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Anchiornithidae ("near birds") is a family of eumaniraptorans which could be the basalmost family of
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s (in the general sense) in the clade Avialae. Anchiornithids have been classified at varying positions in the maniraptoran tree, with some scientists classifying them as a distinct family, a basal subfamily of Troodontidae, members of Archaeopterygidae, or an assemblage of dinosaurs that are an evolutionary grade within Avialae or Paraves.


Description

Anchiornithids share many general features with other Paraves and early avialans. They were small and lightly-built feathered carnivores, similar in biology to ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', early
dromaeosaurids Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...
like ''
Microraptor ''Microraptor'' ( Greek, μικρός, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. ...
'', and particularly troodontids. They are almost exclusively known from
Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...
Chinese deposits, although ''
Ostromia ''Ostromia'' (''Thick feet of John Ostrom'') is a genus of anchiornithid theropod dinosaur from the Late Jurassic Painten Formation of Germany. The genus contains a single species, ''O. crassipes'', named by Christian Foth and Oliver Rauhut ...
'' was discovered in
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
and '' Yixianosaurus'' (a putative member of the group only known from forelimbs) is believed to hail from the early Cretaceous. Most had long legs, arms, and hands, although some ('' Eosinopteryx'') had slightly reduced forelimbs.


Feathering

Although it is practically certain that every anchiornithid possessed advanced pennaceous feathers, there is still much variety in feathering between genera (or between individuals in the case of numerous genera such as ''Anchiornis)''. Most had vaned tail feathers forming a frond-shaped tail, with the tails feathers of '' Caihong'' being particularly long and in some cases asymmetrical. However, ''Eosinopteryx,
Serikornis ''Serikornis'' is a genus of small, feathered anchiornithid dinosaur from the Upper Jurassic Tiaojishan Formation of Liaoning, China. It is represented by the type species ''Serikornis sungei''. Discovery ''Serikornis'', first described in Aug ...
,'' and '' Aurornis'' were preserved with short and downy tail feathering. Some studies on the body feathering of anchiornithids indicate that the feathers were pennaceous, but seemingly lacked barbules, making them "shaggy" or "silky" in life. Long pennaceous feathers were present on the arms of most anchiornithids. However, these feathers were slender, symmetrical, and unspecialized, probably useless for flight. They formed rows which were attached directly to a large fleshy propatagium connecting the upper and lower arm. Most anchiornithids also had dense feathering extending down their legs. A few had short leg feathering, but most (''Anchiornis'' and '' Pedopenna,'' for example) had very long pennaceous feathers on their legs, giving them the moniker of "four-winged dinosaurs", a trait also shared by microraptorians. ''Eosinopteryx'' seemingly lacked any sort of feathers on the lower part of its legs, but its close relative ''Serikornis'' possessed both plumaceous (downy) feathers extending onto its toes as well as pennaceous feathers further up the leg.


Distinguishing features

Foth and Rauhut (2017) established several diagnostic features present in anchiornithids: * Nutrient foramina on the dentary are placed in a deep groove (also in most troodontids and some other coelurosaurs); * Anterior dentary teeth which are smaller, more numerous, and more closely spaced than those in the middle of the tooth row (also in most troodontids); * The front edge of the acromion margin of the
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
is bent or hooked outwards (also in several oviraptorosaurs and more derived avialans); * The inside surface of proximal part of the
fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity ...
is flat (also in
alvarezsaurids Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were hi ...
, therizinosauroids and derived avialans); * Fan-shaped posterior dorsal neural spines (also in compsognathids and some derived avialans); * Extensive large pennaceous feathers on the foot and ankle (also in ''Microraptor'' and '' Sapeornis'').


