''Yanoconodon'' is a monotypic genus of

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

early

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

whose representative species ''Yanoconodon allini'' lived during the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

in what is now

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

fossil of ''Yanoconodon'' was excavated in the

Yan Mountains The Yan Mountains, also known by their Chinese name Yanshan, are a major mountain range to the north of the North China Plain, principally in the province of Hebei. The range rises between the Chaobai River on the west and the Shanhai Pass on ...

about 300 kilometres from

Beijing Beijing, Chinese postal romanization, previously romanized as Peking, is the capital city of China. With more than 22 million residents, it is the world's List of national capitals by population, most populous national capital city as well as ...

in the Qiaotou member of the

Huajiying Formation The Huajiying Formation is a geological formation in Hebei, People's Republic of China. Known for its fossils including feathered dinosaurs, the age of the formation is uncertain. It may represent an early portion of the Jehol Biota, dating to som ...

(which the original authors considered part of the

Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...

) of

Hebei Hebei is a Provinces of China, province in North China. It is China's List of Chinese administrative divisions by population, sixth-most populous province, with a population of over 75 million people. Shijiazhuang is the capital city. It bor ...

Province,

, and is therefore of uncertain age. The Qiaotou Member may correlate with the more well-known

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

, and so probably dates to around 122 Ma ago.Jin, F., Zhang, F.C., Li, Z.H., Zhang, J.Y., Li, C. and Zhou, Z.H. (2008). "On the horizon of ''Protopteryx'' and the early vertebrate fossil assemblages of the Jehol Biota." ''Chinese Science Bulletin'', 53(18): 2820-2827. ''Yanoconodon'' was a

eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...

, a group composing most taxa once classified as " triconodonts" which lived during the time of the

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s. These were a highly ecologically diverse group, including large sized taxa such as ''

Repenomamus ''Repenomamus'' (Latin: "reptile" (reptilis), "mammal" (mammalis)) is a genus of opossum- to badger-sized Gobiconodontidae, gobiconodontid mammal containing two species, ''Repenomamus robustus'' and ''Repenomamus giganticus''. Both species are kn ...

'' that were able to eat small dinosaurs,Hu, Y., Meng, J., Wang, Y. & Li, C. (2005). ''Large Mesozoic mammals fed on young dinosaurs''.

Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

. Vol 433, 12 January 2005, Number 7022, pp91-178,
See commentary on this article (Retrieved 25/6/2007)
the arboreal '' Jeholodens'', the aerial volaticotherines and the spined ''

Spinolestes ''Spinolestes'' is an extinct mammal genus from the Early Cretaceous of Spain. A gobiconodontid eutriconodont, it is notable for the remarkable degree of preservation, offering profound insights to the biology of non-therian mammals.Thomas Marti ...

''. ''Yanoconodon'' is inferred to be a generalized terrestrial mammal, capable of multiple forms of locomotion.Chen, M., Z.-X. Luo, and G. P. Wilson. 2017. The postcranial skeleton of Yanoconodon allini from the Early Cretaceous of Hebei, China, and its implications for locomotor adaptation in eutriconodontan mammals. Journal of Vertebrate Paleontology. . ''Yanoconodons name is composed of two elements: 'Yan' is taken from the

in the north of the

Hebei Province Hebei is a Provinces of China, province in North China. It is China's List of Chinese administrative divisions by population, sixth-most populous province, with a population of over 75 million people. Shijiazhuang is the capital city. It bor ...

near where the

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.Luo, Z., Chen, P., Li, G., & Chen, M. (2007).
A new eutriconodont mammal and evolutionary development in early mammals
'.

. Vol 446, 15 March 2007, .
Paleontologists Discover New Mammal from Mesozoic Era
' a
www.physorg.com
- Retrieved 25/6/2007.

Description

''Yanoconodon'' was a small mammal, barely 5 inches (13 centimetres) long. It had a sprawling posture, and although previously inferred to be semi-aquatic, direct study of its postcrania indicates that ''Yanoconodon'' was likely a terrestrial mammal, and that it has features in common with digging, arboreal, and semiaquatic mammals. ''Yanocodon'' had

lumbar In tetrapod anatomy, lumbar is an adjective that means of or pertaining to the abdominal segment of the torso, between the diaphragm (anatomy), diaphragm and the sacrum. Naming and location The lumbar region is sometimes referred to as the lowe ...

rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...

s, a feature not seen in modern mammals. The closely related

''Jeholodens'' lacks these

s, and it has been suggested that this morphological difference is due to changes in the

Hox genes Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the c ...

, specifically in the ''Hox10'' group. In mice, a triple mutation knocking out all ''Hox10'' genes leads to the presence of

s,Deneen M. Wellik and Mario R. Capecchi. (2003). ''Hox10 and Hox11 Genes Are Required to Globally Pattern the Mammalian Skeleton''.

Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

, Vol. 301, 18 July 2003, no. 5631, pp. 363 - 367, . supporting the theory that these genes have evolved the ability to repress the ancestral lumbar morphology seen in the Eutriconodonts. The ''Yanoconodon'' holotype is so well preserved that scientists were able to examine tiny bones of the middle ear. These are of particular interest because of their "transitional" state: ''Yanoconodon'' has fundamentally modern middle ear bones, but these are still attached to the jaw by an ossified

Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left; also known as Meckelian cartilages), above which the incus and malleus are located. Meckel's cartilage arises from the fir ...

. This is a feature retained from earlier stem mammals, and illustrates the transition from a basal tetrapod jaw and ear, to a mammalian one in which the middle ear bones are fully separate from the jaw. Despite this feature ''Yanoconodon'' is a true mammal''.'' It is thought that the feature was retained during early

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

development, whereas it is lost in most other mammal groups. The intermediate anatomy of the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

of ''Yanocodon'' is said to be a "

Rosetta Stone The Rosetta Stone is a stele of granodiorite inscribed with three versions of a Rosetta Stone decree, decree issued in 196 BC during the Ptolemaic dynasty of ancient Egypt, Egypt, on behalf of King Ptolemy V Epiphanes. The top and middle texts ...

" of mammalian middle ear

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

References

External links

From Jaw to Ear: Transition Fossil Reveals Ear Evolution in Action
by David Biello, Scientific American
Artist's impression of ''Y. allini''
{{Taxonbar, from=Q2418217 Eutriconodonta Early Cretaceous mammals of Asia Transitional fossils Fossil taxa described in 2007 Taxa named by Meng Chen (paleontologist) Taxa named by Peiji Chen Taxa named by Gang Li Taxa named by Zhe-Xi Luo Prehistoric mammal genera