Yanoconodon
Encyclopedia
Yanoconodon is a monotypic genus of extinct
early mammal
whose representative species Yanoconodon allini lived 125 million years ago 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
. Yanoconodon was a Eutriconodont
, a group synonymous to the Triconodonts
which lived during the time of the dinosaur
s. Indeed, some such as Repenomamus
grew so large that they were able to eat small dinosaurs in some cases. In particular, Yanoconodon is considered to be closely related to Jeholodens
.
Yanoconodons 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 in the Yixian Formation
; '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.
Yanoconodon was a small mammal, barely 5 inches (13 centimetres) long. It was lightly built and probably fed on insect
s, worm
s and other invertebrate
s. 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.
The holotype
fossil of Yanoconodon was excavated from the fossil-rich
Yixian Formation, 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
" of mammalian middle ear evolution
. In addition, Yanocodon had lumbar
rib
s, a particularly rare feature in mammals today.
The closely related Eutriconodont Jeholodens lacks these lumbar
rib
s and it has been suggested that this morphological difference is due to changes in the Hox genes, specifically in the Hox10 group. In mice, a triple mutation knocking out all Hox10 paralogues leads to the presence of lumbar
rib
s supporting the theory that these genes have evolved the ability to repress the ancestral Lumbar morphology seen in the Eutriconodonts.
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
early mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
whose representative species Yanoconodon allini lived 125 million years ago during the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
in what is now China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. It is considered to be a transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
due to the formation of its middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
, which is a cross between those of modern mammals and their nearest relatives, the mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
. Yanoconodon was a Eutriconodont
Eutriconodonta
Eutriconodonta is a order of early mammals. Eutriconodonts existed in Asia, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack et al. in 1973 as a replacement name for the paraphyletic Triconodonta....
, a group synonymous to the Triconodonts
Triconodonta
Triconodonta is the generic name for a group of early mammals which were close relatives of the ancestors of all present-day mammals. Triconodonts lived between the Triassic and the Cretaceous. They are one of the groups that can be classified as mammals by any definition...
which lived during the time of the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. Indeed, some such as Repenomamus
Repenomamus
Repenomamus is the largest mammal known from the Cretaceous period of Manchuria, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.-Paleobiology:Repenomamus...
grew so large that they were able to eat small dinosaurs in some cases. In particular, Yanoconodon is considered to be closely related to Jeholodens
Jeholodens
Jeholodens was a primitive mammal belonging to the Triconodonta family, and which lived in present-day China during the Middle Cretaceous about 125 million years ago....
.
Yanoconodons name is composed of two elements: 'Yan' is taken from the Yan Mountains
Yan Mountains
The Yanshan or Yan Mountains are a major mountain range north of the North China Plain, in northern Hebei Province.The range rises between the Chaobai River on the west and the famous Shanhai Pass on the east. It is made up mostly of limestone, granite and basalt. Its altitude ranges from 400 to...
in the north of the Hebei Province
Hebei
' is a province of the People's Republic of China in the North China region. Its one-character abbreviation is "" , named after Ji Province, a Han Dynasty province that included what is now southern Hebei...
near where the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of Yanoconodon was found in the Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...
; '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.
Yanoconodon was a small mammal, barely 5 inches (13 centimetres) long. It was lightly built and probably fed on insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s and other invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s. Like other Eutriconodonts, Yanoconodon probably hunted
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
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.
The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
fossil of Yanoconodon was excavated from the fossil-rich
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
Yixian Formation, in the Yan Mountains
Yan Mountains
The Yanshan or Yan Mountains are a major mountain range north of the North China Plain, in northern Hebei Province.The range rises between the Chaobai River on the west and the famous Shanhai Pass on the east. It is made up mostly of limestone, granite and basalt. Its altitude ranges from 400 to...
about 300 kilometres from Beijing
Beijing
Beijing , also known as Peking , is the capital of the People's Republic of China and one of the most populous cities in the world, with a population of 19,612,368 as of 2010. The city is the country's political, cultural, and educational center, and home to the headquarters for most of China's...
. It is so well preserved that scientists were able to examine tiny bones of the middle ear. The intermediate anatomy of the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
of Yanocodon is said to be a "Rosetta Stone
Rosetta Stone
The Rosetta Stone is an ancient Egyptian granodiorite stele inscribed with a decree issued at Memphis in 196 BC on behalf of King Ptolemy V. The decree appears in three scripts: the upper text is Ancient Egyptian hieroglyphs, the middle portion Demotic script, and the lowest Ancient Greek...
" of mammalian middle ear evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
. In addition, Yanocodon had lumbar
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s, a particularly rare feature in mammals today.
The closely related Eutriconodont Jeholodens lacks these lumbar
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s and it has been suggested that this morphological difference is due to changes in the Hox genes, specifically in the Hox10 group. In mice, a triple mutation knocking out all Hox10 paralogues leads to the presence of lumbar
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s supporting the theory that these genes have evolved the ability to repress the ancestral Lumbar morphology seen in the Eutriconodonts.
External links
- From Jaw to Ear: Transition Fossil Reveals Ear Evolution in Action by David Biello, Scientific American
- Artist's impression of Y. allini.