Ancalagon (worm)
Ancalagon minor is an extinct priapulid worm known from the Cambrian Burgess Shale. Because it superficially resembles the modern-day internal parasites known as the acanthocephalids or "spiny-headed worms," "A. minor" was once thought to be, or once thought to resemble the hypothetical free-living ancestor of acanthocephalids. Along with the other Cambrian worms such as "Ottoia", "Selkirkia, Louisella", "Fieldia", "Scolecofurca", and "Lecythioscopa", the organism may fall into a clade termed "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 generic name is a homage to the dragon Ancalagon of J.R.R. Tolkien's Middle-earth legendarium, in reference to the worm's prominent rows of hooks on its proboscis. The species was previously placed in the genus "Ottoia", as "Ottoia minor", but was removed by Simon Conway Morris, who noted morphological differences. Morphology. "Ancalagon" had a slender, cyndrical, radially symmetric body averaging 6 centimeters in length. Its proboscis was armed with circum-oral hooks at the anterior. There were about 10  of these hooks, equal in size and with prominent bases. Directly posterior was an unarmed space, followed by posteriorly directed spinose hooks. The trunk is annulated with 0.2 to 0.25 millimeter spacing, and carried rows of setae possibly performing sensory functions. The apparent absence of retractor muscles correlates with "Ancalagon"'s inability to significantly invert its proboscis. The organism was probably a burrowing predator.