There are three living groups of amphibians, but one in particular is exceptionally diverse, occupying nearly every landmass on Earth and coming in a dizzying variety of species. From their mysterious origins to their Mesozoic diversity to their modern-day world domination, this episode explores the history of Frogs.
In the news
Colorful insects in amber.
How does metamorphosis affect the evolution of salamanders?
How exactly did the saber-toothed marsupial Thylacosmilus use those teeth?
Venomous caecilians … maybe.
A World Full of Frogs
Frogs are amphibians. Alongside salamanders and caecilians, they are ectothermic (“cold-blooded”), have semi-permeable skin, often live in or near water, typically lay their eggs in water, usually start their lives as larvae that undergo metamorphosis, and do all the other things amphibians do.
Frogs have a number of unique habits: they often get around by hopping, they’re some of the most vocal animals on the planet, and their larval stage – tadpoles – are so dramatically different from the adults that they’ve been called “basically free-living embryos.” But what really sets frogs apart from other amphibians is their diversity. There are over 7,000 living species of frogs on almost every landmass on Earth. They live in rain forests and swamps, mountains and deserts, freshwater and saltwater. They swim in ponds, climb trees, burrow underground, and glide through the air. We live in a world full of frogs.
All modern amphibians – frogs, salamanders, and caecilians – are considered to be closely related to each other, comprising a group called Lissamphibia. But in the past, especially during the Late Paleozoic, the world was home to many other groups of amphibians, and somewhere among that ancient amphibian diversity is the ancestry of Lissamphibia … but we don’t know for sure where.
There are three main hypotheses: 1) that Lissamphibia originated among or adjacent to small amphibamid temnospondyls like Amphibamus and the “frogamander” Gerobatrachus; 2) that Lissamphibia originated instead among the lepospondyls, specifically certain groups with long, caecilian-like bodies; or 3) that Lissamphibia isn’t one closely related group after all, and that frogs and salamanders are temnospondyls while caecilians are lepospondyls. The first hypothesis is the most popular among scientists today, although new research continues to change our ideas.
Part of what makes this so difficult is that the early fossil record of lissamphibians – including frogs – is pretty poor. The earliest true frogs, salamanders, and caecilians in the fossil record are a good 70 million years younger than those possible Paleozoic ancestors.
Frogs Through Time
Triadobatrachus, from Early Triassic Madagascar, is considered by many to be the oldest known lissamphibian. It has a lot in common with frogs, from its peg-like teeth to its short spine to its long hip and foot bones, although it probably wasn’t a jumper. As of this blog post, it’s as close as we have to a well-known Triassic frog, although there’s also the similar Czatkobatrachus from Poland and a recently-described hip bone from Late Triassic Arizona that looks much more like a modern frog.
True frogs show up in the Jurassic. Prosalirus from Early Jurassic Arizona has all the skeletal structures it needs to hop around; Notobatrachus from Middle Jurassic Argentina is known from well over 100 skeletons; and Rhadinosteus from Dinosaur National Monument (Late Jurassic) appears to be a distant relative of modern-day burrowing frogs, just to name a few.
In the Cretaceous, we start seeing many familiar frog families, including relatives of modern-day painted frogs, spadefoot toads, horned frogs, clawed frogs, and more. Specimens in Burmese amber reveal the oldest known rain forest frogs, and more fossils reveal a group called the paleobatrachid frogs which ultimately went extinct only a couple million years ago.
Frogs are ubiquitous and familiar in Cenozoic fossil sites. A 2017 study did a genetic analysis of living frogs, calibrated with fossils, and concluded that three major groups of frogs – Hyloidea, Microhylidae, and Natatanura, together including around 88% of living frog species – first radiated around the end of the Cretaceous Period. It seems that the end-Cretaceous extinction was the impetus for the evolution of global frog diversity as we know it today.
Some More [Technical] Frog Reading
Marjanović and Laurin 2013. Provides an overview of hypotheses about the origins of living amphibians.
Mesozoic Amphibians. This 1994 book chapter is a bit out of date, but provides a nice overview of the frog fossil record.
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