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The Relationships of the Early Cretaceous Ambiortus and
Otogornis (Aves: Ambiortiformes)
ABSTRACT
Ambiortus from the Khurilt beds (Neocomian) of central Mongolia
shows a combination of characters that confirms the assignment
of this fossil to a separate order, Ambiortiformes. Otogornis
genghisi Hou, 1994, of the Yijinhouluo Formation (earliest Cretaceous
or latest Jurassic) of Ordos, China, was first described as
Aves incertae sedis. Ambiortus and Otogornis share specialized
characters, such as a thickened, three-edged acrocoracoid with an
acute top; a flat, wide humeral articular facet of the scapula; ventral
position of a small, oval humeral articular head; and a thin,
long intermediate phalanx of the major wing digit. The generic status
of Otogornis is supported by some other diagnostic characters.
Several advanced characters demonstrate the assignment of
Ambiortus and Otogornis to the Palaeognathae. These two forms
show the occurrence of paleognathous birds in the Early Cretaceous
of Central Asia.
Introduction
The Early Cretaceous Ambiortus dementjevi Kurochkin,
1982, was described as a member of the new family Ambiortidae
and order Ambiortiformes, which was assigned to the
infraclass Carinatae (Kurochkin, 1982). Ambiortus was based
on an associated portion of the skeleton, including the vertebrae,
the shoulder girdle, and some wing bones preserved on
the main slab and counterslab, and also on the distal portion of
the wing bones and feather imprints, which are preserved on an
associated slab. Two papers containing more extensive description
and comparison of this fossil were published later (Kurochkin,
1985a, 1985b). The surprising appearance of this Early
Cretaceous tme bird in the paleontological record made comparison
with other birds very difficult. In these first papers I attempted
to compare Ambiortus with members of the living
Gmidae, Rallidae, Strigidae, Alcedinidae, Momotidae, and, su-
Evgeny N. Kurochkin, Paleontological Institute of the Russian Academy
of Sciences, 123 Profsouznaya Street, 117868 Moscow, GSP-7,
Russia.
Evgeny N. Kurochkin
275
perficially, with the Archaeopterygiformes, Ichthyornithiformes,
Paleogene paleognaths (later described as Lithornithiformes
Houde, 1988), and Wyleyia Harrison and Walker, 1973.
However, further study and a possibility of a direct comparison
with the Enantiornithes, Archaeopteryx, Ichthyornis, Wyleyia,
Paracathartes, Lithomis, Palaeotis, and living Palaeognathae,
and also additional preparation of the holotype of Ambiortus,
showed the published anatomical descriptions and morphological
comparison of this fossil to be incomplete and partly erroneous.
Evidence for the relationships of Ambiortus with the
Palaeognathae was published beginning in 1985 (Kurochkin,
1985a, 1988, 1995a, 1995b). This mainly was based on comparison
with the Paleogene Lithornithiformes studied by
Houde and Olson (1981) and Houde (1988). Olson (1985) emphasized
that Ambiortus shows some decided similarities with
the Paleogene paleognathous birds and also may share some
common characters of the humems with Ichthyornis. Martin
(1987, 1991) united Ambiortus with Apatomis and the Ichthyomithiformes.
Cracraft (1986) concluded that Ambiortus can
be assigned to the Carinatae, in which he included the Palaeognathae,
Neognathae, and Ichthyornis. Sereno and Rao (1992)
have placed Ambiortus outside the Ornithurae without character
evidence. Chiappe (1995) considered Ambiortus to be the
oldest probable ornithurine, yet one of unclear relationships.
Elzanowski (1995) assigned Ambiortus to the Carinatae and
primitive Neornithes, close to Ichthyornis; however, his cladistic
analysis of the skeletal characters also placed the Enantiornithes,
Cathayomis, and Concomis among the Carinatae. Obviously,
Ambiortus has nothing in common with the
Enantiornithes. In that paper, Elzanowski (1995) made mainly
mistaken interpretations of skeletal characters in Ambiortus
that are discussed below. Feduccia (1995, 1996) placed Ambiortus
in the basal Ornithurae together with Gansus, the Hesperornithiformes,
and Ichthyornithiformes.
Thus, controversial phylogenetic conclusions exist concerning
Ambiortus, and it has remained isolated in the avian phylogenetic
tree as the result of incomplete and questionable descriptions
(Kurochkin, 1982, 1985a, 1985b), especially those