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Tree-climbing critter called milestone in mammal evolution

Sept. 7, 2011
Courtesy of the U.S. National Science Foundation
and World Science staff

The ear­li­est known mem­ber of a line­age that in­cludes hu­mans and most other types of mam­mals was a shrew-like crea­ture that likely scur­ried on trees at night as dino­saurs lurked, sci­en­tists have an­nounced.

The find­ings are based on a well-pre­served fos­sil found in Chi­na. A pa­per pub­lished last week in the re­search jour­nal Na­ture claims the ob­ject rep­re­sents a mile­stone in mam­mal ev­o­lu­tion, reached 35 mil­lion years ear­li­er than pre­vi­ously thought.

Artist's re­con­struc­tion of Ju­ra­maia sinen­sis, pic­tured hunt­ing in­sects on a tree fern. (Cre­dit: Mark A. Klin­ger, Car­negie Mus­eum of Na­tur­al His­tory)


The pa­per, by a team of sci­en­tists led by pa­le­on­tol­ogist Zhe-Xi Lu­o of the Car­ne­gie Mu­se­um of Nat­u­ral His­to­ry in Pitts­burgh, de­scribes Ju­ra­maia sinen­sis, a small shrew-like mam­mal that lived in Chi­na 160 mil­lion years ago dur­ing the Ju­ras­sic Pe­ri­od. 

The pa­per adds that Ju­ra­maia, whose tree-climb­ing skills were un­u­sual for mam­mals at the time, fills an im­por­tant gap in the fos­sil rec­ord and helps to cal­i­brate mod­ern, DNA-based meth­ods of dat­ing ev­o­lu­tion.

It’s be­lieved to be the ear­li­est known fos­sil of eu­the­ri­ans, a group that evolved to in­clude all pla­cen­tal mam­mals, which nour­ish their un­born young through a pla­cen­ta. Ju­ra­maia is thought to come from the time when eu­the­ri­an mam­mals branched off from oth­er mam­mals: meta­the­ri­ans, whose de­scen­dants in­clude mar­su­pi­als such as kan­ga­roos, and mono­tremes, such as the plat­y­pus. “Ju­ra­maia, from 160 mil­lion years ago, is ei­ther a great-grand-aunt or a great-grandmoth­er of all pla­cen­tal mam­mals,” Lu­o said.

The fos­sil was found in north­east Chi­na’s Liao­ning Prov­ince. The name Ju­ra­maia sinen­sis means “Ju­ras­sic moth­er from Chi­na.” The fos­sil has an in­com­plete skull, part of the skel­e­ton, and, re­mark­ably, im­pres­sions of re­sid­u­al soft tis­sues such as hair. The com­plete teeth and fore­paw bones en­a­ble pa­le­on­tol­ogists to pin­point that it is clos­er to liv­ing pla­cen­tals on the mam­malian family tree than to the pouched mar­su­pi­als, such as kan­ga­roos, the sci­en­tists said. “Un­der­stand­ing the be­gin­ning point of pla­cen­tals is a cru­cial is­sue in the study of all mam­malian ev­o­lu­tion,” Luo re­marked.

The fos­sil, pre­served on a shale slab, be­longs to the Bei­jing Mu­se­um of Nat­u­ral His­to­ry. (Cred­it: Zhe-Xi Lu­o, Car­ne­gie Mu­se­um of Nat­u­ral His­to­ry)


Mod­ern mo­lec­u­lar stud­ies, such as DNA-based meth­ods, can cal­cu­late the tim­ing of ev­o­lu­tion by an ap­proach called the “mo­lec­u­lar clock” meth­od. But the mo­lec­u­lar clock must be cross-checked and tested by the fos­sil rec­ord.

How and when eu­the­ri­ans branched off from meta­the­ri­ans has posed a quan­da­ry for bi­ol­o­gists be­cause DNA ev­i­dence sug­gested eu­the­ri­ans should have shown up ear­li­er in the fos­sil rec­ord—around 160 mil­lion years ago. But the old­est known eu­the­ri­an was Eo­maia, dat­ed to 125 mil­lion years ago. Ju­ra­maia is thought to pro­vide much ear­li­er fos­sil ev­i­dence to cor­rob­o­rate the DNA find­ings.

