Lungs have been independently lost in some members of all three major amphibian groups – salamanders, frogs, and caecilians. However, lungs have been lost in just one frog and one caecilian species. Lungs are absent in a larger number of salamander species, having been lost in 11 of 89 species of hynobiids and all 497 species of the family Plethodontidae. Lewis et al. (2022) confirm that plethodontids actually initiate lung generation early in their development but the process is reversed through programmed cell death (apoptosis) before they emerge from the egg. Remarkably, the authors show that many of the genes that are involved in lung development in species that keep their lungs are also expressed in the vestigial lungs of plethodontids. This finding suggests that early lung development may be a necessary feature for the development of other organs, such as the heart. Because all plethodontids lack lungs, they were most likely lost in the common ancestor of the group between 66 million years ago (the timing of the first divergence events within the plethodontid crown clade) and 110 million years ago (the timing of the split between Plethodontidae and its sister family, Amphiumidae), thus showing that the retention of the genetic program underpinning lung development have been maintained for tens of millions of years despite the early loss of lungs in the plethodontid common ancestor.
|215 Caecilians||777 Salamanders||7,532 Frogs|