The Echiura, or spoon worms, are a small group of marine animals. They are considered a group of annelids, though they lack the segmented structure found in other members of that group, and are often treated as a separate phylum. The Echiura fossilize poorly and the earliest known specimen is from the Pennsylvanian (called the upper Carboniferous outside North America). However, U-shaped fossil burrows that could be Echiuran have been found, dating back to the Cambrian.