The oldest known chordate, Yunnanozoon lividum Hou et al. 1991,
from the Chengjiang Lagerstatte of Yunnan shows several features in its
anatomy that had not been expected to occur at this stage of evolution. Its
metameric dorsal myomeres were separated by straight myosepta. The notochord
was located ventrad of the muscular blocks instead of being bordered by them.
The pharynx did not contain any filtratory basket but had only seven pairs of
branchial arches. These were composed of rows of minute scleritic segments
that connected the notochord with a rigid ventral trough. The head region was
rather complex in organization and bore a specialized ring-like mouth
apparatus. The presence of sensory organs, perhaps large eyes with sclerotic
rings, is probable. Only in the remarkable elongation of the notochord and
metameric arrangement of oval gonads this early chordate is similar to Branchiostoma.
The anterior part of the muscular blocks of Yunnanozoon resembles a
little the proboscis and collar of enteropneusts and may perhaps be homologous
with these structures, although in Yunnanozoon they are displaced much
behind the mouth. The whole metameric muscular unit is proposed to correspond
to the ‘quilted pneu structure’ of the Ediacaran problematic
fossil Dickinsonia. Monotypic Yunnanozoa classis n., Yunnanozoida ordo
n., and Yunnanozoidae fam. n. are proposed for thei early chordate.