Yanoconodon allini
A pretty interesting little beast, which shows an inner ear structure transitional between reptiles and mammals. (details below).
Like all these discoveries, it shows again the predictive power of evolution. I post I did a year ago, coupled with this discovery, helps define “predictive power.” My earlier post on reptile-mammal transitional fossils obviously did not include Yanoconodon. But, evolutionary theory “predicts” that transitional forms existed between any other forms that might have already found. Sure enough Yanoconodon fits in between the earlier Morganucodon and Steropodon and the later Cimolestes.
[A] new Chinese Cretaceous mammal Yanoconodon [has been discovered], a chipmunk-size nocturnal mammal, adapted to ground-living (terrestrial) and digging (fossorial) habits. It is estimated to weigh about 30 grams (about 1 ounce) from its skeleton about 15 cm (about 5 inches) in length. Yanoconodon belongs to the primitive Mesozoic fossil mammal group known as triconodonts, characterized by three cusps in a straight line on molar teeth (“tri” – three; “con” – cusps; “don” tooth; “triconodont” – mammal with three-cusped tooth).
The ear structure of Yanoconodon is intermediate between those of mammaliaforms (pre-mammalian relatives, such as Morganucodon) on the one hand, and the typical structure of all modern mammals (such as living opossum) on the other hand.
All modern mammals have a middle ear separated from the lower jaw (see example from living opossum). This jaw-ear detachment is an important evolutionary innovation for mammals to have a delicate and highly sensitive ear structure for better hearing, and to have a more robust lower jaw and jaw hinge for better feeding. By comparison, the middle ear bones are a part of the lower jaw and form the jaw hinge in pre-mammalian relatives (see the example of Morganucodon).
The ear bones in Yanoconodon are partly separated from the jaw, and more similar to those modern mammals than to mammaliaforms, but still retain the premammalian condition that the jaw and the ear are connected to each other. Because it is structurally intermediate between the distant mammalian relatives and modern mammals, it provides crucial fossil evidence for a major evolutionary transition for mammalian origins.
Yanoconodon has relatively short limbs of sprawling posture but a more elongate body by six more vertebrae than the 19 or 20 vertebrae of all other terrestrial mammals. Yanoconodon has mobile ribs in the lumbar (waist) vertebrae and gradational transition of thorax (chest) and lumbar (waist) regions, an ancestral feature of this mammal for its relatively advanced position in the mammal family tree. Recent developmental biology studies have documented that the “re-appearance” of these long-lost lumbar (waist) ribs can result from changes in developmental genes in modern mammals; so scientists hypothesize that the convergent evolution of these primitive features in Yanoconodon could evolve as result changes in developmental genes.
More at Pharyngula
Transitional Fossils - Reptiles to Mammals