Yannanozoon lividum, The Earliest Hemichordate
"fossils of the Cambrian Explosion from Chengjiang"

Description: This fine fossil is Yunnanozoon lividum, has been designated as the earliest known hemichordate. Possessing many of the characteristic chordate features and providing an anatomical link between invertebrates and chordates, the Hemichordata is a minor but important phylum in evolutionary biology.

It at one point Yunnanozoon was thought to be a member of the Phylum Chordata, but has been reassigned as mentioned above. This one comes from the Chengjiang deposits near Anning, Yunnan Province, China. The diversity of soft-tissue fossils is astonishing: algae, medusiforms, sponges, priapulids, annelid-like worms, echinoderms, arthropods (including trilobites), hemichordates, chordates, and the first agnathan fish make up just a small fraction of the total.

Numerous problematic forms are known as well, some of which may have represented failed attempts at diversity that did not persist to the present day. As the earliest of the hemichordates, its study casts light upon the early evolution of both the chordates and the vertebrates that sprung from them as the story of life unfolded. The linear structure is the gut, which in some specimens is filled with silt, an indication that it derived sustenance by ingesting the mud of the seafloor.