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concerning the amphicoelous structure of the vertebrae and the
bones of the shoulder girdle. New preparation and observation
of the specimen provide a complete and corrected anatomical
description of this bird.
It was a great surprise to discover that Ambiortus dementjevi
is similar to Otogornis genghisi, which was described by Hou
(1994) from the Ordos Basin at the Chabu Sumu locality, Otog
Qi, Yikezhao-meng, Inner Mongolia, China. The specimen was
collected in the thin, grey green mudstones of the Yijinhuoluo
Formation of the Zidan Group in the Lycoptera-bearing deposits
and represents the earliest Cretaceous or even a Late Jurassic
avian fossil from China (Hou, 1994). Otogornis is based on
associated elements of the forelimb and shoulder girdle (VP-
9607, holotype, Institute of Vertebrate Paleontology and Paleanthropology
(IVPP), Beijing). The slab also displays some
feather impressions. Otogornis was described as Aves incertae
sedis and was compared with Archaeopteryx, Chaoyangia, and
the enantiornithines Sinornis and Cathayornis (Hou, 1994).
Earlier, the same specimen was assigned to the indeterminate
Enantiornithes (Dong, 1993).
ACKNOWLEDGMENTS.—I thank very much L. Hou and Z.
Zhou for permitting investigation of the holotype of O. genghisi
in the IVPP collection. L. Martin prompted me to use for the
investigation the mold from specimen 3790-272, which led to
the discovery of a contact with the counterslab specimen
3790-271- (specimens in the Paleontological Institute, Russian
Academy of Sciences (PIN)). Comparison with the lifhornithids
was made possible by the courtesy of R. Emry in the Department
of Vertebrate Paleontology, National Museum of Natural
History, Smithsonian Institution, Washington, D.C. For
comments on the manuscript and English corrections I am
thankful to S. Lucas, R. Zusi, S. Olson, and two anonymous reviewers.
The stereophotographs were made by S. Morton in the
Faculty of Sciences of Monash University, Clayton, Australia.
The x-radiograph of Ambiortus dementjevi was made by L.
Martin and J. Chom in the Museum of Natural History, University
of Kansas. All drawings, including Otogornis, were made
by the author. This study was supported by a travel grant and
grant 96-04-50822 of the Russian Fund for Fundamental Research,
as well as by funds from the IVPP, Academy of Sciences
of China.
Age of the Khurilt Beds
The geological age of Ambiortus is problematical. It was
found in Mesozoic rocks of the Gobian Altai in central Mongolia
in the Khurilt-Ulan Bulak locality. This discovery caused
some paleontologists to doubt the correct definition of the fossil
and the age of the deposits. At present, no doubts exist about
the advanced avian condition of this specimen. The geologic
age of the Upper Cretaceous lacustrine shales and sandstones
of the Khurilt locality, however, is discussed in contradictory
terms by different experts in biostratigraphy, paleobotany, and
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
paleoentomology, with the dates ranging from the latest Jurassic
to the Aptian.
Numerous and various insects were collected in these beds
(Zherikhin, 1978; Sinitsa, 1993). The insect fauna is very constant
in a number of localities in central Mongolia (Khurilt, upper
members of Kholbotoo, Bon Tsagan). This is known as the
Bon Tsagan assemblage, the youngest among three Lower Cretaceous
assemblages in central Mongolia (Ponomarenko,
1990). Dmitriev and Zherikhin (1988) supported an Aptian age
of the deposits in these localities.
The plant associations of the Khurilt, neighboring deposits of
the Kholbotoo, and of the middle levels of the Bon Tsagan localities
include, following Krassilov (1982), four phytostratigraphic
units in the Upper Cretaceous of Mongolia. The third
unit is the Baierella hastata (Bennettitales; and its cones,
Karkenia mongolica)/'Araucaria mongolica zone, which includes
localities of Shin Khooduk-Anda Khooduk level, and
most of the paper shales of the Bon Tsagan, Kholbotoo Gol,
Khurilt, Erdeni Ula, Shin Khooduk, and Modon Usoo localities.
The sediments of the Khurilt and Kholbotoo localities
were assigned by Krassilov (1982) to the Anda Khooduk Formation.
The plant communities from these localities he correlated
with the Aptian flora of the Russian Far East (Primorye).
Thus, phytostratigraphic data suggest an Aptian age for the
Khurilt beds (Krassilov, 1982).
Based on geological data, Martinson (1973) and Shuvalov
(1975, 1982, 1993) referred the Khurilt and Kholbotoo beds to
the Anda Khooduk Formation, which they correlated with the
Hauterivian-Barremian.
Sinitsa (1993), based on the lithofacial data and ostracod assemblages,
did not provide a definite age for the Khurilt beds
and defined it as a task for future exploration. She specified
that the Khurilt beds belong to the Khurilt Section of the Bon
Tsagan Series. The Khurilt beds are underlaid by the Dund Argalant
Series of the latest Jurassic, which includes the Anda
Khooduk Formation (Tithonian), and are overlapped nearby by
the sediments of the Kholbotoo Section (younger beds of the
Bon Tsagan Series). In more western areas of Mongolia, the
Bon Tsagan Series are overlapped by the Khoolsyn Gol Formation,
which is correlated with the Aptian-Albian.
Perhaps the Mongolian and Chinese Lower Cretaceous shale
sediments were deposited simultaneously. The problem of age
determination of the lacustrine Lower Cretaceous beds in Mongolia
is the same as for the Jehol Group in China (Matsukawa
and Obata, 1994). The Khurilt outcrop and the upper members
of the neighboring Kholbotoo outcrop are very similar to the
grey green, thin-bedded sandy and oil shales and siltstones of
the Jiufotang Formation in Liaoning Province of China. The Jiufotang
Formation is a member of the Jehol Group, which is
subdivided into four lithostratigraphic units: Yixian, Jiufotang,
Shahai, and Fuxin formations (Smith et al., 1995). The Jiufotang
Formation is correlated with the Berriasian-Valanginian
(Li and Liu, 1994), or Tithonian-Valanginian (Lin, 1994), or
even with the Tithonian (Chen and Chang, 1994). On the basis