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Why was a new phylum created for beardworms in the 20th century?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

A new phylum, Pogonophora, was established for beardworms in the 20th century because their unique biological characteristics made them difficult to classify within existing groups. Initially, after the discovery of species such as Siboglinum weberi (1914) and Lamellisabella zachsi (1933), scientists recognized these marine worms as distinct due to their sedentary, tube-dwelling lifestyle and their lack of a mouth and digestive tract in the adult stage.1

In 1937, the class Pogonophora was created for Lamellisabella, and by 1955, after researchers proved a close affinity between Siboglinum and Lamellisabella, the group was elevated to the status of a phylum. This classification reflected the uncertainty regarding their systematic relationships, as they exhibited obscure segmentation and a dorsal nervous system that distinguished them from other polychaetes. However, this classification was later revised; following DNA sequencing in the late 1990s, beardworms were reclassified into the family Siboglinidae within the phylum Annelida by 2001.123