The therian mammals include placental mammals and marsupials. All of these animals can be distinguished by the number, morphology, and replacement pattern of their teeth. Eutherians are the true placental mammals. That means they bear live young that developed in and been nourished by a placenta within the mother's uterus, and unlike marsupials (or metatherians, which have a short-lived placenta) the eutherians' placenta contributes significantly to fetal nourishment. This group became common in central Asia during the Upper Cretaceous, and with exception of Australia (where marsupials rule), they have been the largest and most common land vertebrates following the end of the Mesozoic.
This new paper in describes a new species of eutherian the authors are naming Juramaia sinensis, a shrew-like animal alive during the Jurassic period. The fossil was discovered in the Liaoning Province in northeast China and is comprised of an incomplete skull including teeth, part of the skeleton including forepaw bones, and impressions of residual soft tissues such as hair. Juramaia is among the earliest known eutherians, and is a group that evolved to include all other placental mammals. It has adaptive features such as scansorial forelimbs that are good for climbing. When a majority of your mammalian cousins scuttle around on the ground, the ability to climb works to your advantage in evading predators and finding food. The fossil also provides the ancestral condition for dental and other anatomical features of eutherians.
This fossil find is so important because it "establishes a much older geological time for the split of the metatherian-marsupial and the eutherian-placental lineages than previously shown by the fossil record." J. sinensis is dated at 160 million years. Previously the earliest eutherian record is Eomaia and the metatherian record is Sinodelphys, both of which are 125 million years old. Parsimony analysis show Juramaia to be more closely related to extant placental mammals than to all other metatherians. This placement of Juramaia on the placental side of the marsupial-placental divergence means that the divergence itself must have occurred before Juramaia evolved. Add together the older age of the fossil and the parsimony analysis and you get a marsupial-placental divergence date that was at least 35 million years older than anyone previously thought. As it turns out, getting the timing of this divergence right is critical for calibrating the rates of evolution for all therian mammals. It is particularly useful for scientists doing molecular evolutionary studies and comparative genomics and their work in determining a "molecular clock." So far, there has been a discrepancy between the previous fossil record and the molecular age for the marsupial-placental divergence. Molecular studies have provided estimates of fossil ages between 143-178 million years which did not match up with the fossil evidence at the time. This new fossil now corroborates the molecular findings and sets the minimal divergence time to coincide with the molecular time estimates.
Love when things match up.
Here's the paper:
Luo, Zhe-Xi, et al. (2011) A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature: 476, 442-445. (DOI: 10.1038/nature10291)