Between 15 million and 55 million years ago, India and Asia collided, starting a series of geological events that raised the Himalayas and the Tibetan plateau. Now, a group of well-muscled frogs is revealing how the continents moved.
"Geologists know a lot about that area, but what they haven't been able to do is give a sequence to the timing of the rise of particular mountain masses and particular ridges and pieces," David Wake, a herpetologist and evolutionary biologist at the University of California, Berkeley and a co-author of a new paper detailing the findings, said in a UC Berkeley statement. "We use these frogs as a surrogate for a time machine."
Spiny frogs of the tribe Paini are often called stone frogs in China, because they cling to moss-covered rocks near rapidly flowing streams, the researchers say. When male frogs mate with females, they grab the female from behind. Frogs that live in fast-flowing streams have muscular forearms and sandpaper-like chests to help prevent slimy females from being whisked away by the current. (Images of more odd frogs)
"What we have here is a group of very old frogs that are so fixed to their habitats that they just stuck there, sitting on that mountain mass when it got raised up," Wake said. "They were separated by these uplifts and by the rivers between the mountains into different units, and these give us a fix on the timing of geological events."
Mountains and amphibians
To pinpoint the events, Wake and colleagues conducted a genetic analysis of 24 species of spiny frogs from the tribe Paini. They found that the tribe arose in what is now Indochina and spread into Western China about 27 million years ago, diverging into two groups: Nanorana, now consisting primarily of high-elevation species that dwell in regions up to 15,419 feet in altitude, in Western China; and Quasipaa, consisting of mostly low-elevation species in Indochina and South China.
The Nanorana subgenus isolated in Tibet began to diversify again about 9 million years ago, consistent with the period during which the Tibetan plateau rose above 9,843 feet. These new Nanorana species adapted to the cold, arid, low-oxygen conditions of Tibet.
The researchers found that in Indochina and South China, the Quasipaa frogs were split by the uplift of the Truong Son Mountain Range on the Laos-Vietnam border. This uplift and the opening of the South China Sea probably occurred as the Indian landmass pushed Indochina southeastward, isolating the frogs of Indochina from those of South China.
A third group of spiny frogs was restricted to the Himalayas 19 million years ago as the Tibetan plateau pushed higher.
The sequence of evolution supports a minority view of how the India/Asia collision played out. Rather than merely pushing the Himalayas upward, as some geologists believe, the Indian plate also pushed Southeast Asia and China aside and toward the Pacific Ocean, a process referred to as extrusion or escape tectonics.
"Basically, the frogs were rafting on top of the continents," An Yin, a UCLA geologist who was not involved in the study, said in the statement. "The tectonics control morphological evolution by transporting originally very closely related frogs so far apart they all diverge and develop very differently."
The findings were published online in the Aug. 3 issue of the journal Proceedings of the National Academy of Sciences.