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One of the most unusual baleen whale species alive today is part of a lonely lineage that stretches back up to 20 million years. The species may be the last descendant of an ancient family of whales.

The pygmy right whale (Caperea marginata) 鈥� native to ocean waters in the southern hemisphere 鈥� has long puzzled researchers. Growing to only 6.5 metres long, it is the smallest baleen whale species, says at the Museum of New Zealand Te Papa Tongarewa.

Pygmy right whales are also anatomically distinct, with a proportionally small head and ribs that are broad and flattened. But because they have long baleen plates and skim surface waters for food much like right whales do, the groups have been considered related.

In recent decades, studies on subsets of genes suggested the whales were closer relations to rorquals like blue and fin whales. But there were some lingering doubts, says Marx, because the studies were only sampling a small part of the genome.

Marx and his colleagues sought to confirm the weird whale鈥檚 familial placement with a more detailed genetic picture. They extracted DNA from kidney tissue taken from a stranded pygmy right whale, sequenced its genome and compared it with those from other baleen whales.

The team found that rather than grouping closely with right whales, the pygmy right whale stands alone in an old group that instead diverged from rorqual whales between about 16 million and 20 million years ago.

鈥淭he findings put a final nail in the coffin of the idea that the pygmy right whale is more closely related to the balaenids 鈥撀� right whales and the bowhead,鈥� says at the Natural History Museum in London, who was not involved in the research. The whales鈥� shared feeding styles and head shape are thus the product of evolution working in parallel.

The findings align with that was published in April 2023 from a different research group.

at the National Museum of Natural History in Washington DC says this aligns with what other genetic studies have been pointing to.

Given the timing of Caperea鈥檚 divergence from other whales and certain features of its skeleton, Marx and his colleagues think the tiny titans could be the sole surviving relatives of the cetotheres 鈥� a family of mostly small whales that were once hunted by giant sharks like Otodus megalodon and thought to have disappeared a couple million years ago.

鈥淚 personally think this quite likely, and am convinced by the morphological evidence that has been presented in previous papers,鈥� says Park, but notes it would be great for gaps in the fossil record to be filled showing a physical transition between ancestral cetotheres and Caperea.

Marx thinks that pygmy right whales may have survived into modern times by becoming specialised in a lifestyle differing from their ancestors, be they cetotheres or another lost family. For instance, many ancient sperm whales were large-toothed predators of marine mammals, but today鈥檚 few remaining sperm whale species feed using suction. Similarly, says Marx, pygmy right whales鈥� skim-feeding strategy may have allowed them to escape extinction.