Aves C. Linnaeus 1758 [J. A. Clarke, D. P. Mindell, K. de Queiroz, M. 
Hanson, M. A. Norell, and J. A. Gauthier], converted clade name 
Registration Number: 113 provided lists of many extinct members of the 
crown, as well as a few extinct species that are 
De!nition: !e smallest crown clade containing here regarded to have diverged from the avian 
Struthio camelus Linnaeus 1758 (Palaeognathae), stem. 
Tinamus (originally Tetrao) major (Gmelin 
1789) (Palaeognathae/Tinamidae), Phasianus Diagnostic Apomorphies: With respect to 
colchicus Linnaeus 1758 (Neognathae/Galloans other extant amniotes, Huxley’s (1867: 416– 
erae/Galliformes), and Vultur gryphus Linnaeus 417) diagnosis of Aves still serves (see Comments 
1758 (Neognathae/Neoaves/Accipitriformes). !is below): 
is a minimum-crown-clade de"nition. Abbre-
viated de"nition: min crown ∇ (Struthio cam-  1. “[E]pidermal appendages developed in 
elus Linnaeus 1758 & Tinamus major (Gmelin sacs of the dermis, and having the struc-
1789) & Phasianus colchicus Linnaeus 1758 & ture of feathers. 
Vultur gryphus Linnaeus 1758).  2. [A] remarkably large sacrum, the verte-
brae, through the intervertebral foram-
Etymology: Derived from the Latin vernacular ina of which the roots of the sacral 
for “birds.” plexus (and, consequently, of the great 
sciatic nerve) pass, are not provided 
Reference Phylogeny: Figure 1 in the compre- with expanded ribs abutting against 
hensive molecular analysis of Prum et al. (2015). the ilium externally, and against the 
We selected speci"er species from among those bodies of these vertebrae by their inner 
originally used by Linnaeus to represent uncon- ends. [Instead, these vertebrae are con-
troversial avian subclades in order to bracket nected to the ilia via] slender trans-
the crown clade. Because some of these speci"er verse processes, which seem to answer 
species are not included in the reference phy- to those which unite with the tubercles 
logeny, more inclusive taxa containing them are of the ribs in the dorsal region [in other 
listed parenthetically in the de"nition to facili- reptiles]. 
tate its application.  3. !e broad and expanded part of the 
sternum, which immediately follows the 
Composition: Aves currently contains more coracoidal articular surfaces, receives all 
than 10,000 described species, but could include the sternal ribs. 
as many as 5,000–10,000 more depending upon  4. !e ischia never unite in a median ven-
reassessment of currently recognized subspecies tral symphysis; and both pubes and 
(Barrowclough et al., 2016). !ese include all ischia are directed backwards, approxi-
those listed in Brodkorb (1963, 1964, 1967, mately parallel with one another and 
1971, 1978), Unwin (1993), Mlíkovsky (2002), with the spinal column. 
