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
|