The bone involved in sound generation and echolocation - the spermaceti organ - has reduced in size during the whales' evolution, says a study.
The study of rare fossils from extinct pygmy sperm whales found in Panama revealed an unexpected level of complexity in the evolution of the spermaceti organ that plays a key role in the generation of sound, and in the whale's capacity for echolocation.
The new fossils show that at one time, these small sperm whales had a much larger spermaceti organ, which got downsized at least twice during their evolutionary history.
"The new discovery gives us a better understanding of the ancient distribution of these poorly known relatives of the sperm whale," said Jorge Velez-Juarbe from the Natural History Museum of Los Angeles County.
"Previously we knew of similarly-aged pigmy and dwarf whales from Baja California and Peru, but this new fossil fills in an important geographic gap in the group's ancient distribution and natural history," he added.
Many whales, dolphins, and porpoises have a long fossil record, which helps scientists understand their evolution.
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Yet, the limited fossil record of sperm whale and its kin, the smaller pigmy and dwarf whales, has limited the understanding of their evolution.
The new whale species, named Nanokogia isthmia, is known from the well-preserved skulls of two individuals, which remains were unearthed at a sea cliff along the Caribbean coast of Panama and from rock layers, dated to about seven million years ago.
The reasons of this size reduction remain unclear. Scientists hope to find more complete skeletons of Nanokogia and other closely related species to untangle the question.
The study was published in the open-access journal PLOS ONE.