New relative discovered! In this case, a mammal relative from 160 million years ago

It's this little critter, known as Juramaia sinensis; the name means "Jurassic mother from China," though presumably there were males, too:

Katherine Harmon reports in Jurassic Mammal Moves Back Marsupial Divergence Scientific American Observations 08/24/2011:

Although the exact evolutionary path from the early eutherian mammals has yet to be traced, [Zhe-Xi] Luo [of the Carnegie Museum of Natural History] noted that "Juramaia, from 160 million years ago, is either a great-great-aunt or a great-grandmother of all placental mammals that are thriving today."

Science Codex features a press release from the Carnegie Museum ont he discovery: Juramaia sinensis: 160-million-year-old fossil a new milestone in early mammal evolution 08/24/2011.

Harmon reports:

It also had a strange reproductive strategy—at least for that era. Instead of laying eggs (like a "monotreme" relative the platypus) or carrying tiny young to term in a pouch (like a "metatherian," a marsupial, such as a kangaroo), it was a "eutherian," which means that it bore full-term live young like most of today's mammals do, thanks to a nutrient-rich placenta.

Discovering a creature with such a developed placenta from that period was very significant. From the press release:

"Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution," says Luo. The date of an evolutionary divergence - when an ancestor species splits into two descendant lineages - is among the most important pieces of information an evolutionary scientist can have. Modern molecular studies, such as DNA-based methods, can calculate the timing of evolution by a "molecular clock." But the molecular clock needs to be cross-checked and tested by the fossil record. Prior to the discovery of Juramaia, the divergence point of eutherians from metatherians posed a quandary for evolutionary historians: DNA evidence suggested that eutherians should have shown up earlier in the fossil record—around 160 million years ago. Yet, the oldest known eutherian, was Eomaia (Eomaia was originally described in 2002 by a team of scientists led by Zhe-Xi Luo and Carnegie mammalogist John Wible), dated to 125 million years ago. The discovery of Juramaia gives much earlier fossil evidence to corroborate the DNA findings, filling an important gap in the fossil record of early mammal evolution and helping to establish a new milestone of evolutionary history.

Juramaia also reveals adaptive features that may have helped the eutherian newcomers to survive in a tough Jurassic environment. Juramaia's forelimbs are adapted for climbing; since the majority of the Jurassic mammals lived exclusively on the ground, the ability to escape to the trees and explore the canopy might have allowed eutherian mammals to exploit an untapped niche.

Luo supports this perspective: "The divergence of eutherian mammals from marsupials eventually led to placental birth and reproduction that are so crucial for the evolutionary success of placentals. But it is their early adaptation to exploit niches on the tree that paved their way toward this success."

Tags: evolution, juramaia, mammals