Did you know that every time you go to the beach, you step on millions of dragons? Members of the phylum Kinoryncha, more commonly known as mud dragons, are so small that they live between grains of sand- most are only 1 mm (0.039 in) long! There are currently over 300 recognized species, found throughout the world's beaches and ocean floors.

(Sources - 1, 2, 3)

Kinorhyncha is another phylum of small marine animals that live in mud or sand at all depths. They are commonly called “Mud Dragons”.

Kinorhynchs are segmented, with juveniles having 8 or 9 segments, depending on species, and growing 2 or 3 more with age. Instead of cilia they have hollow, moveable spines along their body and around their head, which they use for locomotion. Like many arthropods, they molt an outer layer of tough cuticle as they grow to adulthood. The head is retractable, and is protected by a set of neck plates called placids when retracted. Some species have simple eyes, but all species use tiny bristles to feel the world around them. They eat diatoms or other organic material found in the mud. They have two sexes which look alike, and larvae are free-living. Most species are around 1 mm or less, though extinct Cambrian species could reach up to 4 cm (1.6 in) long.

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Rotifera: Wheel Animals. Mostly freshwater with some saltwater species, and can live in the water column, within the sediment, or attached to hard surfaces. Some Rotifers even live within the thin layer of moisture covering mosses. Their most distinctive feature is their corona and tentacles, which, when feeding, look like a rotating wheel. Their bodies are entirely soft except for the small jaw-like structures in their throats that help them chew their food. As detritivores, they are important contributors to the nitrogen cycle and breaking down dead organic matter.

Kinorhyncha. This phylum is made up of tiny benthic organisms, known as mud dragons

Priapulida. This phylum is made up of unsegmented marine worms known as penis worms.

Cambrian Explosion #35: Phylum Kinorhyncha

After the slightly unfortunately-shaped priapulids, let's move on to something much safer-for-work: dragons!

More accurately, kinorhynchs, tiny spiky scalidophoran worms with the delightful common name of "mud dragons". These animals weren't even discovered until the mid-1800s and are so small – less than 1mm (0.04") in size – that they're considered to be "meiofauna", wriggling around between grains of sediment using the spines on their heads to pull themselves along.

They're a widespread and abundant phylum with around 300 known living species, but they're also a very understudied group. Only a handful of scientists specialize in them worldwide and most research on them has been done in just the last 60 years, and so there are thought to be many many more species still out there to be discovered.

While these little dragons must have diverged from other scalidophorans at least as far back as the early Cambrian, they have basically no fossil record at all. A ghost lineage of over half a billion years.

Currently the only known exceptions to this are a few incredibly rare fossils from the early Cambrian of China. The recently-discovered Qingjiang fossil deposits (~518 million years ago) include some currently-undescribed specimens up to 4cm long (~1.5"), and if these do turn out to be kinorhynchs they indicate that modern forms may be highly miniaturized versions of much larger ancestors.

And some slightly older fossils from Sichuan, dating to about 535 million years ago, give us another possible ancient mud dragon: Eokinorhynchus rarus.

This species was much smaller than the Qingjiang forms but still twice as big as modern kinorhynchs at around 2mm long (0.8"). It had more body segments than living mud dragons, and a different pattern of spiny armor, suggesting it was probably part of an early stem-kinorhynch lineage rather than a true member of the group.

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Loricifera. This phylum is made up of microscopic animals that attach themselves to gravel. It is one of the most recently discovered groups, first described in the 1980s

Kinorhyncha. This phylum is made up of tiny benthic organisms, known as mud dragons

Part 2 of the InverteFest drawings!

The Kinorhyncha one is referencing the phylum’s nickname of “mud dragons”.

[Image description: Six digital drawings of animals from small phyla.

Picture 1: A Saccoglossus acorn worm curled into a loose spiral.

Picture 2: An Echinoderes kinorhynch with silhouettes of dragon wings drawn on either side of it.

Picture 3: A loriciferan with the caption “Loricifera: Living it up in the anoxic zone!”

Picture 4: A horsehair worm curled into a horseshoe shape, with the caption “Not the actual horseshoe worm, that’s still coming up!”

Picture 5: A blue velvet worm with golden/orange spots crawling on a mossy surface.

Picture 6: The actinotroch larva of a phoronid / horseshoe worm.