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This week let's learn about a weird marine worm and its extinct ancestor!

Further reading:

Eunice aphroditois is a rainbow, terrifying

The 20-million-year-old lair of an ambush-predatory worm preserved in northeast Taiwan

Here's the money shot of the sand striker with its jaws open, waiting for an animal to get too close. The stripy things are antennae:



The fossilized burrow with notes:



Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week we’re going back in time 20 million years to learn about an animal that lived on the sea floor, although we’ll start with its modern relation. It’s called the sand striker and new discoveries about it were released in January 2021.

Ichnology is the study of a certain type of trace fossil. We talked about trace fossils in episode 103, but basically a trace fossil is something associated with an organism that isn’t actually a fossilized organism itself, like fossilized footprints and other tracks. Ichnology is specifically the study of trace fossils caused by animals that disturbed the ground in some way, or if you want to get more technical about it, sedimentary disruption. That includes tracks that were preserved but it also includes a lot of burrows. It’s a burrow we’re talking about today.

Because we often don’t know what animal made a burrow, different types of burrows are given their own scientific names. This helps scientists keep them organized and refer to a specific burrow in a way that other scientists can immediately understand. The sand striker’s fossilized burrow is named Pennichnus formosae, but in this case we knew about the animal itself before the burrow.

The sand striker is a type of polychaete worm, and polychaete worms are incredibly successful animals. They’re found in the fossil record since at least the Cambrian Period half a billion years ago and are still common today. They’re also called bristle worms because most species have little bristles made of chitin. Almost all known species live in the oceans but some species are extremophiles. This includes species that live near hydrothermal vents where the water is heated to extreme temperatures by volcanic activity and at least one species found in the deepest part of the ocean that’s ever been explored, Challenger Deep.

A polychaete worm doesn’t look like an earthworm. It has segments with a hard exoskeleton and bristles, and a distinct head with antennae. Some species don’t have eyes at all but some have sophisticated vision and up to eight eyes. Some can swim, some just float around, some crawl along the seafloor, and some burrow in sand and mud. Some eat small animals while others eat algae or plant material, and some have plume-like appendages they use to filter tiny pieces of food from the water. Basically, there are so many species known—over 10,000, with more being discovered almost every year, alive and extinct—that it’s hard to make generalizations about polychaete worms.

Most species of polychaete worm are small. The living species of sand striker generally grows around 4 inches long, or 10 centimeters, and longer. We’ll come back to its size in a minute. Its exoskeleton, or cuticle, is a beautifully iridescent purple. It doesn’t have eyes, instead sensing prey with five antennae. These aren’t like insect antennae but look more like tiny tentacles, packed with chemical receptors that help it find prey.

The sand striker lives in warm coastal waters and spends most of its time hidden in a burrow in the sand. It’s especially common around coral reefs. While it will eat plant material like seaweed, it’s mostly an ambush hunter.

At night the sand striker remains in its burrow but pokes its head out with its scissor-like mandibles open. When the chemical receptors in its antennae detect a fish or other animal approaching,