"Long before it's in the papers"
January 27, 2015


Impact that killed dinos may have nearly knocked off mammals, too

Dec. 17, 2014
Courtesy of Pensoft Publishers
and World Science staff

The di­no­saurs’ extinction 66 mil­lion years ago is thought to have opened the way for mam­mals to dom­i­nate the land. But a new study claims many of them died off too.

“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,” said Steve Bru­satte of the Uni­vers­ity of Ed­in­burgh in the U.K., one of the au­thors of a re­port on the find­ings.

A cast of a fossil of Asiatherium reshetovi, a meta­ther­ian species that lived mil­lions of years ago. The white line at low­er left marks one cen­ti­me­ter (0.4 inches). (Credit: Thom­as Wil­liam­son)

Among mam­mals, the study ar­gues, the brunt of the dis­as­ter seems to have hit a group known as me­ta­the­ri­ans—extinct rel­a­tives of liv­ing mar­su­pi­als (“mam­mals with pouch­es,” such as opos­sums and kan­ga­roos.) These thrived in the shad­ow of the di­no­saurs dur­ing the Cre­ta­ceous pe­ri­od, just be­fore the ex­tinc­tion. 

The stu­dy, pub­lished in the re­search jour­nal Zoo­keys, finds these once-abun­dant mam­mals nearly fol­lowed the di­no­saurs in­to ob­liv­i­on.

When a 10-km (6-mile)-wide as­ter­oid struck what is now Mex­i­co, un­leash­ing a glob­al cat­a­clysm, some two-thirds of all me­ta­the­ri­ans liv­ing in North Amer­i­ca per­ished, ac­cord­ing to the re­search­ers. These ca­su­al­ties, they said, in­clud­ed more than 90 per­cent of spe­cies liv­ing in the north­ern Great Plains, the best ar­ea in the world for pre­serv­ing lat­est Cre­ta­ceous mam­mal fos­sils.

Me­ta­the­ri­ans, the sci­en­tists added, would nev­er re­cov­er their pre­vi­ous di­vers­ity, which is why mar­su­pi­als are rare to­day and largely re­strict­ed to ar­e­as in Aus­tral­ia and South Amer­i­ca. Tak­ing ad­van­tage of the met­a­the­rian de­mise were the pla­cen­tal mam­mals: spe­cies that give live birth to well-de­vel­oped young. They are al­most eve­ry­where to­day and in­clude eve­ry­thing from mice to men.

“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,” said Thom­as Wil­liam­son of the New Mex­i­co Mu­se­um of Nat­u­ral His­to­ry and Sci­ence, lead au­thor of the paper. 

The study re­views the Cre­ta­ceous ev­o­lu­tion­ary his­to­ry of me­ta­the­ri­ans and pro­vides a family tree for these mam­mals based on the lat­est fos­sil records, which re­search­ers said al­lowed them to study ex­tinc­tion pat­terns in un­prec­e­dent­ed de­tail.

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The extinction of the dinosaurs 66 million years ago is thought to have opened the way for mammals to dominate the land. But a new study shows many of them died off too. “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,” said Steve Brusatte of the University of Edinburgh in the U.K., one of the authors of a report on the findings. Among mammals, the study argues, the brunt of the disaster seems to have hit a group known as metatherians--extinct relatives of living marsupials (“mammals with pouches,” such as opossums and kangaroos.) These thrived in the shadow of the dinosaurs during the Cretaceous period, just before the extinction. The study, published in the research journal Zookeys, finds these once-abundant mammals nearly followed the dinosaurs into oblivion. When a 10-km (6-mile)-wide asteroid struck what is now Mexico, unleashing a global cataclysm, some two-thirds of all metatherians living in North America perished, according to the researchers. These casualties, they said, included more than 90% of species living in the northern Great Plains, the best area in the world for preserving latest Cretaceous mammal fossils. Metatherians, the scientists added, would never recover their previous diversity, which is why marsupials are rare today and largely restricted to areas in Australia and South America. Taking advantage of the metatherian demise were the placental mammals: species that give live birth to well-developed young. They are almost everywhere today and include everything from mice to men. “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,” said Thomas Williamson of the New Mexico Museum of Natural History and Science, lead author of a report on the findings. The study reviews the Cretaceous evolutionary history of metatherians and provides a family tree for these mammals based on the latest fossil records, which researchers said allowed them to study extinction patterns in unprecedented detail.