PDF (Lo-Res) - Smithsonian Institution Libraries

NUMBER 89 277
of fossil fishes, the Yixian and Jiufotang formations are correlated
with the Late Jurassic and Neocomian, in agreement with
the fish faunas of Japan, Kazakhstan, and western Europe (Fan,
1996).
Published radiometric ages for the base of the Yixian Formation
include 137±7 Ma using K/Ar and 142.5 Ma using Rb/Sr
(Wang, 1983; Wang and Diao, 1984), which corresponds with
a Berriassian age, using the time scale of Harland et al. (1989).
For the Fuxin Formation, K/Ar ages range from 100 Ma to 137
Ma (Mao et al., 1990), corresponding with an Aptian-Valanginian
age. New age dates were reported for the Yixian Formation,
however, that are based on a laser 40 Ar/ 39 Ar study of single
mineral grains (Smith et al., 1995). This study estimated the
age of the lower Yixian Formation as 121.2±0.3 Ma and
121.3±2.3 and 121.4±0.7 Ma for the upper Yixian Formation.
Smith et al. (1995) also tested the absolute ages using 40 Ar/
39 Ar in very fine crystallites of glaucony from the white lacustrine
Ershilipu sediments that occupy a stratigraphic position
between the upper and lower parts of the Yixian Formation.
The resulting ages of 122.1 ±0.2 Ma and 122.5±0.3 Ma agree
with the strict chronostratigraphic constraints imposed by the
40 Ar/ 39 Ar ages of the upper and lower Yixian Formation. Thus,
Smith et al. (1995) provided an integrated age range of
121.1-122.9 Ma for the Yixian Formation, which corresponds
to the Barremian using the time scale of Harland et al. (1989),
and these dates are much younger than the K/Ar and Rb/Sr
dates of other authors.
Thus, insects and plants suggest an Aptian age for the Khurilt
deposits, but the geological data and ostracods indicate a
Neocomian age. The probably contemporaneous Yixian and Jiufotang
formations in northeastern China are assigned to the
Neocomian on faunistic, plant, and radiometric data, although
the laser 40 Ar/ 39 Ar study gives a Barremian age. I am inclined
to accept a Neocomian age for the Khurilt deposits.
Comprehensive Description of Ambiortus
Ambiortus dementjevi is represented on three slabs. The
main slab (PIN 3790-271+) bears the cervical and thoracic vertebrae,
furcula, left scapula and coracoid, a portion of the sternum,
some thoracic ribs, the proximal portion of the left humerus,
distal portions of the radius and ulna, ulnare, the
proximal portion of the left carpometacarpus, and phalanges of
the major wing digit (Figures 1, 3-5). This slab also shows an
isolated impression of a feather vane about 12 mm long and the
probable impression of the soft body of the specimen, with
small contour feathers that surround the body impression. The
feathers and body impression are better represented on the
counterslab (PIN 3790-271-). The counterslab bears small fragments
of the vertebrae and a portion of the major metacarpal,
and it has a good mold of the humems, coracoid, clavicle, and
carpometacarpus. A small associated slab (PIN 3790-272)
shows just the mold of three phalanges of the major left wing
digit, the mold of the ulna and major metacarpal, and a frag-
FlGURE 1.—Main slab with Ambiortus dementjevi Kurochkin, 1982, holotype PIN 3790-271+; Khurilt Ulan
Bulak locality, central Mongolia, Neocomian. Stereopairs. (Scale bar= 1 cm.)