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Ichthyornis dispar

Reptilia - Avetheropoda

Taxonomy
Ichthyornis dispar was named by Marsh (1872). Its type specimen is YPM 1450, a partial skeleton (vertebrae, wings and legs), and it is a 3D body fossil. Its type locality is Bow Creek; Sugar Bowl Mound, which is in a Coniacian/Santonian marine chalk in the Niobrara Formation of Kansas.

Synonyms
Synonymy list
YearName and author
1872Ichthyornis dispar Marsh
1872Graculavus anceps Marsh pp. 364-365
1872Colonosaurus mudgei Marsh p. 406
1873Graculavus agilis Marsh
1873Ichthyornis dispar Marsh p. 117
1876Ichthyornis victor Marsh
1880Ichthyornis dispar Marsh p. 120
1880Ichthyornis victor Marsh p. 121
1880Ichthyornis agilis Marsh p. 197
1880Ichthyornis anceps Marsh p. 198
1880Ichthyornis validus Marsh p. 198
1890Ichthyornis dispar Zittel p. 835
1915Graculavus anceps Shufeldt p. 19
1915Graculavus agilis Shufeldt p. 20
1933Ichthyornis dispar Lambrecht p. 581
1933Ichthyornis agilis Lambrecht p. 583
1933Ichthyornis anceps Lambrecht p. 583
1933Ichthyornis validus Lambrecht p. 584
1934Ichthyornis victor Swinton p. 4
1960Ichthyornis dispar Wetmore p. 3
1962Plegadornis antecessor Wetmore p. 1 fig. 1
1972Ichthyornis dispar Gingerich p. 471
1972Angelinornis antecessor Kashin
1975Ichthyornis antecessor Olson p. 105
1981Plegadornis anteccessor Thurmond and Jones p. 164
1991Ichthyornis dispar Chatterjee p. 328
1992Ichthyornis antecessor Parris and Echols
1992Ichthyornis dispar Parris and Echols
2002Ichthyornis dispar Chiappe p. 462
2002Ichthyornis validus Chiappe p. 462
2002Ichthyornis victor Chiappe p. 462
2004Ichthyornis dispar Clarke p. 21
2004Ichthyornis dispar Padian p. 214

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RankNameAuthor
kingdomAnimalia()
Triploblastica
Nephrozoa
Deuterostomia
phylumChordataHaeckel 1847
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
Sarcopterygii
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda()
Reptiliomorpha
Anthracosauria
Batrachosauria()
Cotylosauria()
Amniota
Sauropsida
classReptilia
subclassEureptilia()
Romeriida
Diapsida()
Eosuchia()
Neodiapsida
SauriaGauthier 1984
Archosauromorpha(Huene 1946)
RankNameAuthor
Crocopoda
ArchosauriformesGauthier 1986
Eucrocopoda
Archosauria()
informalAvemetatarsalia
Ornithodira
Dinosauromorpha
Dinosauriformes
Dinosauria()
Saurischia()
Theropoda()
Neotheropoda
AverostraPaul 2002
Tetanurae
orderAvetheropoda
suborderCoelurosauria
Maniraptora
Paraves
classAves
PygostyliaChiappe 2002
Ornithothoraces
OrnithuromorphaChiappe et al. 1999
subclassOrnithurae
genusIchthyornis
speciesdispar

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Diagnosis
ReferenceDiagnosis
J. A. Clarke 20041. Quadrate: single large pneumatic foramen located on the anteromedial surface of the corpus of the quadrate, lying close to the pterygoid articulation (‘‘1’’ in fig. 4). This condition is not encompassed by either character 39 or 40 (appendix 1) as it was not observed in any of the other included taxa (although per the recommendation in Part II, Methods, it will be included in subsequent analyses). The anteromedial position of the foramen (close to the pterygoid articulation) is considered a local autapomorphy of Ichthyornis (contra Witmer, 1990) with this condition also observed in some Neoaves (Witmer, 1990).

2*. Cervical vertebrae: amphicoelous or ‘‘biconcave’’. The conformation of the articular surfaces of the cervical vertebral centra in Ichthyornis is unambiguously optimized as derived (‘‘2’’ in fig. 4). While amphicoelous cervical articulations are developed in nonavialan theropods and Archaeopteryx, the cervical articular surfaces are heterocoelous in Patagopteryx deferrariisi, Hesperornis regalis, Baptornis advenus, Apsaravis ukhaana, Lithornis, and Aves. Other basal avialan taxa (e.g., Confuciusornis sanctus) exhibit an intermediate condition (appendix 2, character 52). The cervical articulations in Ichthyornis (described in detail in the Anatomical Description) are optimized not as homologous with the amphicoely in more basal theropods, but as a unique transformation of a heterocoelous conformation. However, both the anterior and posterior articular surfaces of all cervical vertebrae are ovoid and flat with a central concavity (fitting the definition of amphicoely). By contrast, heterocoelous articulations involve an anterior surface that is broadly concave mediolaterally but convex dorsoventrally and a posterior surface convex mediolaterally and concave dorsoventrally.

