Eomaia scansoria - earliest eutherian mammal

Eomaia scansoria: discovery of oldest known placental mammal

Alec MacAndrew

In Nature 416 816 - 822, Ji et al report a fossil of the earliest known eutherian (placental) mammal (1).



		Eomaia scansoria (Chinese Academy of Geological Sciences (CAGS) 01-IG-1a, b; holotype). a, Fur halo preserved around the skeleton (01-IG-1a, many structures not represented on this slab are preserved on the counter-part 01-IG-1b, not illustrated). b, Identification of major skeletal structures of Eomaia. c, Reconstruction of Eomaia as an agile animal, capable of climbing on uneven substrates and branch walking. Taken from ref (1)

This fossil was found in Lower Cretaceous Yixian formation. It is extremely well preserved, and both part and counter-part were recovered. It is a complete cranial and post-cranial skeleton of a small (large mouse-sized) mammal that, dated at 125Myr, represents the earliest eutherian (placental) animal fossil found. (The previous oldest, Murtoilestes is dated at 120M years).

The new species is named Eomaia (Dawn Mother in Greek) scansoria (climber in Latin).

The new fossil clearly has some transitional features. In particular:

Epipubic bones extending forwards from the pelvis (normally found in marsupials but not in placental mammals, except in early transitionals). Reilly and White (4) have recently identified a non-reproductive function for the epibubis in primitive mammals. They show that the epipubic bone is part of a kinetic linkage extending from thefemur, by way of the pectineus muscle, to the epipubic bone, throughthe pyramidalis and rectus abdominis muscles on one side of theabdomen, and through the contralateral external and internal obliquemuscles to the vertebrae and ribs of the opposite side. This muscleseries is activated synchronously as the femur and contralateralforelimb are retracted during the stance phase in locomotion.The epipubic bone acts as a lever that is retracted (depressed)to stiffen the trunk between the diagonal limbs that support thebody during each step. The epipubuc bone therefore has more of a function in locomotion than in reproduction. The stiffening function of the epipubic bone appears to be theprimitive conditions for mammals.

A narrow pelvic outlet which indicates that a short gestation was followed by a parental nurture of the young suspended from the abdomen after the fashion of modern marsupials.

There are many other transitional mammalian features, such as 5 incisors in the upper jaw which is diagnostic of metatherians (marsupials) although the molar to premolar ratio (it has more pre-molars than molars) places it firmly amongst the eutherians (placentals).

Given this mosaic of metatherian and eutherian features, along with many other derived and primitive features, it is clear that Eomaia has transitional characteristics.

The animal seems to be specialised for climbing and the speculation is that this ability allowed placentals (and marsupials who also have climbers in their early lineage) to out-compete the many sub-classes of mammals that have become extinct (only three sub-classes survive today: placentals; marsupials, like kangaroos etc; and monotremes like the duck billed platypus - but the Cretaceous was a melting pot of mammalian sub-classes most of which do not survive to today).

Some of the reasons that the fossil points to a climbing or tree-living habit are:

1: Fore and hind feet of Eomaia show similar proportion and curvature of the phalanges (the bones making up the fingers and toes) to the grasping feet of extant arboreal mammals (opposum, flying lemur and arboreal primates).

2: The first joint of the forefoot digit is curved and it shows compelling evidence of the attachment of a strong muscle that closes the grasp around branches (absent in ground-dwellers)

3: The length of the intermediate phalanx (second joint) of the forefoot as a percentage of the proximal (first joint) varies according to the habit of the animal. It ranges from around 53% in fully terrestrial mammals (eg Metachirus) to 126% in fully arboreal mammals (the flying lemur, Cynocephalus). Scansorial mammals have a lower ratio than fully aboreal animals(eg, the tree shrew, Tupaia, has 61%). E scansoria is, at 79%, more arboreal than scansorial.

4: On the hindfoot, digits (toes) 4 and 5 are longer than digits 1,2 and 3: this is a scansorial pattern.

5: On the hindfoot, the bones of the digits 4 and 5 are longer than those of digit 3; In terrestrial mammals, the bones of digits 2 and 3 are longer than those of digits 4 and 5

6: Turning to the claws, they have the curved shape and show the insertion of strong flexor (grasping) muscles and are very much likethe dormouse (Glis glis) and the tree shrew (Tupaia).

7: Clear evidence of insertion of strong climbing muscles in the shoulder blade

8: The tail is twice as long as the rest of the spine, and the tail vertebrae are elongated

9: The trapezoid and capitate are small and in the same proportion to the hamate and trapezium (all of these are wrist bones) as in extant climbing and tree dwelling mammals

The palaeontological age of the fossil matches very well with the molecular data which suggests a diversification of the major clades within the placental sub-class at 110Myr. This age is estimated by analysing the genetic divergence between the most distant extant placental mammals according to a phylogenetic analysis.

The extraordinarily well-preserved fossil includes a detailed fur halo.

See also Anne Weil's supporting article in the issue of Nature (2)

The eomaia fossil

Half of the fossil of Eomaia scansoria, which is preserved on two facing slabs of rock from China's Yixian Formation from ref (2)

1. Ji et al, The earliest known eutherian mammal, Nature 416, 816 - 822

2. Weil, Mammalian evolution: upwards and onwards, Nature 416, 798 -799

3. Nature Science Update article on the find

4. Reilly and White, Hypaxial motor patterns and the function of epipubic bones in primitive mammals, Science 299, 400 - 402