Spartobranchus tenuis was an endobenthic (burrowing) enteropneust (or acorn worm) from the Burgess Shale. It is the earliest known enteropneust worm, predating the previous example by 200 Ma. Although common in the Walcott quarry, with a few thousand specimens known, it was nicknamed "Ottoia" tenuis until scientifically described in March 2013.
Unlike living enteropneusts, Spartobranchus made fibrous tubes in the sediment. It looked much like its modern counterparts, except for the fact that it had a posterior bulb that could probably expand to anchor the worm in its burrow so that it would not be extracted by predators such as Opabinia.
Spartobranchus was a hemicordate, and the third hemicordate from the Burgess Shale (one is Chaunograptus, a benthic graptolite, the other was Oesia, a vermiform animal that appears to belong somewhere in hemicordata). Spartobranchus was a deposit feeder and may have played a role similar to modern earthworms.
Although previously thought to have evolved from the colonial pterobranch hemicordates, fossils of Spartobranchus tell us that enteropneusts were present in the Cambrian Period and common in at least one place, the Burgess Shale community. The fact that S. tenuis built tubes suggests pterobranchs evolved from enteropneusts that built similar tubes.
Here are some cool images of the fossils:
Note from The Mom: New blog post?! Art didn't dictate this to me, but wrote it out himself.
Tubicolous enteropneusts from the Cambrian period
Figure 2: Spartobranchus tenuis (Walcott, 1911) individuals associated with tubular structures, from the Burgess Shale.
Burgess Shale worm provides crucial missing link