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Baleen Whales’ Ancestors Were Toothy Suction Feeders

Skeletal anatomy provides crucial information on archaeocete-mysticete transition

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The remains of the oldest known baleen whale have risen from the coastal desert of Peru, already known for its extraordinary fossils as the Leviathan. The skeleton have been found inside 36 million years old rocks, and it will help to shed light on the origin and evolution of the mysticete group, which includes the blue whale, the humpback whale, and the right whale. The discovery has been made by an international research group of paleontologists and geologists from the Universities of Pisa and Camerino and the Natural History Museums of Paris, Brussels and Lima and recently published in the international journal Current Biology (DOI: 10.1016 / j. cub.2017.04.026).

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Mystacodon reconstruction A Gennari: This illustration shows two Mystacodon selenensis individuals diving down to catch eagle rays along the seafloor of a shallow cove off the coast of present-day Peru. Reconstruction by Alberto Gennari.

"What we have found is a whale very different from those that now swim in our seas - says Giovanni Bianucci, paleontologist of the Department of Earth Sciences of the Pisa University who participated in the digging and study of the fossil – Our Peruvian whale shows primitive characters as the presence of the hind limbs, even if extremely small (we found a tiny innominate bone), and robust teeth which gave this whale the name "Mystacodon", which means "mysticete with teeth". The name of the species "selenensis" evokes Selene, the goddess of the moon, for Media Luna, the locality where the fossil was discovered. Moreover, this baleen whale, as most of those lived in the past, was much smaller than the living ones: only about 4 meters, few if compared to the 30 and more meters achieved by the blue whale. By studying this skeleton we have come to the conclusion that Mystacodon selenensis probably fed by sucking small preys on sandy bottoms. This type of feeding is supported by the peculiar tooth wear due to the accidental swallowing of sand when catching the prey. Even pectoral fins show a peculiar mobility, probably useful to direct and balance the body when the whale was moving near the bottom. "

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This photo shows members of the excavation team (from the Museo de Historia Natural, Lima and the Muséum National d’Histoire Naturelle, Paris) gathered around a plaster jacket surrounding part of the skeleton of Mystacodon selenensis at the Media Luna locality in the Pisco Basin, Peru. Photo by Giovanni Bianucci.

To understand the real importance of this discovery we must trace back to the origins of the two large, still living, groups of cetaceans: the odontocetes (dolphins, killer whale and sperm whale) and the mysticetes. The odontocetes (toothed whales) have developed a biosonar that allows them to detect prey, such as fishes and squids, even in low light conditions, while mysticetes (baleen whales) have replaced their teeth with baleen to filter small organisms from the water mass or sandy bottoms. These two important 'innovations' have allowed cetaceans to diversify and colonize all marine environments. But at what specific moment of the evolutionary history of whales these two large groups originated is still a mystery. Genetic studies carried out on extant cetaceans suggest that this important event occurred around 40 million years ago, but fossil finds in rocks of this age are very rare and, in fact, the oldest fossil toothed whale known is 'only' 29 million years old, and the oldest baleen whale was, up to our discovery, 34 million years old. Therefore, Mystacodon selenesis, with its 36 million years of age and its primitive characters, such as the presence of teeth and hind limbs, it is very important to fill the gap in the knowledge of the early whale history of this fascinating group of marine mammals.

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This photo shows members of the excavation team digging around the skeleton of Mystacodon selenensis at the Media Luna locality in the Pisco Basin, Peru. Photo by Giovanni Bianucci.

"One of the most important aspects of this research - explains Claudio Di Celma, geologist of the School of Science and Technology of Camerino University, who has been involved in the stratigraphic study of this fossil - was to provide an age as accurate as possible of the specimen. For this reason, many rock samples were collected in the various outcropping layers, including the one containing the whale skeleton. In these samples we found the microfossils which allowed our colleague Etienne Steurbaut to date the whale remains to 36 million years ago".

"This research and others we have been carrying on in the last 10 years and more, always in collaboration with several foreign institutions - concludes Giovanni Bianucci - confirm the extraordinary paleontological importance of the Peru coastal desert and its outstanding fossils. It is a fossil deposit unique at a worldwide scale, that documents in great detail 40 million years of evolution of marine vertebrates".

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Media Luna locality in the Pisco Basin, Peru.

  • 11 May 2017

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