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Xidazoon
''Xidazoon stephanus'' is an extinct vetulicolian chordate described by Shu, et al. (1999) based on fossils found in the Qiongzhusi (Chiungchussu) Formation, Yu'anshan Member (Eoredlichia zone), Lower Cambrian, Haikou, (Kunming), about 50 km west of Chengjiang, China.
The fossils show that the body of the animal was divided into two parts. The anterior part of the body is moderately inflated, with a prominent mouth circlet. It has faint transverse divisions towards the front, but is otherwise smooth. The mouth circlet consists of about 30 plates divided into inner and outer regions. The anterior section has five structures on each side, which are interpreted as gills. A dark region running close to the ventral and posterior margins is interpreted as an endostyle. The condition of the anterior portion of the fossils suggests that it was thin-walled, i.e., that the anterior portion was largely hollow. The posterior part of the body tapers towards front and back (diamond-shaped), and is divided into seven segments covered in cuticle with three less well-defined segments at the anterior end.〔Shu Degan. (2005) "On The Phylum Vetulicolia." ''Chinese Science Bulletin.''〕 There are short spines at the posterior tip. The authors describe an alimentary canal with terminal openings and a rectum with what might be dilator muscles.
Based on a comparison of the incomplete/damaged holotype with the incomplete/damaged holotype of ''Pomatrum ventralis'', researchers Aldridge, et al., proposed that ''X. stephanus'' was a junior synonym of ''P. ventralis'' because the anterior portions of the two species are largely identical.〔Aldridge, et al. (2007) "The Systematics and Phylogenetic Relationships of Vetulicolians." ''Paleontology.'' Volume 50〕 However, other researchers point out that, although the anterior portions may be extremely similar, the posterior portions of the carapaces differ in shape and width, and, more importantly, the tails differ greatly, in that in ''X. stephanus'', it is composed of seven segments and leaf- or diamond-shaped, while in ''P. ventralis'', the tail is oar-shaped, and is composed of many more segments, especially near its base.〔Shu Degan. (2005) "On The Phylum Vetulicolia." ''Chinese Science Bulletin.''〕
==See also==
* Maotianshan shales
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
■ウィキペディアで「Xidazoon」の詳細全文を読む
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