Frontal view from below of the skull of a Llanocetus denticrenatus in the Sant Hall of Oceans in the Smithsonian Museum of Natural History in Washington, D.C. The name is a tribute to Dr. George Llanos, and is combined with the Latin name for whale ("cetus"). "Denticrenatus" means "small-toothed." It is an intermediate form between toothed and baleen whales.

Llanocetus denticrenatus is the oldest known mysticete (or baleen whale). It was discovered in the La Meseta Formation on Seymour Island in Antarctica in 1989. Only the skull has been unearthed so far; the skeleton has yet to be fully unearthed and described. It probably lived 34 to 35 million years ago in colder seas near the Antarctic. It had tiny peg-like teeth which jutted out in a fan-like spread from a larger tooth (which was covered over by the gums). From these teeth grew primitive baleen (stuff like your fingernails are made of). These baleen-growing teeth were very widely separated within the jaw.

The skull is long and narrow, somewhat looking like a dolphin's. The upper jaw is exceptionally slender (more so than the lower jaw), and the lower jaw is exceptionally wide at the rear.