You've probably already seen the following image, as it's been splashed all over the media recently:
From Franzen et al. (2009) PLoS One 4(5): e5723 doi:10.1371/journal.pone.0005723.g001
NB 'Plate B' is the 'counterplate' of A; while A is complete & genuine, it seems that B was altered to make it appear more complete (& thus more attractive to prospective purchasers) while in private ownership. The parts of B marked '1' & '2' are the genuine article.
Meet 'Ida' (Darwinius masillae), a juvenile female primate who died when she was less than a year old, to become an exquisite fossil in Eocene rocks that are 47 million years old . So complete is her preservation that even the remains of her last meal are present, while the dark material surrounding the skeleton is the remains of soft tissues (muscles & skin). And while the skeleton is described as 'lightly crushed' by the research team, the only bit that's actually missing is the lower left leg & foot. But how can we be so sure about details such as age & sex?
We can be sure that Ida is female because she lacks a baculum, or penis bone, which supports the penis during copulation. Most mammals have them, but not humans (nor horses & hyenas, marsupials, rabbits & hares, whales & dolphins). As for Ida's age - well, CT images of her remains clearly show that her jaws contain both permanent (adult) & deciduous ('baby', or milk) teeth. (The same images also clearly identified the fake & genuine parts of plate B.) And because the first & second molars had erupted, she was almost certainly weaned - something supported by the remains of leaves & fruit in her gut. However, a comparison of Ida's pattern of tooth eruption with that of other primates suggested she was probably less than a year old at time of death. (How's that for forensic palaeoanthropology?)
The paper by Franzen & his colleagues provides a thorough description of Ida's remains, & some interesting background regarding the area where she was found, & the history of the fossil after it was unearthed. (You can read it for yourselves, if you want to, as PLoS One is an open-access journal.) OVerall she seems lemur-like, although she lacks the tooth comb (forward-facing incisor teeth) & toilet claw commonly found in the prosimians (which also include the lorises & bushbabies). Partly because of these missing features, the team state that Darwinius isn't itself a lemur, but belongs to an extinct group of lemur-like primates that later gave rise to anthropoids (a view that runs counter to most current interpretations).
On a worrisome note - the release of the PLoS paper was preceded by a rather overblown press release, which described Ida as 'the most significant scientific discovery of recent times'. This really was over the top. D.massillae is certainly an extraordinary fossil - extraordinary in the sense of its superb preservation & level of detail. But it's not as if it represents a hither-to unsuspected missing link, or something that will shake our understanding of the roots of our own family tree. While it's true to say that the paper itself makes no claims about missing-links, the associated promotional materials from the History Channel certainly do. Personally I can't see why we've got this level of sensationalism - it certainly doesn't make for good science communcation. Brian Switek has an excellent article about this on his blog, & Carl Zimmer has also criticised both the paper itself and the way the whole thing was make public, and provides comments from other primate experts on the phylogenetic position of Darwinius.
(Nice to see that today's print edition of the NZ Herald presented a more level-headed view than some other media commentaries, following on from its original story of a few days ago :-) The Herald article - originally from the Independent - concludes with the statement that Ida is not 'the link' because there is never going to be one missing link between humans and their primate ancestors. Niether is Ida our direct ancestor. She belonged to a branch that evolved parallel to the ancestral line of primates that eventually gave rise to humans. Ida is an important and fascinating discovery at the roots of the primate lineage but the discovery could be mired in hype and exaggerated claims - such as Ida being our 'earliest ancestor'. She was not 'the link' but simply one of many links in the long and complicated descent of man.)
J.L.Franzen, P.D.Gingerich, J.Habersetzer, J.H.Hurum, W.von Koenigswald & B.H. Smith (2009) Complete primate skeleton from the Middle Eocene of Messel in Germany: morphology and palaeobiology. PLoS One 4(5): e5723 doi:10.1371/journal.pone.0005723