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Anchiornithidae is a family of small Paraves . Anchiornithids have been classified at varying positions in the paravian 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.
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 . 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.
In their description of Wiehenvenator Rauhut and colleagues had informally called the group as " Anchiornithosaurs" which they placed outside of Avialae.Rauhut, O.W.M., Hübner T.R., and Lanser, K., 2015, "A new theropod dinosaur from the late Middle Jurassic of Germany and theropod faunal turnover during the Jurassic", Libro de resúmenes del V Congreso Latinoamericano de Paleontología de Vertebrados. 62
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, Junglefowl, 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 "all maniraptoran 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.
In 2019 with the description of the Late Jurassic genus Hesperornithoides, 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 and Yixianosaurus being classified as a troodontid and a dromaeosaurid respectively, Pedopenna found in many possible positions within the Paraves phylogeny, and Ostromia 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).
Cau (2024) found support for the inclusion of Scansoriopterygidae in Anchiornithidae.
Chinese anchiornithids discovered outside of the Tiaojishan Formation include Yixianosaurus longimanus, which was found in the 125 million-year-old Early Cretaceous Yixian Formation. Fujianvenator prodigiosus was discovered in the 148 to 150 million-year-old Zhenghe Biota of southeastern China, which was dominated by aquatic and semi-aquatic fossils such as fish and turtles indicative of a lacustrine swamp environment. Only one genus of anchiornithid has been found outside of China: Ostromia, which is found in the Painten Formation from Riedenburg, Bavaria, Germany.
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