New salamander species provide new answers to old questions in evolutionBy Catherine Gianaro
Medical Center Public Affairs
For more than three years, Shubin, Chairman and Professor of Organismal Biology and Anatomy, his Chicago colleagues and scientists from Peking University in Beijing, China, have been collecting thousands of salamander fossilsmany of which preserve the entire skeleton and impressions of soft tissuesfrom seven excavation sites in China. Prior to the 1996 discovery of the Chinese sites, scientists had complete salamander fossils dating back only to the Tertiary period, which began 65 million years ago.
Its remarkable to have the earliest-known salamanders with so much diversity, so many specimens and such high-quality preservation, said Shubin, an author of the study.
Usually when you find the earliest-known animal, you only have one representative. But we have thousands. Its 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 a paper published in the March 27 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, Pa. 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 salamander100 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.
The new cryptobranchid shows extraordinary morphological similarity to its living relatives, noted the study authors. Indeed, extant cryptobranchid salamanders can be regarded as living fossils whose structures have remained little changed for more than 160 million years.
Shubin said, What this tells us 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 new information about the salamanders 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 discuss one such well-preserved fossil that reveals the animals 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 animals internal gills remain.
Salamanders are living fossilsthey 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 detailthe bones in their wrists are the same, the way their skulls are formedintricate 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 dont know why.
The National Geographic Society supported the researchers fieldwork, 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.