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Mammals could have gone extinct along with the dinosaurs

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Posted December 18, 2014

A new study introduced an interesting twist to the usual story that dinosaur extinction some 66 million years ago gave way to the flourishing of mammals. As a matter of fact, many mammalian species were drastically affected by the 10-kilometre wide asteroid that hit a territory in modern-day Mexico as well.

A cast of the fossil remnants of Asiatherium reshetovi, one of the metatherian species that used to live on the planet millions of years ago. (scale bar: 1cm) Credit: Dr Thomas Williamson

A cast of the fossil remnants of Asiatherium reshetovi, one of the metatherian species that used to live on the planet millions of years ago. (scale bar: 1cm) Credit: Dr Thomas Williamson

The ancestors of present-day marsupials (or ‘mammals with pouches’, such as opossums) are called metatherians. They thrived alongside the great variety of dinosaur species during the Cretaceous period. An international team of researchers has shown that as a result of the asteroid impact at the end of the period, around two-thirds of North American metatherians died out.

This mass extinction meant that the metatherian mammals would not go back to the levels of diversity that characterised the Cretaceous period. As a consequence, there is only a small number of marsupials living today, and even those are mostly found in somehow unusual geographical areas, such as Australia and South America. If it were not for the few mammalian species that survived the devastating environmental aftereffects of the asteroid, mammals could have disappeared off the face of the Earth along with the dinosaurs.

This diagram is showing how severely metatherian mammals were affected when an asteroid hit Earth at the end of the Cretaceous, 66 million years ago. In North America, the number of metatherian species dropped from twenty species within the last million years of the Cretaceous Period, to just three species in the first million years of the Paleogene Period. Credit: Dr Thomas Williamson

This diagram is showing how severely metatherian mammals were affected when an asteroid hit Earth at the end of the Cretaceous, 66 million years ago. In North America, the number of metatherian species dropped from twenty species within the last million years of the Cretaceous Period, to just three species in the first million years of the Paleogene Period. Credit: Dr Thomas Williamson

Dr. Thomas Williamson, who is lead author of the report, and is based at the New Mexico Museum of Natural History and Science, commented: ‘This is a new twist on a classic story. It wasn’t only that dinosaurs died out, providing an opportunity for mammals to reign, but that many types of mammals, such as most metatherians, died out too – this allowed advanced placental mammals to rise to dominance.’

Placental mammals are abundantly common all over the world, and include all sorts of creatures from mice to people. They are characterised by the fact that their young develops extensively within the womb and is hence already quite advanced when born.

Dr. Steve Brusatte, who is an author of the study, and is based at the University of Edinburgh’s School of GeoSciences, said: ‘The classic tale is that dinosaurs died out and mammals, which had been waiting in the wings for over 100 million years, then finally had their chance. But our study shows that many mammals came perilously close to extinction. If a few lucky species didn’t make it through, then mammals may have gone the way of the dinosaurs and we wouldn’t be here.’

The report is published in an open access journal, and also presents the most accurate and extensive metatherian family tree as a result of including the latest fossil finds. The precise evolutionary history and extinction patterns of these fascinating mammals are also discussed in great detail.

Reference: Williamson TE, Brusatte SL, Wilson GP (2014) The origin and early evolution of metatherian mammals: the Cretaceous record. ZooKeys 465: 1-76.

Read more at: Phys.org

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