Selkirkia lived in a cone-shaped tube which had a split at the end. This split was the opening that would have allowed feces to go out of the tube. If it did not have the split, feces would have built up inside the tube and eventually poisoned Selkirkia, then killed it. But that's just my hypothesis.
|Click image to see a video of Selkirkia|
When Selkirkia died and rotted away, agnostid trilobites dug up the tube and then sheltered in it. Scientists know that because some specimens of Selkirkia's tube have agnostids in them. The tubes inhabited by agnostids are horizontal, but the tubes with Selkirkia still in them are vertical in the rock.
Selkirkia was very similar to its relative Paraselkirkia, but there was one huge difference between the two. Paraselkirkia had a ring of spines on the head that were pointing backwards. Selkirkia did not have this.