The dinosaurs wiped out from Earth gave rise to most of the frogs alive today.
A new study by Chinese and American biologists shows that if the calamity had not wiped the planet clean of most terrestrial life 66 million years ago, 88 percent of today’s frog species wouldn’t be here. Nearly nine out of 10 species of frog today have descended from just three lineages that survived the mass extinction.
The findings to be published in the journal Proceedings of the National Academy of Sciences, show that frog evolution pinpointed the blossoming of the main frog lineages to about 35 million years earlier, in the middle of the Mesozoic era.
The new analysis of 95 genes from frogs within 44 of 55 living families shows that these three lineages started to take off precisely at the boundary between the Cretaceous and Paleogene periods — the K-Pg boundary, formerly called the KT boundary — when the last mass extinction occurred, and not 100 million years ago.
According to herpetologist and co-author David Wake, a University of California, Berkeley professor of the graduate school and a curator of the Museum of Vertebrate Zoology, new frog species likely radiated rapidly throughout the world because so many environmental niches were available after the animals occupying them disappeared.
“We think the world was quite impoverished as a result of the KT event, and when the vegetation came back, angiosperms dominated. That’s when trees evolved to their full flowering,” Wake said. “Frogs started becoming arboreal. It was the arboreality that led to the great radiation in South America in particular.”
Trees are an ideal habitat for frogs not only because they allow them to escape from terrestrial predators, but also because their fallen leaves provide protection while the frogs are on the ground, breeding habitat and plenty of food, such as insects. Trees and other flowering plants took off in the late Cretaceous, and were ready for exploitation by frogs after they recovered from the extinction.
Another adaptation that became popular was direct development, that is, producing young without a tadpole stage, which is standard for about half of all frog species today.
“The majority of the frogs that thrive now are thriving because of direct development of eggs in terrestrial situations,” he said. “It is a combination of direct development and use of arboreal habitat that accounts for a great deal of the radiation.”
Two of the three surviving lineages came out of Africa, which remained intact as the continents shifted around over the ensuing eons, with the breakup of Pangea and then Gondwana to form the continents.
Today’s frogs, comprising more than 6,700 known species are, however, under severe stress around the world because of habitat destruction, human population explosion and climate change, possibly heralding a new period of mass extinction, says the new study.