Baleen Whales First Evolved Large Body Size in Cold Southern Waters, New Fossil Shows – Sci.News

Baleen whales (mysticetes) include the largest animals on the Earth. How they achieved such gigantic sizes remains debated, with previous research focusing primarily on when they became large, rather than where. Now, paleontologists have described an edentulous baleen whale (chaeomysticete) fossil from South Australia. With an estimated body length of 9 m, it is the largest baleen whale from the Early Miocene. Analysing body size through time shows that ancient baleen whales from the southern hemisphere were larger than their northern counterparts.

Early Miocene mysticete gigantism. Image credit: Ruairidh Duncan

Until now, it was believed that the beginning of the Ice Age in the northern hemisphere about 3 million years ago kickstarted the evolution of truly gigantic baleen whales.

The new study, led by Dr. James Rule from Monash University and Natural History Museum, London, reveals that in fact this evolutionary leap in size happened as early as 20 million years ago and at the polar opposite, in the southern hemisphere.

The major discovery came from research into a 16 to 21-million-year-old fossil cared for in the Museums Victoria collection.

The specimen the front end of the lower jaw of a large edentulous baleen whale was recovered from a cliff face on the bank of the Murray River in South Australia in 1921 but was largely unrecognized in the collection.

In their study, Dr. Rule and colleagues show how whales evolved into gigantic sizes first in the southern hemisphere, not the northern, and have had larger body sizes in the south for their entire evolutionary history some 2030 million years.

The findings underscore the vital importance of the Australian and wider southern hemisphere fossil record for piecing together the global picture of whale evolution.

The previous ruling hypothesis was based on fossils primarily found in the northern hemisphere, but the Murray River whale fossil disrupts that theory.

The southern hemisphere, and Australia in particular, have always been over-looked frontiers for fossil whale discovery, said Dr. Erich Fitzgerald, a paleontologist at the Museums Victoria Research Institute.

Fossil whale finds in the south, like the Murray River whale, are shaking up the evolution of whales into a more accurate, truly global picture of what was going on in the oceans long ago.

The researchers discovered that the tip of the baleen whale jaw is scalable with body size.

They estimated the length of this baleen whale to be around 9 m.

The largest whales alive today, such as the blue whale, reach the length of a basketball court, Dr. Rule said.

Around 19 million years ago the Murray River whale, at 9 m long, was already a third of this length. So, baleen whales were well on their way to evolving into ocean giants.

The results appear in the Proceedings of the Royal Society B.

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James P. Rule et al. 2023. Giant baleen whales emerged from a cold southern cradle. Proc. R. Soc. B 290 (2013): 20232177; doi: 10.1098/rspb.2023.2177

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Baleen Whales First Evolved Large Body Size in Cold Southern Waters, New Fossil Shows - Sci.News