Science  02 Sep 2011:
Vol. 333, Issue 6047, pp. 1205

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  1. 'Jurassic Mother' Found in China

    A new, 160-million-year-old fossil discovery pushes back the earliest appearance of placentals, the peculiar group of mammals to which we and many other mammal species belong.

    Distant cousin.

    A skeleton and body of Jurassic mammal Juramaia.


    Living mammals are split into three subgroups: egg-laying monotremes, pouched marsupials, and placental mammals, which includes everything from humans to bats to whales. Determining when marsupials and placentals diverged has been problematic: Fossil discoveries point to about 125 million years ago, whereas genetic differences among living mammals suggest that the split happened even earlier.

    Now the discovery of a partial skeleton of a small, shrewlike mammal, described online 24 August in Nature, pushes back the date of the divergence to 160 million years ago. Found in the famous fossil beds of Liaoning, China, the newly discovered little mammal has been named Juramaia sinensis, or “Jurassic mother from China.”

    Juramaia probably ate insects and was a skilled climber. Based on the arrangement of its teeth and on characteristics of its arms and wrists, Juramaia belonged to a group of animals called eutherians, a lineage that includes placentals and their forebears, says lead author Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

  2. Supernova Ready For Closeup

    Bright light.

    Newfound supernova PTF11kly is in the Pinwheel Galaxy.


    A supernova discovered 23 August in the nearby Pinwheel Galaxy, just 21 million light-years from Earth, is no mere flash in the pan. Astronomers are calling the stellar explosion, detected at Palomar Observatory in California and dubbed PTF11kly, a Rosetta Stone for deciphering the origin and evolution of type 1a supernovas, a class of stellar eruptions critical for probing the expansion history of the universe because they all have similar luminosities. The new-found supernova is the closest type 1a spotted since 1986, and it was caught earlier in time—just hours after it blew up—than any other such explosion, says Andy Howell of the University of California, Santa Barbara. Expected to reach maximum brightness on 7 September, the supernova can be seen with a small backyard telescope near evening twilight in northern skies. On 27 August, the Hubble Space Telescope recorded ultraviolet spectra of the eruption, which is expected to provide new clues about the star that exploded.

  3. 'Time Cells' Weave Events Into Memories

    Neurons in a part of the brain called the hippocampus encode the “gaps” between sequential events as precisely as the events themselves, new research shows. The hippocampus, a center for navigation and memory, contains “place cells” that fire when a person or animal is at a certain location. The new findings suggest that the hippocampus may also contain “time cells” that encode “episodic” memories of events.

    Howard Eichenbaum and colleagues at Boston University trained rats to perform a two-part task with a 10-second delay in the middle while fitted with surgically implanted electrodes that recorded neural activity in the hippocampus. Recordings from 300 hippocampal neurons showed that during the delay, about a third of the cells that fired did so one after another throughout the delay even though nothing was happening, the team reported 25 August in Neuron. The researchers surmised that the hippocampus was encoding the passage of time during the empty period, bridging the gap between the two phases of the test.

    “The study shows how we can retain our memory of distinct events while also being aware of time passing in the background,” says Wendy Suzuki of New York University.