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Slimy, worm-shaped, and poorly-understood, they’re the most mysterious of the modern amphibians, and they’re full of bizarre surprises from their heads to their butts. This episode, we explore Caecilians.

In the news
The sabertoothed Thylacoleo had weird eyes that somehow worked fine
A new look at the preposterously long neck of Mamenchisaurus
Fossil skull represents the oldest and largest known kangaroo rat
Retraction! Those Eocene army ants aren’t Eocene! Read more here and here.
Huge thanks to our News guests: Sabrina and Garret from I Know Dino!

The Weirdest Amphibians?

Modern amphibians all belong to a group called Lissamphibia, with three subgroups: frogs and toads, salamanders, and the weird worm-like caecilians.

Caecilians live mainly in the tropics of Asia, Africa, and South and Central America, and most of them spend much of their time in the soil eating worms and insects. Compared to frogs and salamanders, caecilians have a narrower range and fewer species (with only about 200 known species), and they’re generally much poorly understood by scientists. They’re also just so strange. Their compact, sturdy heads have reduced eyes and a pair of tentacles they use to sense their surroundings. Their bodies are elongate, limbless, and covered in folds that make them look like earthworms. And most species have no tails! They are anatomically quite unique.

And that’s not to mention the whole eating-the-skin-off-their-mothers thing.

Mysterious Past

Very little is known about caecilian evolution, in part because their fossil record is extremely poor. According to a 2020 review, there are only about a dozen records of caecilians fossils, and most of them are isolated vertebrae.

There are a few exceptional cases, however, which give us insights into early caecilians, namely the Early Cretaceous Rubricocaecilia, the Early Jurassic Eocaecilia, and the Late Triassic Funcusvermis. These fossils demonstrate various stages along the evolutionary pathway that gave rise to caecilians as we known them, progressively developing their characteristic tentacles, reduced eyes, and lack of limbs. These are still just three well-preserved species over a time span of more than 200 million years, but when it comes to a group like caecilians, we take what we can get.

For a long time, there has been debate about where modern amphibians came from. Recent fossil evidence suggests that frogs and salamanders originated from a group of ancient amphibians called dissorophoid temnospondyls, but whether or not caecilians arose within that same group has been unclear. The most recent caecilian fossil discovery, Funcusvermis from 2023, supports a link between caecilians and those dissorophoids, suggesting that they did indeed evolve from the same ancestors as their fellow modern amphibians.

Learn More

Look here to see some of the identifying characteristics of caecilians, including their weird butts.

Caecilians, an overview (semi-technical)
The Amazing Caecilians (semi-technical)

Finding giant caecilians in Ecuador cloudforest

Caecilians might have a venomous bite

New caecilian family discovered in India
Introduced caecilians found in Florida

‘Funky Worm’ Fossil Helps Explain The Mysterious Origins of Amphibians

A review of the fossil record of caecilians (technical, open access)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 3 – Snakes
• Episode 77 – Fins to Feet: the Fish-Tetrapod Transition
• Episode 91 – Frogs
• Episode 151 – Tails

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