Anchiornithidae ("near birds") is a family of eumaniraptorans which could be the basalmost family of birds (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.
|Skeletal restoration of Anchiornis huxleyi by Scott Hartman, 2017|
Xu et al. 2016 sensu Foth & Rauhut, 2017
Xu et al., 2009
Anchiornithids share many general features with other Paraves and early avialans. They were small and lightly-built feathered carnivores, similar in biology to Archaeopteryx, early dromaeosaurids like Microraptor, and particularly troodontids. They are almost exclusively known from Late Jurassic Chinese deposits, although Ostromia was discovered in Germany 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.
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, 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.
Foth and Rauhut (2017) established several diagnostic features present in anchiornithids:
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, 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.
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, Gallus, Troodon, Dromaeosaurus, Unenlagia, or Epidexipteryx".
In 2017 Foth and Rauhut in their re-evaluation of the Haarlem Archaeopteryx specimen (which they classified it in its own distinct genus Ostromia) found that the anchiornithids are a distinct family closer to the ancestry of birds. They provided their own definition of Anchiornithidae as "as all maniraptoran theropods that are more closely related to Anchiornis huxleyi than to Passer domesticus, Archaeopteryx lithographica, Dromaeosaurus albertensis, Troodon formosus, or Oviraptor philoceratops."
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 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.
A good majority of the known anchiornithid fossils are have been recovered from the Tiaojishan Formation in Liaoning, China dating back to 160 million years. The climate during this period of time would have been subtropical to temperate, warm and humid based on the plant life present in the Tiaojishan Formation. This environment was dominated plant by gymnosperm trees. There were ginkgopsids like Ginkoites, Ginkgo, Baiera, Czekanowskia, and Phoenicopsis. There were also conifers like Pityophyllum, Rhipidiocladus, Elatocladus, Schizolepis, and Podozamites. Also, Lycopsids like Lycopodites and Sellaginellities, horsetails (Sphenopsida) like Equisetum, cycads like Anomozamites, and ferns (Filicopsida) like Todites and Coniopteris. Only one genus of anchiornithid has been found outside of China: Ostromia, which is found in the Painten Formation from Riedenburg, Bavaria, Germany.
Anchiornis is a genus of small, four-winged paravian dinosaur. The genus Anchiornis contains only the type species Anchiornis huxleyi, named for its similarity to modern birds. Anchiornis fossils have been only found in the Tiaojishan Formation of Liaoning, China, in rocks dated to the Late Jurassic, about 160 million years ago. Anchiornis is known from hundreds of specimens, and given the exquisite preservation of some of these fossils, it became the first Mesozoic dinosaur species for which almost the entire life appearance could be determined, and an important source of information on the early evolution of birds. Anchiornis huxleyi translates to "T.H. Huxley's near-bird" in Greek.Aurornis
Aurornis is an extinct genus of anchiornithid theropod dinosaurs from the Jurassic period of China. The genus Aurornis contains a single known species, Aurornis xui (). Aurornis xui may be the most basal ("primitive") avialan dinosaur known to date, and it is one of the earliest avialans found to date. The fossil evidence for the animal pre-dates that of Archaeopteryx lithographica, often considered the earliest bird species, by about 10 million years.Aurornis xui was first described and named by Pascal Godefroit, Andrea Cau, Hu Dong-Yu, François Escuillié, Wu Wenhao and Gareth Dyke in 2013. The generic name is derived from the Latin word aurora, meaning "daybreak" or "dawn", and the Ancient Greek ὄρνις (órnis) meaning "bird". The specific name, A. xui, honors Xu Xing. A recent study of specimens of the avialan Anchiornis has found that the traits exhibited by Aurornis fall within the range of variation in Anchiornis, warranting their synonymization.Avialae
Avialae ("bird wings") is a clade of flying dinosaurs containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to modern birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used (see below).
Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is the earliest known avialan which may have had the capability of powered flight. However, several older avialans are known from the late Jurassic Tiaojishan Formation of China, dated to about 160 million years ago.Caihong
Caihong (Chinese: 彩虹; pinyin: cǎihóng; literally: 'rainbow') is a genus of small paravian theropod dinosaur from China that lived during the Late Jurassic period.Eosinopteryx
Eosinopteryx is an extinct genus of theropod dinosaurs known the late Jurassic period of China. It contains a single species, Eosinopteryx brevipenna.Ostromia
Ostromia 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 in 2017.Paraves
Paraves are a widespread group of theropod dinosaurs that originated in the Late Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and scansoriopterygids, the group also contains the avialans, among which are the over ten thousand species of living birds. Primitive members of Paraves are well known for the possession of an enlarged claw on the second digit of the foot, which was held off the ground when walking in some species.Pedopenna
Pedopenna (meaning "foot feather") is a genus of small, feathered, maniraptoran dinosaur from the Daohugou Beds in China. It is possibly older than Archaeopteryx, though the age of the Daohugou Beds where it was found is debated. A majority of studies suggest that beds probably date from between the late Middle Jurassic (168 million years ago) and early Late Jurassic Period (164-152 million years ago).The name Pedopenna refers to the long pennaceous feathers on the metatarsus; daohugouensis refers to the locality of Daohugou, where the holotype was found. Pedopenna daohugouensis probably measured 1 meter (3 ft) or less in length, but since this species is only known from the hind legs, the actual length is difficult to estimate. Pedopenna was originally classified as a paravian, the group of maniraptoran dinosaurs that includes both deinonychosaurs and avialans (the lineage including modern birds), but some scientists have classified it as a true avialan more closely related to modern birds than to deinonychosaurs.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. Its name means "Ge Sun's silk bird", a reference to the plumulaceous-like body covering evident in the fossil. The specimen's nickname, "Silky", refers to the striking resemblance of the delicate hindlimb filaments to the modern Silky breed of domestic chicken.Serikornis, first described in August 2017, is noteworthy for the variety of feather types represented in its holotype, a single complete articulated skeleton preserved on a slab along with extensive integumentary structures. The specimen's feather imprints include wispy bundles along the neck, short and symmetrical vaned feathers on the arms, and both fuzz and long pennaceous feathers on its hindlimbs. While its anatomy and integument share features with birds as well as derived dromaeosaurs such as Microraptor, cladistic analysis places the genus within the cluster of feathered dinosaurs near the origin of avians. It was unlikely to be a flier.Xiaotingia
Xiaotingia is a genus of anchiornithid theropod dinosaur from early Late Jurassic deposits of western Liaoning, China, containing a single species, Xiaotingia zhengi.Yixianosaurus
Yixianosaurus (meaning "Yixian lizard") is a maniraptoran theropod dinosaur genus from the Early Cretaceous of China.
The type species, Yixianosaurus longimanus, was formally named and described by Xu Xing and Wang Xiaolin in 2003. Its partial skeleton was discovered in 2001, in Liaoning at Wangjiagou in northeastern China. The generic name refers to the Yixian Formation. The specific name means "with a long hand" from Latin longus, "long", and manus, "hand".