‘X-rated’ Acorn Worms Discovered: “Phallus” Worm Is Evolutionary Missing Link between two lineages of acorn worms, a new study says.

A phallus-shaped worm that lived 505 million years ago is heads above the rest—it’s a “missing link” between two lineages of acorn worms, a new study says.

Dubbed Spartobranchus tenuis, the odd creature is a type of soft-bodied marine animal that’s rarely preserved in the fossil record. The new specimen was first discovered in the early 1900s in an area called the Burgess Shale, a fossil-rich area in Canada‘s Yoho National Park. (Also see“Pictures: New Deep-Sea Worms Found—Have Big ‘Lips.’”)

The new worm species S. tenuis outside their tubes (above) and inside. Illustration by Marianne Collins

But the fossil went mostly unnoticed until a few years ago, when evolutionary biologist Jean-Bernard Caron of the University of Toronto “stumbled on drawers full of these worms” at the Smithsonian Institution in Washington, D.C.

“I said, ‘Oh my gosh.’ I noticed a lot of these worms in bizarre-shaped rings, like mini Michelin tires in the rock,” said Caron, a co-author of the study.

After Caron and colleagues looked more closely at the fossils, they realized the newfound worm “really connects a lot of dots” in the evolution of hemichordates.

Solving an Evolutionary Puzzle

Hemichordates, a group of marine invertebrates that includes S. tenuis, are closely related to modern starfish and sea urchins, as well as to chordates, or animals with backbones—such as primates. (Watch a video of a sea cucumber that fights with its guts.)

Two modern acorn worms, Harrimania planktophilus. The total length of a relaxed and uncoiled animal is approximately 32 millimeters. Photograph courtesy C.B. Cameron

There are two main branches within the hemichordates: enteropneusts and pterobranchs, Caron said. Pterobranchs live in colonies while enteropneusts don’t.

“That has always puzzled evolutionary biologists—what is the common ancestor of the hemichordates?” he said. (See more pictures of marine worms that fire “glowing blobs.”) 

The Burgess Shale region, with Wapta Mountain in the background. Photograph courtesy JB Caron

Now, they’ve found it in S. tenuis, an enteropneust that lived 200 million years before the previous earliest known specimen. 

The giveaway, Caron said, was that S. tenuis fossils were found with tubular structures. Modern-day pterobranchs live in these colonial tubes, but modern-day enteropneusts don’t. Finding the tubes with S. tenuis suggests the tubes were lost as enteropneusts evolved, but were retained over time in the pterobranchs.

These modern pterobranchs live in tubes similar to those preserved in the S. tenuis fossils. Photograph courtesy C.B. Cameron

What’s more, “understanding the origin of chordates can help us understand our own origins,” since we all shared an as yet-unknown worm-like ancestor, noted Caron, whose study was published today in the journal Nature.

Phallic Shape Withstood the Test of Time

S. tenuis lived in a different world—during the Cambrian period, Canada was tropical due to its position near the Equator. Other than that, though, the four-inch-long (10-centimeter-long) creature seems astonishingly similar to modern acorn worms. (See a prehistoric time line.)

“One of the things that blew my mind about this thing is that most animals in the Burgess Shale look nothing like modern-day animals, but this is so clearly an acorn worm,” said study co-author Christopher B. Cameron of the Université de Montréal.

“Except for losing the tube, the animal is virtually unchanged in 505 million years.”

For instance, S. tenuis and modern acorn worms both have flexible bodies with a long, narrow trunk that ends in a bulbous structure, which may serve as an anchor to pull itself backward into its tube quickly if there’s a threat. (See a picture of a mushroom named for its phallic shape.)

An undescribed species of modern acorn worm. Photograph by C.B. Cameron

The scientists also suspect that, like modern acorn worms, S. tenuis “would have been a recycler of organic material—a bit like earthworms in our gardens,” Caron said.

You probably don’t see acorn worms very often, though they’re widespread worldwide and likely burrow under the sand of your favorite beach.

The telltale signs of their presence are tiny sausage-shaped sand pellets that the animals push up to the surface, essentially the garbage from their work filtering the sand, Caron said.

So the next time you’re taking a long walk on the beach, think about the worm relatives busy at work, just under your feet.

Research article:
Osborn, K.J., Kuhnz, L.A., Priede, I.G., Urata, M., Gebruk, A.V., and Holland, N.D. (2011). Diversification of acorn worms (Hemichordata, Enteropneusta) revealed in the deep sea. Proceedings of the Royal Society - B, November 16, 2011; doi:10.1098/rspb.2011.1916