3*. Caudal vertebrae: anterior free caudal vertebrae with well-developed prezygapophyses clasping the dorsal surface of preceding vertebra (‘‘3’’ in fig. 4; appendix 1, character 66:2). As Marsh (1880) noted, in Ichthyornis a reverse of the typical zygapophysial articulation is developed with elongate prezygapophyses clasping the dorsal surface of the preceding vertebra in the anterior caudal vertebrae. The postzygapophyses, by contrast, are extremely weakly developed. They are flat facets on the posterodorsal surface of the neural arch that are in contact with the ventral surface of the prezygapophysis of the succeeding vertebra. By contrast, well-developed pre- and postzygapophyses (appendix 1, character 66:0) are present in the anterior caudal vertebrae of the outgroups and Confuciusornis sanctus. Both the pre- and postzygapophyses are short (even apparently noncontacting in the avian taxa included in this analysis as well as in Hesperornis regalis; appendix 1, character 66:1) but show no sign of a reverse articulation. In Ichthyornis, the pre- and postzygapophyses are short relative to the outgroup condition, but have a reverse articulation developed. This conformation is also observed within Neoaves (e.g., Charadriiformes such as Vanellus melanopterus). And, while its distribution deserves further scrutiny, it is not present in any taxa included in this analysis other than Ichthyornis. Even if development of a reverse articulation is found to be ancestral to Neoaves, it would remain most parsimoniously optimized as an autapomorphy of Ichthyornis.

4*. Scapula: The presence of an extremely diminutive acromion process (‘‘4’’ in fig. 4) is unambiguously optimized as an autapomorphy of Ichthyornis. The acromion in Ichthyornis is minute (fig. 37); it does not extend anteriorly beyond the bosslike articular surface for the coracoid (appendix 1, character 103:0). The acromion also does not extend anterior to this articulation in Chauna torquata in what is optimized as a separate evolution of this morphology. In all other included taxa for which this character could be scored, the acromion extends well anterior to the articular surface for the coracoid (see appendix 1, character 103, regarding outgroup condition and Hesperornithes).

5. Humerus, bicipital crest, pit-shaped fossa for muscular attachment located directly at the distal end of the bicipital crest (‘‘5’’ in fig. 4): The condition in Ichthyornis is currently currently not seen in any other avialan taxa. Considering the fossa/scar seen in Enantiornithes, Apsaravis ukhaana, Ichthyornis, and Aves on the bicipital crest (see appendix 1, character 115) as potential homologues is different from specifying a transformation series for this morphology. The latter would require that the directly distal position seen only in Ichthyornis is a necessary intermediate condition between an anterodistal position seen in more basal taxa (see appendix 1, characters 115, 116) and the posterodistal position in Aves. As the condition in Ichthyornis dispar is seen in no other taxon, it is used in the diagnosis. However, because this optimization is currently ambiguous (due to missing data in Limenavis patagonica and Gansus yumenensis and of uncertain homology with the highly transformed condition in Hesperornithes), this character is not used in the definition of the clade name ‘‘Ichthyornis’’ although it is preserved in the holotype of Ichthyornis dispar (YPM 1450).

6*. Ulna: the dimensions of the dorsal condyle (appendix 1, character 132) are such that the length of the trochlear surface along the posterior surface of the distal ulna is approximately equal to the width of the trochlear surface taken across its distal end (‘‘6’’ in fig. 4). While these dimensions are also seen developed within Neoaves (Clarke and Chiappe, 2001) as well as apparently in Gobipteryx minuta (Kurochkin, 1996; although this morphology is extremely poorly preserved in that taxon), this character is unambiguously optimized as a local autapomorphy of Ichthyornis. All included taxa for which this character could be scored have a dorsal condyle with the posterior extent of the trochlear surface less than its distal width. In the outgroup taxa, the trochlear surface has no extension up the posterior edge of the ulna (see appendix 1, characters 131, 132).

7. Radius: an oval scar located on the posteroventral surface of the distal radius, in the center of a depression (depressio ligamentosa in Aves; Baumel and Witmer, 1993). The depression is also seen in Ichthyornis, but no conspicuous oval scar is developed in the included Aves. The ovoid scar (‘‘7’’ in fig. 4) was not observed in any other avialans. However the posteroventral surface of the distal radius is not visible in Apsaravis ukhaana and not preserved in Patagopteryx deferrariisi. A scar appears to be absent in Baptornis advenus and Confuciusornis sanctus.

8. Carpometacarpus: A large tubercle is developed close to the articular surface for the first phalanx of the second digit where the deep tendinal groove for the m. extensor digitorum communis ends as this tendon passes distally to insert on the first phalanx in the crown clade (Stegmann, 1978). This robust tubercle (‘‘8’’ in fig. 4) is not present in any of the included Aves, YPM 1734, Limenavis patagonica, or more basal taxa where this portion of metacarpal II is preserved (e.g., Neuquenornis volans or Confuciusornis sanctus). Some Charadriiformes (Stegmann, 1978) have a tubercle in approximately the same position as Ichthyornis. Stegmann (1978) related this feature to the attachment of part of the lig. digito-metacarpale, part of which constrains the passage of the m. extensor digitorum communis.

9*. Phalanx II:1: the presence of an internal index process (Stegmann, 1978; appendix 1, character 152:1). An internal index process (‘‘9’’ in fig. 4) is seen within Neoaves (e.g., Charadriiformes; Stegmann, 1978) but not in any of the avian taxa included in the phylogenetic analyses (i.e., the galloanserine or palaeognath exemplars used; see Part II, Material and Methods). This process is not present in Iaceornis marshi (YPM 1734; see below), Limenavis patagonica (Clarke and Chiappe, 2001), Apsaravis ukhaana (Norell and Clarke, 2001), or more basal taxa (e.g., Confuciusornis sanctus, Chiappe et al., 1999; see also appendix 1, character 152:1).