A type of algae, called diatoms, have been key to the evolution of the diversity of whales, according to a new study.
The research by Felix Marx of the University of Otago in New Zealand and Dr Mark Uhen of George Mason University in the US is published today in the journal Science.
"The fossil record clearly shows that diatoms and whales rose and fell in diversity together," says Marx, whose research was part of a PhD project under the supervision of Associate Professor Ewan Sordyce.
Marx and Uhen looked at the diversity of dolphins and whales (cetaceans) in the fossil record dating back 30 million years.
They then compared this with records of climate change and estimates of various food sources in the ocean.
Marx and Uhen measured the abundance of two different types of algae: nanoplankton and diatoms, which are key "primary producers" of the ocean - converting sunlight into food.
They found diatoms were the key to cetacean diversity: The greater the diversity of diatoms found in the fossil record [a proxy for diatom abundance] the greater the diversity in species of whales and dolphins, says Marx.
Short food chain
Marx says the importance of diatoms is linked to their larger size, compared to nanoplankton.
The larger the primary producer, the fewer the links in the food chain between it and the top predator, and less energy is lost on the way.
This suits a whale, says Marx.
"You get a relatively large diatom, a krill can come along and eat the diatom and then a whale can come along and eat the krill. So you have two steps in the food chain," he says.
The food chain based on nanoplankton by contrast is longer and more energy is lost on the way to the top predator. This means the top predator has to eat a lot more to survive.
"If you have a really long food chain, with really small primary producers, you can't sustain large top predators like whales because they just couldn't get enough energy out of that," says Marx.
Marx says one factor that would have boosted diatom diversity in the past was continental drift - in particular the isolation of Antarctica.
He says this led to the onset of the Antarctic Circumpolar Current 30 to 40 million years ago, which brings up nutrient-laden water from the deep, and fertilises the diatoms.
Influence of climate
Previous research has found that in the modern ocean, there is an optimum ocean temperature for cetacean diversity.
"Today it's around 21°C that you get the most cetaceans," says Marx.
Based on this Marx and Uhen reasoned that they should find a relationship between ocean temperatures and cetacean diversity looking back in time.
But, using oxygen isotope records as a chemical fingerprint of oceanic temperature and global ice volumes through time, they found no straightforward relationship between climate change and cetacean diversity.
Rather, the effect of climate depended on the abundance of diatoms, says Marx.
Nonetheless, he says, the findings suggest it will be important for scientists to consider the role of diatoms when modelling the long-term effect of climate change on cetaceans.
Fossil record confirmed
While it is well known that ancient cetaceans evolved from four-legged land animals, Marx says there have been many questions over the evolution of whales since then.
One of the problems is that scientists weren't sure whether the fossil records for cetaceans was biased by the amount of preserved sedimentary rock at different time periods through history.
After all, the more rock we can search the more fossils we find, says Marx.
He says the new research helps show that the diversity reflected in the preserved rock is not biased, but indeed representative of the evolution of cetaceans.