Molecular phylogeny suggests Mysticeti split from Odontoceti (toothed whales) between 26 and 17 million years ago between the late Oligocene or middle Miocene, but the earliest Mysticeti fossils date to at least 34 million years ago. Their evolutionary link to archaic toothed cetaceans (Archaeoceti) remained unknown until the extinct "Janjucetus hunderi" was discovered in the early 1990s in Victoria, Australia. While, unlike a modern baleen whale, "Janjucetus" lacked baleen in its jaw, the anatomy shows sufficient similarity to baleen whales. It appears to have had very limited apparent biosonar capabilities. Its jaw contained teeth, with incisors and canines built for stabbing and molars and premolars built for tearing. These early mysticetes were exceedingly small compared to modern baleen whales, with species like "Mammalodon" measuring no greater than . It is thought that their size increased with their dependence on baleen. However, the discovery of a skull of the toothed "Llanocetus", the second-oldest mysticete, yielded a total length of , indicating filter feeding was not a driving feature in mysticete evolution. The discovery of "Janjucetus" and others like it suggests that baleen evolution went through several transitional phases. Species like "Mammalodon colliveri" had little to no baleen, while later species like "Aetiocetus weltoni" had both baleen and teeth, suggesting they had limited filter feeding capabilities; later genera like "Cetotherium" had no teeth in their mouth, meaning they were fully dependent on baleen and could only filter feed. However, the 2018 discovery of the toothless "Maiabalaena" indicates some lineages evolved toothlessness before baleen.