What is the longest tusk of a narwhal actually an elongated tooth of?

The longest narwhal tusk is usually an elongated left upper canine.

Narwhals are toothed whales with a distinctive arrangement of teeth. In most males, the left upper canine grows forward through the upper lip and forms the familiar spiral tusk. It can reach about 3 metres in length. Females usually do not develop a tusk, although some females grow one, and a small number of males develop two.

The tusk is not a horn. It is a tooth with a hard outer layer and a living, sensory-rich interior. Scientists have found nerve pathways that may allow the tusk to detect changes in the surrounding water, including temperature and chemical conditions. Its exact roles in social behavior and competition are still studied.

The right upper canine normally remains small or stays inside the mouth. Narwhals can therefore be mistaken for animals with a single horn, but the structure is anatomically a tooth. The species lives mainly in Arctic waters around Canada, Greenland, and Russia and is closely related to the beluga whale.