Yanoconodon

Yanoconodon is a monotypic genus of extinct early mammal whose representative species Yanoconodon allini lived during the Mesozoic in what is now China. It is considered to be a transitional fossil due to the formation of its middle ear, which is a cross between those of modern mammals and their nearest relatives, the mammaliaformes. It was found in the Qiaotou member of the Huajiying Formation (which the original authors considered part of the Yixian Formation) of Hebei Province, China, and is therefore of uncertain age. The Qiaotou Member may correlate with the more well-known Early Cretaceous Yixian Formation, and so probably dates to around 122 Ma ago.[1] Yanoconodon was a Eutriconodont, a group synonymous to the Triconodonts which lived during the time of the dinosaurs. Indeed, some such as Repenomamus grew so large that they were able to eat small dinosaurs in some cases.[2] In particular, Yanoconodon is considered to be closely related to Jeholodens.[3]

Yanoconodon's name is composed of two elements: 'Yan' is taken from the Yan Mountains in the north of the Hebei Province near where the holotype of Yanoconodon was found; 'Conodon' is an often used as a mammalian taxonomic suffix meaning 'cuspate tooth'. Its species name, "allini," is derived from mammalian researcher Edgar Allin, who was notable for his research on the mammalian middle ear.[3][4]

Yanoconodon was a small mammal, barely 5 inches (13 centimetres) long. It was lightly built and probably fed on insects, worms and other invertebrates. Like other Eutriconodonts, Yanoconodon probably hunted at night out of the danger posed by predatory dinosaurs during the day. Like most early mammals, Yanoconodon had short, sprawling legs and claws that were most likely used for burrowing underground or digging.[3][4]

The holotype fossil of Yanoconodon was excavated in the Yan Mountains about 300 kilometres from Beijing. It is so well preserved that scientists were able to examine tiny bones of the middle ear. The intermediate anatomy of the middle ear of Yanocodon is said to be a "Rosetta Stone"[4] of mammalian middle ear evolution. In addition, Yanocodon had lumbar ribs, a particularly rare feature in mammals today.[5]

The closely related Eutriconodont Jeholodens lacks these lumbar ribs and it has been suggested that this morphological difference is due to changes in the Hox genes, specifically in the Hox10 group.[3] In mice, a triple mutation knocking out all Hox10 paralogues leads to the presence of lumbar ribs[6] supporting the theory that these genes have evolved the ability to repress the ancestral Lumbar morphology seen in the Eutriconodonts.