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An outline of a pygmy right whale compared to a diver. Art by Chris huh, image from Wikipedia.

There’s Something about Caperea marginata

Of all the baleen whales in the sea, the pygmy right whale is arguably the most mysterious. Technically-known as Caperea marginata, this 20-foot cetacean is rarely seen in its South Ocean habitat. And even when a stranded specimen turns up, the whale continues to vex scientists. Molecular and anatomical data conflict about just what sort of baleen whale Caperea marginata actually is, and the frustrating marine mammal has often been regarded as the sole member of its own subfamily. Yet University of Otago whale experts Ewan Fordyce and Felix Marx have proposed a startling solution to the question of the pygmy right whale’s identity.

Zoologists have long known that “pygmy right whale” is a misnomer. As Fordyce and Marx point out in the paper, Caperea marginata “differs from right whales … in its external form and osteological features in all parts of the skeleton.” The small, southern whale just happens to superficially resemble the larger, better-known right and bowhead whales. Indeed, as Fordyce and Marx found when they reviewed a “wealth” of Caperea marginata in New Zealand museum collections, the whale appears to be a cetothere – a variety of baleen whale that was thought to have gone extinct about 2 million years ago.

In particular, the cryptic whale has two distinct ear characteristics that have are only seen among a recently-extinct group of whales called herpetocetines, themselves part of the larger cetothere group. Indeed, when Fordyce and Marx ran an analysis of whale relationships drawing from 166 traits among 23 taxa, Caperea marginata fell out among the cetotheres. A switch in our understanding has resurrected a lineage of whales that was thought to have entirely disappeared.

When cetotheres first evolved is a sticky problem. Their fossil record goes back at least 12 million years, Fordyce and Marx note, but, based on estimates from molecular data, they may have emerged even earlier. Future fossil finds will test the timing estimates. Still, based upon the divergence of other cetotheres, Fordyce and Marx propose that the Caperea marginata lineage split from its relatives around 9 million years ago. Given the general lack of Caperea fossils, they expect that the ancient whales also inhabited the southern hemisphere, where the marine record is patchy and not as well-studied.

But what did Caperea marginata survive while its relatives perished? A significant clue, Fordyce and Marx suggest, may be that the rare whale is a case of evolutionary convergence with true right whales. Caperea marginata didn’t look like other cetotheres and presumably had different habits. This shift in diet and behavior away from its close relatives might have allowed the marine mammal to survive. Even though Fordyce and Marx have discerned the place of Caperea marginata in the whale family tree, paleontologists and zoologists are faced with the new enigma of how this exceptionally rare whale persisted through the climatic and oceanic changes that wiped out its kin.

[Check out this video of a Caperea marginata dissection, featuring Ewan Fordyce.]