“These sci­en­tists have used the rich fos­sil mam­mal rec­ord to test ev­o­lu­tion­ary hy­pothe­ses pro­posed by their col­leagues stu­dying liv­ing mam­mals us­ing ge­net­ic data,” said Chuck Ly­deard, pro­gram di­rec­tor in the En­vi­ron­men­tal Bi­ol­o­gy di­vi­sion at the U.S. Na­tional Sci­ence Founda­t­ion, which funded the re­search.

Ju­ra­maia al­so shows fea­tures that may have helped the eu­the­ri­an new­com­ers sur­vive in a tough Ju­ras­sic en­vi­ron­ment, the re­search­ers said. Its fore­limbs are adapted for climb­ing; since the ma­jor­ity of Ju­ras­sic mam­mals lived only on the ground, the abil­ity to es­cape to the trees and ex­plore the for­est can­o­py might have al­lowed eu­the­ri­ans to ex­ploit an un­tapped niche. “The di­ver­gence of eu­the­ri­an mam­mals from mar­su­pi­als even­tu­ally led to the pla­cen­tal birth and re­pro­duc­tion that are so cru­cial for the ev­o­lu­tion­ary suc­cess of pla­cen­tals,” Lu­o said. “But it is their early adapta­t­ion to ex­ploit niches on trees that paved their way to­ward this suc­cess.”


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The earliest known member of a lineage that includes most types of mammals—including us—was a shrew-like creature whose tree-climbing talents may have helped it avoid dinosaurs, scientists have announced. The findings are based on a well-preserved fossil found in China. A paper published last week in the research journal Nature said the object represents a milestone in mammal evolution, reached 35 million years earlier than previously thought. The paper, by a team of scientists led by paleontologist Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, describes Juramaia sinensis, a small shrew-like mammal that lived in China 160 million years ago during the Jurassic Period. The paper adds that Juramaia fills an important gap in the fossil record and helps to calibrate modern, DNA-based methods of dating evolution. It’s believed to be the earliest known fossil of eutherians—the group that evolved to include all placental mammals, which provide nourishment to unborn young through a placenta. Juramaia is thought to provide fossil evidence of the date when eutherian mammals diverged from other mammals: metatherians, whose descendants include marsupials such as kangaroos, and monotremes, such as the platypus. “Juramaia, from 160 million years ago, is either a great-grand-aunt or a great-grandmother of all placental mammals,” Luo said. The fossil was found in northeast China’s Liaoning Province. The name Juramaia sinensis means “Jurassic mother from China.” The fossil has an incomplete skull, part of the skeleton, and, remarkably, impressions of residual soft tissues such as hair. Juramaia’s complete teeth and forepaw bones enable paleontologists to pinpoint that it is closer to living placentals on the mammalian family tree than to the pouched marsupials, such as kangaroos, the scientists said. “Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution,” Luo remarked. Modern molecular studies, such as DNA-based methods, can calculate the timing of evolution by an approach called the “molecular clock” method. But the molecular clock must be cross-checked and tested by the fossil record. Eutherians’s branching off from metatherians has posed a quandary for biologists because DNA evidence suggested eutherians should have shown up earlier in the fossil record—around 160 million years ago. But the oldest known eutherian was Eomaia, dated to 125 million years ago. Juramaia is thought to provide much earlier fossil evidence to corroborate the DNA findings. “These scientists have used the rich fossil mammal record to test evolutionary hypotheses proposed by their colleagues studying living mammals using genetic data,” said Chuck Lydeard, program director in the Environmental Biology division at the National Science Foundation, which funded the research. Juramaia also shows features that may have helped the eutherian newcomers survive in a tough Jurassic environment, the researchers said. Its forelimbs are adapted for climbing; since the majority of Jurassic mammals lived only on the ground, the ability to escape to the trees and explore the forest canopy might have allowed eutherian mammals to exploit an untapped niche. “The divergence of eutherian mammals from marsupials eventually led to the placental birth and reproduction that are so crucial for the evolutionary success of placentals,” Luo said. “But it is their early adaptation to exploit niches on trees that paved their way toward this success.”