Problematica
As problematica, or organisms of uncertain systematic position (lat. incertae sedis), we classify the organisms with so specific body morphology or poor preservation that does not enable their exact affiliation into the current biological or palaeontological systematics. Mostly, the poor preservation of all so far available remains cause these troubles. Sometimes, the organisms are relatively well documented but their morphology is too odd that their position in the phylogenetical tree is obscure. A good case is enigmatic animal Tullimonstrum gregarium.
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Česky 
Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]