports the lab analyses of the fossils.
The researchers plan to return to the sites this summer.
Materials provided by University Of Chicago Medical Center. Note: Content may be edited for style and length.
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