Ancalagon is a genus of worm known from the Cambrian Burgess Shale, and assigned to the Priapulids. It also bears similarity to the hypothetical ancestor of a group of parasitic worms, the Acanthocephala. Two specimens of Ancalagon (genus) are known from the Greater Phyllopod bed, where they comprise < 0.01% of the community.
Along with the other Cambrian worms Ottoia, Selkirkia, Louisella, Fieldia, Scolecofurca, and Lecythioscopa, the organism may fall into a clade termed the archaeopriapulida, a stem group to the Priapulids proper. However, the morphological similarity of these organisms to their modern cousins is remarkable, especially for the Burgess Shale. A phylogenetic analysis does not provide a great deal of resolution to the relationships between these basal worms.
The genus takes its name from the dragon Ancalagon the black (Which was so large when it died it crushed Thangorodrim - a humongous mountain that was created by Melkor) , who is featured in J.R.R. Tolkien's Middle-earth legendarium, in reference to the worm's strong row of hooks on its proboscis.