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Image 1: Yanoconodon allini
Reconstruction artwork: Mark A. Klingler/CMNH

Life reconstruction of the new Chinese Cretaceous mammal Yanoconodon, as a chipmunk-size nocturnal mammal, adapted to ground-living (terrestrial habits) and digging (fossorial habits). It is estimated to weigh about 30 grams (less than 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).

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Image 2: Yanoconodon allini Fossil Photograph.
Photo: Dr. Zhe-Xi Luo/CMNH

New Mesozoic mammal Yanoconodon allini. The original fossil (type specimen) is preserved in a slab and a counter slab of layered siltstones. The fossil is about 15 cm (about 5 inches) in length. Collected in 2003 by the team of Chinese and American scientists, deposited in the Nanjing University Paleontology Laboratory collection (NJU-P06001A, B).

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Image 3: Discovery site of Yanoconodon allini.
Map: Mark A. Klingler/CMNH

Fossil site of Yanoconodon is in Fengning County of Hebei Province of China, about 300 km north of the Chinese capital Beijing. Yanoconodon was named for the “Yan” mountains in the Hebei Province (“cono” and “don” mean “cusp” and “tooth”). This newly discovered mammal is from the Early Cretaceous Yixian Formation. The Yixian and equivalent beds were dated to be 125 to 122 million years in the adjacent areas of the northeastern China.

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Image 4: Yanoconodon allini skeleton illustration.
Illustration: Mark A. Klingler/CMNH

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.

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Image 5: Relationship of Yanoconodon to other mammals and new evidence on evolution of mammalian ear and jaw from Yanoconodon.
Illustration: Mark A. Klingler & Zhe-Xi Luo/CMNH

(for background on ear structure see http://en.wikipedia.org/wiki/Ear#Middle_ear)

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.

Moreover, the connected ear-jaw structure of Yanoconodon is similar to the ear structure of modern mammals at embryonic and fetal growth stages. This phenomenon is known as “paedomorphosis” (“paedo:” child-like; “morphosis:” similarity) and is caused by timing and rate change in growth. The analysis of the new fossil suggests that the evolutionary pattern of the mammalian ear is directly related to timing changes in growth.

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