Scientists from the University of Otago in New Zealand have created one of the most comprehensive reports detailing the evolutionary history of the baleen whale, the largest animal to have ever lived.

In their research, Dr. Felix Marx and Professor R. Ewan Fordyce provide a complete report discussing the family tree of the baleen whales, including both living and extinct species, within the last 40 million years.

While there have been other attempts at creating a comprehensive family tree before, Marx and Fordyce assert that their study is the largest and most complete, directly calibrating their data using many dated fossils. It shows which of the whales are related and the exact period when every branch of the family tree—whether extinct or still living—first came into existence.

Through the help of this new family tree, researchers can now provide estimates on several factors. The first is how many types of baleen whales have existed; the second is on the similarities and differences between various ancestries of the baleen whale when it comes to their overall body shape; and third is how quickly the baleen whales evolved at a given time in the last 40 million years.

"We find that the earliest baleen whales underwent an adaptive radiation, or sudden 'evolutionary burst,' similar to that of 'Darwin's finches' on the Galapagos Islands," Fordyce said.

According to the researchers, the beginning stages of the whale evolution occurred during a period of global cooling. He said that this is also when the Southern Ocean opened, contributing to the rise of a strong current circling the Antarctic. This same current still provides many of the nutrients found in the world's oceans today.

Fordyce stated that the early whales were different from their descendants today. Their teeth were the most noticeable difference, with some whales having well-developed teeth unlike the filters that baleen whales used. The whales that had teeth often hunted for larger prey.

Fordyce explained that the toothed baleen whales eventually vanished after a few million years. The theory is that this was caused by the developing competition between the whales and other toothed marine mammals such as seals and dolphins. This left the filter-feeding whales behind.

The disappearance of the whales happened somewhere between 30 million and 23 million years ago. This coincided with the point in history when the current circling the Antarctic was at its strongest, bringing more nutrients into the ocean and making filter feeding sustainable.

Marx and Fordyce noted that the filter-feeding whales were able to thrive and remain diverse until the number of lineages suddenly fell around 30 million years ago. This was possibly caused by the extinction of the small baleen whale species, leaving only the larger ones we see today.

During the ice ages, the distribution of available food changed, and the shallow water habitats of animals shifted as well until they disappeared.

This prompted the long-distance migration of animals, traveling from their polar feeding areas to their equatorial breeding grounds.

Fordyce pointed out that this phenomenon of long-distance migration is the hallmark of all existing baleen whales.

The study of the University of Otago is published in the Royal Society Open Science.

Photo: Christopher Michel | Flickr