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These are the absence of the bicipital crest and intumescence
(character 11); the absence of the pneumatic foramen in the
pneumotricipital fossa, being expressed only as an undivided
tricipital depression (character 12); a small ventral tubercle on
the proximal end of the humems; the presence of a ligamental
fossa instead of a transversal ligamental furrow (character 15);
a rounded cross-section of the shaft of the radius; and the presence
of an ungual phalanx on the major wing digit. These characters
demonstrate that the Ambiortiformes have a common origin
with other orders of paleognathous birds.
Otogornis differs from Ambiortus in having a smaller procoracoid
process; a deep groove on the lateral side of the shoulder
end of the scapula; a wide scapular blade (narrow in Ambiortus);
a flat, elongated excavation along the cranial side of the
deltopectoral crest; and the presence of a capital groove, which
is divided into two furrows (Figures 6, 7). Differences in the
detailed morphology of the scapula and humems support their
separate generic status, although it could be argued that they
are only two species of a single genus.
Ambiortus dementjevi is smaller than Otogornis genghisi.
The maximum width of the proximal end of the humems of A.
dementjevi is 13.0 mm, and the maximum width across the
most projecting edge of the deltopectoral crest is 11.2 mm. The
same measurements in O. genghisi are 15.8 mm and 12.2 mm,
respectively.
Chen, Peiji, and Zhenlu Chang
1994. Nonmarine Cretaceous Stratigraphy of Eastern China. Cretaceous
Research, 15:245-257, 4 figures.
Chiappe, Luis M.
1995. First 85 Millions Years of Avian Evolution. Nature. 378:349-355, 6
figures.
Cracraft, J.
1986. The Origin and Early Diversification of Birds. Paleobiology,
12:383-399, 1 table, 4 figures.
Dmitriev, V.Yu., and V.V. Zherikhin
1988. [The Changes of Diversity in the Insect Families on the Data by the
Method of Accumulated Occurrences.] In A.G. Ponomarenko, editor,
Cretaceous Biocenotic Crisis and the Evolution of Insects,
pages 208-215, 1 table, 1 figure. Moscow: Nauka. [In Russian.]
Dong, Zhi-Ming
1993. A Lower Cretaceous Enantiornithine Bird from the Ordos Basin of
Inner Mongolia, People's Republic of China. Canadian Journal of
the Earth Sciences, 30:2177-2179, 1 table, 2 figures.
Elzanowski, A.
1995. Cretaceous Birds and Avian Phylogeny. Courier Forschungsinstitut
Senckenberg, 181:37-53, 4 figures.
Fan, Jin
1996. New Advances in the Late Mesozoic Stratigraphic Research of
Western Liaoning, China. Vertebrata Palasiatica, 34(2): 102-122, 3
tables. [In Chinese, with English summary.]
Literature Cited
Conclusions
Ambiortus from central Mongolia and Otogornis from the Ordos
Basin, China, show a close relationship based on the shared,
specialized characters 8, 10, and 16 in the structure of the forelimb
and shoulder girdle (Table 1). At the same time, Ambiortus
and Otogornis show some differences in the shoulder girdle and
the forelimb that support their separate generic status.
The relationships of Ambiortus and Otogornis with other
birds are determined by comparison with the Ichthyomithes,
Neornithes, Palaeognathae, and Neognathae. Ambiortus and
Otogornis share an advanced condition of characters 7 and 9,
which are common to the Ornithurae. Ambiortus shares with
the Neornithes an advanced condition of characters 1 and 3,
which are unknown for Otogornis. At the same time, the Ambiortiformes
share with the Palaeognathae (including Lithomithiformes)
such specialized characters as 5, 6, 13, and 14,
which suggests their assignment to the parvclass Palaeognathae,
sensu Kurochkin (1995b). No common advanced characters
were found for the Ambiortiformes, Ichthyomithes, and
Enantiornithes. This study confirms that the Ambiortiformes
are not closely related to the Ichthyomithes or the Neognathae
and are totally unrelated to the Enantiornithes.
The Early Cretaceous Ambiortiformes were flying palaeognathous
birds. Thereby, they document an early diversification
of ornithurine birds into two main evolutionary branches:
Palaeognathae and Neognathae.
Feduccia, Alan
1995. Explosive Evolution in Tertiary Birds and Mammals. Science,
267:637-638, 2 figures.
1996. Origin and Evolution of Birds, x+420 pages. New Haven and London:
Yale University Press.
Harland, W.B., R.L. Armstrong, A.V. Cox, L.E. Craig, A.G. Smith, and D.G.
Smith
1989. A Geologic Time Scale 1989. xvi+263 pages. Cambridge: Cambridge
University Press.
Harrison, C.J.D., and CA. Walker
1973. Wyleyia: A New Bird Humerus from the Lower Cretaceous of England.
Palaeontology, 16:721-728, 2 figures.
Hou, Lianhai
1994. A Late Mesozoic Bird from Inner Mongolia. Vertebrata Palasiatica,
32(4):258-266, 3 figures, 1 plate.
Houde, P.
1988. Paleognathous Birds from the Early Tertiary of the Northern Hemisphere.
Publications of the Nuttall Ornithological Club, 22: vii+148
pages, 27 tables, 41 figures.
Houde, P., and S.L. Olson
1981. Paleognathous Carinate Birds from the Early Tertiary of North
America. Science, 214:1236-1237, 2 figures.
Krassilov, V.A.
1982. Early Cretaceous Flora of Mongolia. Palaeontographica, series B,
Paldophytologie, 181:1-43, 1 table, 11 figures, 20 plates.