Systematic history

In 2015 Chatterjee created Tetrapterygidae in the second edition of his book ''The Rise of Birds: 225 Million Years of Evolution'', where he included ''
Xiaotingia ''Xiaotingia'' is a genus of anchiornithid theropod dinosaur from Middle Jurassic or early Late Jurassic deposits of western Liaoning, China, containing a single species, ''Xiaotingia zhengi''. Discovery ''Xiaotingia'' is known from the holoty ...
'', '' Aurornis'', '' Anchiornis'', and even ''Microraptor''; together they were proposed to be the sister group of the Avialae. However this family is invalid as must include the genus ''Tetrapteryx'', which is the junior synonym of '' Grus'' – therefore Tetrapterygidae is a junior synonym of Gruidae. In their description of '' Wiehenvenator'' Rauhut and colleagues had informally called the group as "Anchiornithosaurs" which they placed outside of Avialae. The clade was originally named as "Anchiornithinae" by Xu ''et al.'' (2016) and defined as for "the most inclusive clade including '' Anchiornis'' but not ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', '' Gallus'', '' Troodon'', '' Dromaeosaurus'', '' Unenlagia'', or '' Epidexipteryx''". In 2017 Foth and Rauhut in their re-evaluation of the Haarlem ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'' specimen (which they classified it in its own distinct genus ''
Ostromia ''Ostromia'' (''Thick feet of John Ostrom'') is a genus of anchiornithid theropod dinosaur from the Late Jurassic Painten Formation of Germany. The genus contains a single species, ''O. crassipes'', named by Christian Foth and Oliver Rauhut ...
'') found that the anchiornithids are a distinct family closer to the ancestry of birds. They provided their own definition of Anchiornithidae as "all maniraptoran
theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
s that are more closely related to '' Anchiornis huxleyi'' than to '' Passer domesticus'', '' Archaeopteryx lithographica'', '' Dromaeosaurus albertensis'', '' Troodon formosus'', or ''
Oviraptor philoceratops ''Oviraptor'' (; ) is a genus of oviraptorid dinosaur that lived in Asia during the Late Cretaceous period. The first remains were collected from the Djadokhta Formation of Mongolia in 1923 during a paleontological expedition led by Roy Chapman ...
''." During the description of '' Halszkaraptor,'' Cau ''et al.'' (2017) incorporated many putative anchiornithids into two different large-scale phylogenetic analyses. The first analysis was a comprehensive study of theropod dinosaurs originally designed by Lee ''et al.'' for a 2014 paper on miniaturization in theropods leading up to the evolution of birds. Cau ''et al.'''s usage of this first analysis found support for Anchiornithidae being a distinctive family of avialans. The strict consensus tree of the first analysis is given below: The second analysis was first used in a different paper on theropod size published by Brusatte ''et al.'' in 2014. This analysis (which was updated by Cau ''et al.'' during a 2015 study on the affinities of '' Balaur bondoc'') focused specifically on coelurosaurs and found that anchiornithids (represented only by ''Anchiornis, Xiaotingia, Aurornis'', and ''Eosinopteryx'' in the analysis) were troodontids rather than avialans, in contrast to the first analysis. The description of ''Caihong'' by Hu ''et al.'' (2018) also implemented the Brusatte analysis and found the same result. However, this study also implemented an analysis performed by Xu ''et al.'' (2015) during the description of '' Yi qi''. This analysis placed anchiornithids (or as the study calls them, members of "Anchiorninae") either as troodontids or unresolved paravians, depending on whether parsimony or bootstrap analyses are used. An analysis used in the description of the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
bird '' Alcmonavis'' by Rauhut ''et al.'' (2019) recovered anchiornithids (represented in the analysis by ''Eosinopteryx'', ''Anchiornis'', and ''Ostromia'') as the most basal avialans. ''Xiaotingia'' and ''Pedopenna'' were placed as more advanced avialans closer to ''Archaeopteryx''. In 2019 with the description of the Late Jurassic genus ''
Hesperornithoides ''Hesperornithoides'' (meaning "western bird form"; nicknamed "Lori") is a genus of troodontid theropod dinosaur that lived in North America during the Late Jurassic period. Discovery Although several Troodontid teeth were found, with the trood ...
'', Hartman ''et al.'', using every named Mesozoic maniraptoromorph (with the addition of 28 unnamed specimens), which they scored 700 characters and 501 operational taxonomic units, found that most of the anchiornithids are members of Archaeopterygidae, with only ''
Xiaotingia ''Xiaotingia'' is a genus of anchiornithid theropod dinosaur from Middle Jurassic or early Late Jurassic deposits of western Liaoning, China, containing a single species, ''Xiaotingia zhengi''. Discovery ''Xiaotingia'' is known from the holoty ...
'' and '' Yixianosaurus'' being classified as a troodontid and a dromaeosaurid respectively, '' Pedopenna'' found in many possible positions within the Paraves phylogeny, and ''
Ostromia ''Ostromia'' (''Thick feet of John Ostrom'') is a genus of anchiornithid theropod dinosaur from the Late Jurassic Painten Formation of Germany. The genus contains a single species, ''O. crassipes'', named by Christian Foth and Oliver Rauhut ...
'' described too late to include in the analysis. Below is their phylogeny: The cladogram below shows the results of the phylogenetic analysis by Cau (2020).