Clements (2007), Mayr (2009), del Hoyo  5. !e proximal constituent of the tarsus 
et al. (2013), and Gill and Donsker (2018), is anchylosed [sic] with the tibia into 
Aves 
one tibio-tarsal bone; the distal element both branches of the basal split within the crown 
of the tarsus similarly unites with the clade (Gauthier, 1986). A century later, discov-
second, third, and fourth metatarsal ery of the stem bird Archaeopteryx lithographica 
bones, and gives rise to the tarso-meta- (von Meyer, 1861; Owen, 1863) with its mosaic 
tarsal bone. !e metatarsal of the hallux of ancestral (e.g., teeth, long bony tail) and 
is shorter than the others, and does not derived (e.g., feathers, wings) characters, engen-
reach the tarsus. dered controversy regarding the circumscription 
 6. [H]ot blood … a single aortic arch, of Aves (reviewed in de Beer, 1954). A con-
and remarkably modi"ed respiratory sensus eventually emerged that Archaeopteryx 
organs.” should be considered part of Aves, as "rst pro-
posed explicitly by Haeckel (1866), who also 
Additional apomorphies have been listed proposed Ornithurae to distinguish the “living 
by Cracraft (1988) and Kurochkin (1995). … true birds” (p. 140) from Archaeopteryx. A 
Currently, there are no unambiguous synapo- loose association between Aves and at least part 
morphies of Aves that will distinguish members of its stem lineage remained in steady use for 
of the crown from all known members of its more than a century following Haeckel (1866), 
stem (see Comments). although the exact clade to which the name 
applied became more varied with the discovery 
Synonyms: Ornithurae Haeckel 1866 (approxi- of additional fossil intermediates, increasing 
mate); Neornithes Gadow 1892 (approximate); knowledge of phylogenetic relationships, and 
see review in Gauthier and de Queiroz (2001) changing taxonomic philosophies. More spe-
and Comments below. ci"cally, the name Aves became associated with 
at least "ve di$erent nested clades (see Gauthier 
Comments: !e ease with which Aves (the and de Queiroz, 2001, for a review): (1) the 
crown) could be diagnosed in the mid-nineteenth total clade of birds (e.g., Patterson, 1993); (2) 
century depended entirely upon missing data, as the clade characterized by pinnate feathers (e.g., 
does its ease of diagnosis relative to other extant James and Pourtless, 2009; Feduccia, 2013); (3) 
taxa today. However, due both to the discov- the clade characterized by %ight (e.g., Ji and Ji, 
ery of an array of intermediate forms (e.g., Cau, 2001; Xu et al., 2009); (4) the clade stemming 
2018), and to the fact that extinct species closest from the Archaeopteryx node (e.g., Padian and 
to the crown are so incompletely preserved, it is Chiappe, 1998); and (5), the crown clade (e.g., 
now much more di#cult to distinguish members Gauthier, 1986). 
of the crown (Aves) from the nearest members of Some contemporary systematists (those 
its stem. !e poorly known extinct species most adopting one of the "rst four uses of Aves listed 
closely related to the crown were placed outside above) prefer the name Neornithes Gadow 1892 
the crown when considered individually (e.g., for the crown. !is name was proposed by 
Clarke, 2004). Nevertheless, when considered Gadow (see also Gadow, 1893) as an explicit 
together, there was so little overlap among their replacement name for Ornithurae to distinguish 
preserved remains that there were no longer any extant birds as well as the Cretaceous toothed 
unambiguously optimized morphological apo- birds (Marsh, 1880), which were unknown 
morphies for Aves. !is remains true today. to Haeckel (1866), from Archaeopteryx. 
Linnaeus proposed the name Aves for a group Subsequently, however, the associations of these 
composed entirely of extant species representing names diverged. Ornithurae came to be applied 
1248 
Aves 
to a more inclusive clade than the crown, i.e., to even though they were addressing relationships 
those stem avians sharing apomorphies of the within the crown (Livezey and Zusi, 2007: 42). 
tail to which the name implicitly refers (e.g., Despite e$orts to promote the use of 
Gauthier, 1986; Elzanowski, 1995; Gauthier Neornithes, a preference for using Aves when 
and de Queiroz, 2001; Clarke, 2004; Zhou referring to the crown remains strong. Recent 
and Zhang, 2006). Neornithes was "rst applied landmark molecular phylogenetic studies, for 
explicitly to the crown by Walker (1981), and example, which were necessarily restricted 
that practice has been followed frequently in the to the crown clade, have not used Neornithes 
palaeontological literature (e.g., !ulborn, 1984; or “neornithine” but referred to their topic as 
Chiappe, 1995; Dyke and van Tuinen, 2004). “avian” diversi"cation and their target clade 
But palaeontologists have also used Neornithes as Aves (Hackett et al., 2008; Jetz et al., 2012; 
for more inclusive clades, such as for the crown Burleigh et al., 2015; Prum et al., 2015). Jarvis 
plus Ichthyornithes (e.g., Martin, 1983), or the et al. (2014) used both Neornithes and “avian” 
crown and all stem birds closer to the crown when referring to the crown clade. Other stud-
than to Enantiornithes (e.g., Elzanowski, 1995). ies have avoided using either Aves or Neornithes 
Given that most published papers on birds but continued to use “avian” (e.g., McCormack 
deal only with extant species, however, the et al., 2013; Reddy et al., 2017). 
