Xidazoon stephanus[1] Pomatrum is an extinct vetulicolian, the senior synonym of Xidazoon; the latter taxon was 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.

A dark region running close to the ventral and posterior margins is interpreted as an endostyle.

The authors describe an alimentary canal with terminal openings and a rectum with what might be dilator muscles.

Pomatrum is a member of the Didazoonidae, which one 2024 study has found to be a paraphyletic grade of the vetulicolians closest to crownward chordates as shown in this simplified cladogram:[4] Banffia† Heteromorphus† Skeemella† Nesonektris† Vetulicolidae† Didazoon† Pomatrum† Yuyuanozoon† Yunnanozoon† Cathaymyrus† Pikaia† Chordata(crown group) An earlier study in 2014 place vetulicolians as the sister-group to tunicates, but was unable to resolve any relationships among vetulicolians as a group:[5] Vetulicola† Beidazoon† Ooedigera† Pomatrum† / Xidazoon† Didazoon† Yuyuanozoon† Banffia† Heteromorphus† Nesonektris† Tunicata Based on a comparison of the incomplete/damaged holotype with the incomplete/damaged holotype of Pomatrum ventralis, a comprehensive examination of the vetulicolian systematics published in 2007 accepted a 2002 proposal that X. stephanus was a junior synonym of P. ventralis because the anterior portions of the two species are largely identical.

[1] This proposal has been accepted by a number of other workers,[5][8] although in some cases, notably including one co-authored by all authors of the 2007 comprehensive examination, only tentatively.