Palaeoecology

A good majority of the known anchiornithid fossils have been recovered from the Tiaojishan Formation in
Liaoning Liaoning () is a coastal province in Northeast China that is the smallest, southernmost, and most populous province in the region. With its capital at Shenyang, it is located on the northern shore of the Yellow Sea, and is the northernmo ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
dating back to 160 million years. The climate during this period of time would have been
subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...
to
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
, warm and
humid Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
based on the plant life present in the Tiaojishan Formation. This environment was dominated plant by
gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
trees. There were
ginkgo ''Ginkgo'' is a genus of non-flowering seed plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian, 270 million years ago, and is now the only living genus with ...
psids like ''Ginkoites'', ''
Ginkgo ''Ginkgo'' is a genus of non-flowering seed plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian, 270 million years ago, and is now the only living genus with ...
'', ''Baiera'', ''Czekanowskia'', and ''Phoenicopsis''. There were also
conifer Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All ext ...
s like ''Pityophyllum'', ''Rhipidiocladus'', ''Elatocladus'', ''Schizolepis'', and ''Podozamites''. Also, Lycopsids like ''Lycopodites'' and ''Sellaginellities'', horsetails ( Sphenopsida) like ''Equisetum'',
cycads Cycads are seed plants that typically have a stout and woody ( ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or ...
like ''Anomozamites'', and ferns ( Filicopsida) like ''Todites'' and ''Coniopteris''. The only Chinese anchiornithid to not have been discovered in the Tiaojishan Formation is ''Yixianosaurus longimanus'', which was found in the 125 million-year-old Early Cretaceous
Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...
, in the same area. Only one genus of anchiornithid has been found outside of China: ''
Ostromia ''Ostromia'' (''Thick feet of John Ostrom'') is a genus of anchiornithid theropod dinosaur from the Late Jurassic Painten Formation of Germany. The genus contains a single species, ''O. crassipes'', named by Christian Foth and Oliver Rauhut ...
'', which is found in the Painten Formation from
Riedenburg Riedenburg () is a town in the district of Kelheim, in Bavaria, Germany. It is situated on the river Altmühl, northwest of Kelheim and northeast of Ingolstadt. Prunn Castle ( de) is located in Riedenburg. The town is on the Rhine–Main– ...
,
Bavaria Bavaria ( ; ), officially the Free State of Bavaria (german: Freistaat Bayern, link=no ), is a state in the south-east of Germany. With an area of , Bavaria is the largest German state by land area, comprising roughly a fifth of the total l ...
,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
.


References

{{Taxonbar, from=Q35643398 Prehistoric paravians Middle Jurassic first appearances Aptian extinctions Prehistoric dinosaur families