Olfaction written in bones: New insights into the evolution of the sense of smell in mammals

(Press-News.org) The sense of smell is vital for animals, as it helps them find food, protect themselves from predators and interact socially. An international research team led by Dr Quentin Martinez and Dr Eli Amson from State Museum of Natural History Stuttgart has now discovered that certain areas of the brain skull allow conclusions to be drawn about the sense of smell in mammals. Particularly significant is the volume of the endocast of the olfactory bulb, a bony structure in the skull that is often well preserved even in very old fossils. This volume is closely related to the number of intact odour receptor genes – an important indicator of olfactory ability. This allows the sense of smell to be estimated even in extinct species such as early whales, sabre-toothed cats or the Tasmanian tiger, also known as the thylacine. The study, which provides a reliable method for reconstructing the sense of smell in extinct mammals, was published in the journal ‘Proceedings of the National Academy of Sciences’ (PNAS).

From the brain to the genes – the connection between anatomy and genomics
It is a major challenge for scientists to understand the development of the sense of smell, especially in long-extinct animals whose behaviour can no longer be observed today. In mammals, the volume of the braincase cortex roughly corresponds to the volume of the brain. The present study shows that the larger the front part of the braincase, which contains the olfactory bulb, the more functional odour receptor genes the animal has – an important indication of the development of the sense of smell. Since the bony braincase is well preserved in many fossils, researchers can reconstruct the development of the sense of smell even in long-extinct species.

‘Our approach – from the brain to the genes – combines the anatomy of the skull with genetic information. This helps us to better understand the evolution of the sense of smell in mammals,’ explains Dr Quentin Martinez, scientist at the State Museum of Natural History Stuttgart and lead author of the study.

Extensive skull analysis: from shrews to elephants
For this comprehensive study, the research team examined skulls from all mammalian orders using computed tomography (CT).

"Our samples ranged from the ten-gram shrew to the five-ton African bush elephant and included the endocranium of elephants, whales, rhinos, primates and many other species. Scanning extremely large skulls in particular required unusual CT scanning equipment and was a technical challenge. Trying to scan an elephant or whale skull can be quite an adventure," says Dr Eli Amson, palaeontologist at the State Museum of Natural History Stuttgart and expert on fossil mammals.

What could extinct mammals smell?
With the help of comprehensive anatomical and genetic studies and detailed analyses of fossils and bones, researchers have been able to assess the olfactory abilities of a wide variety of extinct mammals.

"Among other things, we examined fossils of early whales from the Eocene, sabre-toothed cats and the Tasmanian tiger, as well as other extinct species. We found it particularly exciting that some of the early whales still had a clearly pronounced olfactory bulb. This suggests that they had a good sense of smell – in contrast to today's toothed whales such as dolphins, whose olfactory bulb has shrunk considerably in the course of evolution. Early whales from the Eocene therefore probably had a very good sense of smell," says Dr Quentin Martinez.

A new window on the evolution of the senses
By linking anatomical features of the skull with genetic information, the study provides a better understanding of the development of the sense of smell over the course of evolution. It yields new insights into the lifestyle and ecological adaptations of today's mammals and those that lived millions of years ago. These findings are an important basis for research into sensory evolution and open up new perspectives for interpreting the palaeoecology and behaviour of extinct mammals.

Background:

State Museum of Natural History Stuttgart:
The State Museum of Natural History Stuttgart is a future-oriented research and communication institution. Its research collections, the archives of diversity, comprise over 12 million objects. The museum researches the evolution of life, analyses the biodiversity of various ecosystems and communicates research findings to the public.
http://www.naturkundemuseum-bw.de

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