Species: Eomaia scansoria Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP & Georg JA, 2002
Place: Yixian Formation, Liaoning Province
Country: China
Age: Barremian, Lower Cretaceous
Remarks: This stunning find is an early candidate for the Find of the Century competition. The fossil is 10 cm in length and virtually complete. An estimate of the bodyweight is between 20-25g.
"First of all, Eomaia is not a placental mammal. It is an early, primitive representative of the lineage that eventually led to placental mammals," said Anne Weil, [1].
The abstract does use the terminology "eutherian (placental)". However, the narrowness of the hips suggests an animal which gave birth to live young, but the babies were not well developed. This strongly indicates there was no placenta.
The Nature paper states, "the epipubis is present." This is highly unusual for eutherids, though not completely unknown from early representatives. Otherwise, this is a feature of marsupials, monotremes and non-mammalian therapsids.
Are the authors sure this is a eutherid? Yes, damned sure. Many justifications are given. The most readibly comprehensible is: "a typical eutherian dental formula, 5.1.5.3/4.1.5.3 (incisors, canines, premolars, molars)," (also extracted from the Nature article). Assuming I understand this correctly, (which is not necessarily so), that means five premolars. This is not the typical number for all eutherids. Most have four, but it's typical for basal members.
"On the basis of 268 characters sampled from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous, Eomaia is placed at the root of the eutherian tree with Murtoilestes and Prokennalestes. Clearly, these three taxa are closer to living placentals than to living marsupials. Eomaia is placed in Eutheria by numerous apomorphies in the dentition, the wrist and the ankle", (Qiang et al 2002, p.820).
There are even traces of hair. The previous record for such a feature was about 60Ma - this fossil is around 65 million years older. My flabber is gasted and I shall allow myself an explanation mark! This is not to suggest that previous mammals had been Kojak lookalikes. Skeletal evidence suggests hair possibly appeared in non-mammalian ancestors back in the deep Triassic / Upper Permian. Fur as good as never, ever fossilizes. Liaoning is a freak location.
By some happy quirk of fate, Nature chose this week to allow me free access to the full on-line magazine. As well as the description, this edition also contains an excellent commentary by Anne Weil, which nicely places Eomaia into perspective. It's in English rather than paleo-speak:
"Eomaia - 'Dawn Mother' - is exceptionally well-preserved for a 125-million-year-old. Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible."
Reference: Ji et al (2002), The earliest known eutherian mammal. Nature (416), p.816-822.
(This information is derived from [1] MESOZOIC MAMMALS; 'basal' Eutheria, an internet directory. As that's my webpage, there are no issues of copyright.)