Yunnanozoon lividum, Earliest Known Hemichordate from Chengjiang
"in association with Eldonia eumorpha"

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. At one point Yunnanozoon was thought to be a member of the Phylum Chordata, but has been reassigned as mentioned above. Yunnanozoon is similar to the form Haikouella, which is almost certainly a vertebrate.

This one comes from the site of the original discovery of the Chengjiang Biota, Maotianshan (Mao Tian Hill). 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.

The second animal was originally described as a medusoid under the name Stellostomites eumorphus, but subsequently assigned as a species of Eldonia, a genus represented in the somewhat younger Burgess Shale Fauna by Eldonia ludwigi. Some believe there is sufficient difference between them to warrant a new genus for the Chengjiang material. Just as its affinities are enigmatic, so too is its lifestyle. Its medusoid shape is suggestive of a pelagic lifestyle, but other researchers think it led a benthic existence, passively lying on the mud surface. This taxon has only been recorded from the Chengjiang Biota.

Also see: Chengjiang Biota List Chengjiang Fossils