preference for Neornithes for the crown does None of the uses of the name Aves is opti-
not accord well with the most widespread use mal in all respects. Exclusion of Archaeopteryx 
of the name Aves (Gauthier and de Queiroz, lithographica from Aves is disruptive given that 
2001). Google Scholar (Aug. 2018) yields more this species was included in that taxon by many 
than one million records for searches on “Aves” authors for more than 150 years. Associating Aves 
or “avian”; by contrast, searches on “Neornithes” with pinnate feathers would be even more disrup-
or “neornithine” (sic) yield approximately 2,500 tive, as such feathers are now known to diagnose 
and 800 records, respectively. Contrasting a much larger clade than the Archaeopteryx node 
usage seems to follow disciplinary boundaries, that includes more traditional “non-avian” the-
with palaeontological authors being more likely ropods such as Velociraptor (e.g., Clarke, 2013). 
to use Neornithes than ornithologists focused on Applying the name to the total clade requires 
extant birds. Unfortunately, some authors who including even more distantly related extinct 
have explicitly chosen to use the name Neornithes forms not traditionally included in Aves such as 
for the crown have not been entirely successful sauropodomorphs (e.g., Brontosaurus), ornith-
at avoiding inconsistent use of the name Aves. ischians (e.g., Triceratops), and pterosaurs (e.g., 
For example, consider the title of the paper Rhamphorhynchus). Linking the name Aves to the 
"Phylogenetic Relationships among Modern origin of “%ight” is hard to maintain in the face 
Birds (Neornithes)" with the subtitle "Toward of abundant discoveries that show this complex 
an Avian Tree of Life," and one section of that character was not a simple apomorphy diagnosing 
paper entitled "!e Challenge of Resolving a single node (e.g., Feo et al., 2015). Additionally, 
Avian Relationships" (Cracraft et al., 2004). the fraction of the scienti"c community that 
Similarly, authors who analyzed the “Higher- needs to distinguish between the origin of volant 
order phylogeny of modern birds (!eropoda, dinosaurs and the crown clade is exceeding small. 
Aves: Neornithes)” refer to "Broad a#nities of In sum, most authors continue to use “Aves” or 
long standing among the avian orders ... that “avian” to discuss aspects of bird biology—such 
were not supported by the present analysis," as their genomes—that have been documented 
1249 
Aves 
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is rarely used when discussing extant birds Phylogenet. Evol. 84:53–63. 
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Authors Mark A. Norell; Division of Paleontology; American 
Museum of Natural History; New York, NY 
Julia Clarke; Department of Geological Sciences; 10024, USA. Email: norell@anmh.org. 
Jackson School of Geosciences, !e University Jacques A. Gauthier; Department of Geology and 
of Texas, Austin, TX 78712-1722. Email: Geophysics; Yale University; 210 Whitney 
Julia_Clarke@jsg.utexas.edu. Ave, New Haven, CT 06520-8109, USA. 
David P. Mindell; Museum of Vertebrate Zoology; Email: jacques.gauthier@yale.edu. 
University of California; Berkeley, CA 94720-
3160, USA. Email: dpmindell@gmail.com. Date Accepted: 14 October 2018 
Kevin de Queiroz; Department of Vertebrate 
Zoology; National Museum of Natural History; 
Smithsonian Institution; Washington, DC Primary Editor: Philip Cantino 
20560-0162, USA. Email: dequeirozk@si.edu. 
Michael Hanson; Department of Geology and 
Geophysics; Yale University; New Haven, CT 
06520-8109, USA. Email: michael.hanson@ 
yale.edu. 
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