User:Giant Blue Anteater/Yunnanozoon

Note: this is a draft of Giant Blue Anteater's planned edit to the Yunnanozoon article incorporating a new understanding of the species based on new research, which was created as a subpage here so he can make incremental edits until the draft is complete and is ready to replace the content of that article.

Yunnanozoon (Yunnan + Greek ζῷον zôion "animal") is an extinct genus of enigmatic animal from the Lower Cambrian, Chengjiang biota of Yunnan province, China. Since the description of the only known species, Yunnanozoon lividum (from Latin lividum "livid"; together with the generic name meaning "livid animal of Yunnan," with respect to the color of the fossils), in 1991, it was generally understood to be a type of deuterostome, with authors interpreting it as a chordate, a hemichordate, a vertebrate, or a relative of the clade Vetulicolia (which are now known to be chordates themselves through the discovery of Nesonektris). However, a recent analysis of its morphology by Cong et al. has ruled out its classification as a chordate, and whether it is a even a deuterostome is currently unclear; however, it is unmistakably a bilaterian, and thus the authors suggest that any attempt to classify Yunnanozoon should take into consideration a broader bilaterian context.

Description
Yunnanozoon has a bilaterally-symmetrical, fusiform body consisting of an anterior region and a segmented trunk. The anterior region consists of a series of seven paired arch-like structures with combed edges on both sides, enclosed by paired sac-like coverings and attached at the dorsal and ventral ends to anteroposteriorly oriented rods. The arches bear numerous filaments; the anteriormost pair of arches is thicker with some fusion of the filaments; the posterior two pairs and their coverings extend into the axial zone of its body, lying ventral to the axial segments. The ventral rod extends nearly half the length of its body; the posterior portion of the dorsal rod is covered by a convex triangular shield. The dorsal and axial parts of the trunk are segmented, covered by a possible cuticle, the dorsal part of the body comprising about 24 segments. Posterior to the ventral rod is a series of paired circular structures. A ventral coiled gut is connected with the space between the anterior rods and filamentous arches, the latter structure interpreted as a pharynx.

Interpretation
Yunnanozoon 's body plan has been subjected to many different interpretations.

Phylogeny
Yunnanozoon is similar to the form Haikouella. However, it was considered a different genus from Yunnanozoon because of perceived anatomical differences from Haikouella, including a smaller stomach and much larger (1 mm) pharyngeal teeth. It is by no means certain whether Yunannozoon possessed features such as a heart, gills, etc., which are seen in well-preserved specimens of Haikouella. Yunnanozoon somewhat resembles the Middle Cambrian Pikaia from the Burgess shale of British Columbia in Canada. Thirteen pairs of symmetrically arranged gonads have been identified, as have possible gill slits. However, some authors think that Yunnanozoon is closely related to the chordate Haikouella and that Yunnanozoon is probably a chordate rather than a hemichordate. A close relationship between Yunnanozoon and the taxon Vetulicolia has also been proposed.

It is now considered that Haikouella is a junior synonym of Yunnanozoon.