杏吧原创

Bowhead and right whales are among the biggest animals alive today, but a new look at how they evolved suggests that these two closely related groups ballooned in size independently, and probably for different reasons.

Both are balaenids, a type of baleen whale (a classification that includes the gigantic blue whale), but they aren鈥檛 shaped like other baleens. They have stocky bodies and immense heads that can take up a third of their body length, and their mouths are the largest on the planet, equipped with great sheets of hair-like bristles called baleen that they used to filter food from the water.

Equations often used to estimate the size of ancient, extinct baleen whales didn鈥檛 take into account these peculiar proportions, says Michelangelo Bisconti at the University of Turin in Italy.

鈥淏alaenids are fundamental [ecological] players that shape the energy flow in the ocean ecosystems,鈥� says Bisconti, so understanding how they evolved is crucial for unravelling ocean prehistory.

Now, he and his colleagues have analysed the proportions of living and fossil balaenid whale species, estimating and mapping changes in size and shape to different time points in their evolutionary history and ancient environmental changes.

The team found that right whales (Eubalaena) expanded first 鈥� to more than 14 metres long 鈥� about 6 million years ago, possibly as a result of a new influx of stable plankton food in their environment. But bowheads (Balaena) grew large a few million years later, possibly as an adaptation to the Arctic habitat, which cooled considerably 3 million years ago.

The findings show that some baleen whales may have evolved extreme sizes due to differing environmental pressures, even among closely related species.

Felix Marx at the Museum of New Zealand Te Papa Tongarewa in Wellington says seeking environmental explanations for this pattern isn鈥檛 something that has been explored for balaenids before, but he notes that there is a big gap in the balaenid fossil record between 7 and 20 million years ago.

鈥淚f you were to find a fossil now that鈥檚 say, 10 or 12 million years old, it could change that whole story,鈥� says Marx. Bisconti says he is hopeful that such fossils of these odd whales will turn up, shedding further light on their evolution.

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