Even after dino dieoff, our mammal forebears laid low
Mar 30,2007 00:00 by Bend_Weekly_News_Sources

When the cat's away, the mice will play. And for some­what si­m­i­lar rea­sons, bi­ol­o­gists have long be­lieved that the ex­tinc­tion of di­no­saurs caused the great flour­ish­ing of mam­mals on Earth - a pro­cess that pro­duced spe­cies in­clud­ing ours.

That's not quite the way things hap­pened, a study has found.

 
A Cape Hy­rax (Pro­ca­via ca­p­en­sis), a small Af­ri­can mam­mal that looks like a ro­dent but is ac­tu­al­ly re­lat­ed to ele­phants. Their com­mon an­ces­tor lived 83 mil­lion years ago, long be­fore the di­no­saurs died out. - Photo: Rich­ard Gren­yer 
A com­plete new fam­i­ly tree trac­ing the his­to­ry of all Earth's 4,500 mam­mals shows they did­n't start to di­ver­si­fy right af­ter the di­no­saurs' de­mise, as con­ven­tion­al wis­dom holds, re­search­ers say. Rath­er, the pro­cess took at least 10 mil­lion years to start in ear­nest.

The sci­en­tists, with Im­pe­ri­al Col­lege Lon­don and the Zo­o­log­i­cal So­ci­e­ty of Lon­don, de­scribed the find­ings in the March 29 is­sue of the re­search jour­nal Na­ture.

They found that many of the ge­net­ic an­ces­tors of the mam­mals liv­ing to­day ex­isted 85 mil­lion years ago, and large­ly sur­vived a me­te­or crash thought to have killed the di­no­saurs 65 mil­lion years ago. Through­out the Cre­ta­ceous era, when di­no­saurs reigned, these mam­mal spe­cies had been rel­a­tively few, pre­sum­a­bly blocked from di­ver­si­fying and evolv­ing in di­no­saur-dominated habi­tats.

The fam­i­ly tree in­di­cates that af­ter the mass ex­tinc­tion, some mam­mals did un­der­go a quick di­ver­si­fi­ca­tion and ev­o­lu­tion, the sci­en­tists said. But most of these groups have since ei­ther died out, such as An­drewsarchus—an ag­gres­sive wolf-like cow—or de­clined in di­ver­si­ty, such as the group con­tain­ing sloths and ar­madil­los. 

The re­search­ers con­tend that our ac­tu­al "an­ces­tors," and those of liv­ing mam­mals, be­gan to di­ver­si­fy around the time of a sud­den in­crease in the tempe­rature of the plan­et—10 mil­lion years af­ter the di­no­saur dis­as­ter.

An­dy Purvis of Im­pe­ri­al Col­lege said: "For the first 10 or 15 mil­lion years af­ter the di­no­saurs were wiped out, pre­s­ent-day mam­mals kept a very low pro­file, while these oth­er types of mam­mals were run­ning the show. It looks like a lat­er bout of 'global warm­ing' may have kick-started to­day's di­ver­si­ty—not the death of the di­no­saurs.

"This discovery rewrites our un­der­stand­ing of how we came to evolve on this plan­et, and the study as a whole gives a much clear­er pic­ture than ev­er be­fore as to our place in na­ture."