Although Kermit the Frog has always struggled with body image, in evolutionary terms, the frog body plan is a rather successful one. With a short, stout body, protruding eyes and strong, flexible limbs with webbed feet, the world can be your swamp. The frog body plan has remained rather similar for almost 200m years, and with only limited tweaks in anatomy, frogs (Anura) have managed to occupy a range of different habitats, from muddy pools in Alaska to tree tops in the tropics. Currently, over 6700 species are known from all continents except Antarctica, which makes frogs one of the most diverse and species-rich groups of tetrapods. Never change a good thing. However, this limited variation in the frog body plan over time and space has made it difficult for biologists to reconstruct the evolutionary history of frogs and to sort out who is related to who.



Frogs are amphibians, and the oldest member of the frog lineage – the stem-frog Triadobatrachus massinoti which lived during the Early Triassic (~250m years ago) in what is now Madagascar – still retained primitive features, such as a tail and the likely inability to jump, that distinguish it from modern frogs. By the Early Cretaceous (131-120m years ago), the first members of the modern frogs have evolved, such as the three-dimensionally preserved Liaobatrachus zhaoi from the Yixian Formation in China (Dong et al., 2013).

The rise of molecular techniques enabled scientists to use DNA instead of morphology to try to unravel the frog family tree. Initial studies focused only on a limited number of genes, and as a result, age estimates for certain groups of frogs varied wildly. Moreover, these studied did little to understand relationships within frog groups, particularly for those groups that contain massive numbers of species, such as the Hyloidea which includes the glass frogs and poison-dart frogs.

A new study by Yan-Lie Feng and colleagues from Sun Yat-Sen University in Guangzhou, China, set out to change this by using an extensive molecular dataset that included 95 different genes from 156 species of frogs. Their dataset represents a major leap compared to previous studies, and has resulted in the best supported timescale of frog evolution thusfar.

Specimens of Liaobatrachus zhaoi, the earliest modern frog, from the Yixian formation in China. Photograph: Dong L, Roček Z, Wang Y, Jones MEH (2013) Anurans from the Lower Cretaceous Jehol Group of Western Liaoning, China. PLOS ONE

By using such a large number of genes and species, this new analysis indicates that the major frog groups are younger than previously thought. The last common ancestor of all living frogs (crown-group Anura) is estimated to have lived during the Upper Triassic at 210m years ago. This is in contrast to previous studies that placed the last common ancestor much further back in time at around 250 million years.

When plotting the diversification of frogs on a geological time scale, it becomes clear that diversification events in frogs coincide with break-ups of major prehistoric landmasses. The first split within Anura is that between the Neobatrachia and other anurans. This split occurs at ~ 180 Mya (Middle Jurassic), at around the same time as the breakup of Pangaea into the two supercontinents Laurasia (northern hemisphere) and Gondwana (southern hemisphere). A second break up occurred at around 135m years ago, when two major lineages of Neobatrachia split into Proceola, containing the superfamily Hyloidea, and Diplasiocoela including the Ranoidea. This split coincides with the separation of South America and Africa and the spreading of the South Atlantic Ocean sea floor in the Early Cretaceous.

Interestingly, three major lineages of frogs, the Hyloidea, Microhylidae and Natatanura, have a near-synchronous origin at around 66m years ago. That estimate overlaps with a major extinction event, the Cretaceous – Tertiary (K/T) mass extinction, during which two-thirds of life on earth, including the non-avian dinosaurs, marine reptiles and pterosaurs, went extinct.

Although there is little fossil evidence to show how the K/T mass extinction impacted frogs, it is not unlikely that a number of frog species went the way of the dinosaurs. However, when researchers looked at the rate at which species originated during that time period, the analysis indicated that there was a surge in frog diversification immediately following the K/T boundary. Quite ribbiting, perhaps, is the fact that 88% of current frog species originated in this relatively short time period after the K/T mass extinctions.

Mass extinctions leave behind a wasteland of empty ecological real estate. Species that survive can take advantage of this empty ecological space, and as different organisms invade different niches, they adapt and diversify. This is why mass extinction events are often followed by periods of rapid adaptive radiation and speciation.

The demise of non-avian dinosaurs and many other groups at the end of the Cretaceous triggered explosive radiations of mammals (Alroy, 1999) and birds (Ksepka et al., 2017). This new study on frogs shows that the aftermath of the K/T mass extinction may have provided new ecological opportunities for amphibians as well. Particularly, the increase in forest habitats after the massive loss of vegetation that happened at the K/T boundary is thought to have played a major role in enabling adaptive radiations for arboreal taxa. Truly arboreal species of frogs are limited to groups that originated after the K/T boundary, demonstrating how mass extinctions in the past have shaped the current diversity of frogs. However, as past performance is no guarantee for future success, it remains to be seen how frogs will do in the next round of mass extinction.

References

Alroy, J. 1999. The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation. Systematic Biology 48 (1).

Dong, L., Roček, Z., Wang, Y., and Jones, M.E.H. 2013. Anurans from the Lower Cretaceous Jehol Group of Western Liaoning, China. PLoS ONE 8 (12)

Feng, Y-J., Blackburn; D.C., Liang, D., Hillis, D.M., Wake, D.B., Cannatella, D.C., and Zhang, P. 2017. Phylogenomics reveals rapid, simultaneous diversification of three major clades of Gondwanan frogs at the Cretaceous – Paleogene boundary. PNAS 114(29)

Ksepka D.T., Stidham, T.A., and Williamson, T.E. 2017. Early Paleocene landbird supports rapid phylogenetic and morphological diversification of crown birds after the K–Pg mass extinction. PNAS 114 (30)