Episode 91 – Frogs

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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
A small sample of frog diversity. Top left: Argentine horned frog. Image: Max [CC BY-SA 4.0]; Top right: One of many species of rain frog. Image: Boistel et al. 2011 [CC BY 2.5]; Bottom left: Poison dart frog. Image: Ltshears [CC BY-SA 3.0]; Bottom right: Orange-thighed frog tadpole. Image: Rainforest_harley [ CC BY-SA 2.0]
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.

Skeleton
Part of the job of a vertebrate paleontologist is getting to know skeletons, and frogs have very strange skeletons. Their spines are unusually short (usually only 6-10 vertebrae) and they have no ribs; their leg, toe, and ankle bones are long and often hollow; their hips are jointed for flexibility and braced for strength. Frogs use these unusual bodies for swimming, climbing, and burrowing, but their skeletons are particularly well-adapted for saltatory locomotion – jumping!

Amphibian Origins

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.

Gero
Early Permian Gerobatrachus, the so-called “frogamander,” is not a frog or salamander or even a true lissamphibian, but many paleontologists think it and its close temnospondyl cousins are very near the ancestors of frogs and other living amphibians. Image: Nobu Tamura [CC BY-SA 3.0]
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.

Triad
The rock in this photo was split to reveal the top and bottom of Triadobatrachus, which is considerd by many to be the oldest known batrachian lissamphibian, making it a close relative or ancestor of frogs and salamanders. Image: Ascarrunz et al 2016 [CC BY 3.0]
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.

CretFrogs
Left: Electrorana, the oldest known rain forest frogs, discovered in Burmese amber, ~100 million years old. Image: Xing et al 2018 [CC BY 4.0]. Right: Two specimens of the fossil tadpole Shomronella, from Israel. As of this writing, this site yields the oldest tadpoles described in scientific literature. Image: Roček & Van Dijk 2016 [CC BY-SA 4.0]
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.

Beelz
Beelzebufo, the “Devil toad” from Late Cretaceous Madagascar is a fossil relative of modern-day horned frogs. It gets its name from it’s enormous size and bony, ornamented skull. It also has the best scientific name of any organsm (says David). Image: Nobu Tamura [CC BY 3.0]
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.

The latest chapter in the story of frogs is a grim one. During our modern biodiversity crisis, frogs and other amphibians are among the most vulnerable animals.

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.

Gardner 2019. The Fossil Record of Tadpoles.

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