The invasion of land by the tetrapods – four-limbed animals that include mammals, reptiles and amphibians – was surely one of the most evocative events in animal evolution. The march onto terra firma began some 365 million years ago and was driven by a suite of innovative adaptations that allowed back-boned animals to live out of water.
Lungs were among the most crucial of these for they allowed the first land-lubbers to extract oxygen from the surrounding air. That ability is so important that it’s rare for tetrapods to lose their lungs completely. Until now, the only groups that we know have done so are two families of salamanders and a lone species of caecilian (a type of burrowing worm-like amphibian).
Now, David Bickford and colleagues form the National University of Singapore have expanded that list with the discovery that a species of frog – the Bornean flat-headed frog (Barbourula kalimantanensis)- also lacks lungs of any sort.
The small species, also known as the Kalimantan jungle toad, is one of the most primitive of all frogs. It’s almost completely aquatic and lives in fast-flowing streams on the island of Borneo. Bickford found nine specimens on a recent expedition to the island in 2007 and before then, only two specimens had ever been found in almost 30 years of searching. Through dissections carried out right there in the field, Bickford confirmed that the frog has no lungs.