The generic name refers to the dragon Ancalagon, who was mentioned in The Silmarillion by J. R. R. Tolkien. In The Silmarillion, Ancalagon was described as being one of the more powerful servants of the malevolent being, Morgoth, having been bred to be the fiercest, mightiest, and largest dragon in all of Middle-earth. According to Tolkien, "Ancalagon" translates from Sindarin as being anc 'jaw', alag 'impetuous'.^[3] The species name, "saurognathus," translates as "lizard jaw."

Leigh Van Valen studied the New Mexico mesonychid Dissacus saurognathus and eventually came to the conclusion that D. saurognathus differed enough from the sympatric D. navajovius, and from other members of the genus to merit its own genus. Van Valen attempted to erect the new genus "Ancalagon" for "Dissacus" saurognathus in 1978.^[4] However, he later found that the name was already occupied by Ancalagon minor, a Middle Cambrian priapulid worm described in 1977 by Simon Conway Morris. As a result, he renamed "Ancalagon" saurognathus to Ankalagon saurognathus in 1980.^[5]

The main feature that distinguishes A. saurognathus from the ancestral Dissacus species is its size: Ankalagon grew to be as large as a bear,^[6] as compared to the coyote or jackal-sized species of Dissacus. In fact, the only North American mesonychids that surpassed Ankalagon in size were the larger species of the Early Eocene genus, Pachyaena, such as P. gigantea and P. ossifraga, which, too, grew to the size of bears.^[6]

Evidence of sexual dimorphism comes from an analysis of tooth and jaw size in two specimens of this genus, compared with unrelated, extant Carnivora (where the body size and sex of living individuals can be recorded and compared with their tooth size). Though the two Ankalagon jaws are of very different sizes, and one has much larger canines, the first two molars are close to the same size. As the size of the first two molars varies little in individuals of the same species even when body size is different, the study suggests two conclusions: first, that these two adult individuals are the same species, so Ankalagon had significant sexual dimorphism. Second, that size/weight estimates of Ankalagon and other fossil mammals with sexual dimorphism may not be trustworthy, since estimates are usually made with a formula based on the size of the first two molar teeth, and these teeth may be the same size in individuals of much different body sizes.^[2]

The strong jaw musculature specialized for up-and-down biting rather than side-to-side grinding movement, and the triangular, laterally compressed premolars and molars with carnassial notches of Ankalagon are typical of mesonychids. Though no living group of animals has similar structures, these features suggest that A. saurognathus was carnivorous. Paleontologists believe that mesonychids would not have been able to slice meat as effectively as other carnivorous animals, but large genera like Ankalagon would have used their pointed teeth to grab a chunk of meat and their unusually strong jaw muscles to pull it free from a large carcass, perhaps bracing it with their front feet.^[7] Whether the genus was active hunters, scavengers, or both is unknown.

The evidence of sexual dimorphism in Ankalagon suggests that they formed either permanent social groups (like a pride of lions) or temporary associations (like a lek of grouse) of one male and several females; all living species of Carnivora as well as Primates that show sexual dimorphism in canine size form mating groups of this type.^[2]