Geoscience Reference
In-Depth Information
P
ALEOECOLOGY OF THE
C
HINLE
G
ROUP
The Chinle Group was deposited on the
Colorado Plateau on a low relief floodplain
that was cut by numerous meandering and
braided streams and included lacustrine
environments. Siliclastic sediment was
derived from two major mountain belts
bordering the enclosed Chinle basin, the
Mogollan Highlands of southern Arizona
and southwestern New Mexico, and the
ancestral Rocky Mountains to the north.
Situated in a back-arc basin on the western
margin of the Pangean landmass, between
about 5˚ and 15˚ N, it represents a tropical
hot and humid climate with exceptionally
strong monsoonal weather patterns causing
well-defined seasons, punctuated by
periods of drought and also by volcanic
activity.
Different horizons and different
locations within the Chinle Group reflect a
variety of environments of deposition
within the floodplain (Long and Padian,
1986). Normally the lower unit of the
Petrified Forest Formation is dominated by
a 'typical' fauna of large metoposaurian
amphibians and crocodile-like phytosaurs
in what would seem to be a definite aquatic
association on a frequently flooded
floodplain. Occassionally, however, this
lower unit is dominated by associations of
terrestrial aetosaurs either with phytosaurs
or with other archosaurs, suggesting a less
than fully aquatic regime.
The
Placerias
Quarry, on the other hand,
is unusual among sites in the lower unit in
that it is totally dominated by terrestrial
rather than amphibious or aquatic
vertebrates.
Placerias
itself occurs with
aetosaurs, rauisuchids, and small dinosaurs.
This site was traditionally regarded as a
permanent bog, marsh, or pond (Camp and
Welles, 1956), with
Placerias
pictured as a
hippopotamus-like marsh dweller. However,
Fiorillo
et al
. (2000) suggested instead that
the site was only seasonally flooded from
nearby channels, and that the ensuing
drought concentrated these terrestrial
animals around a diminishing water supply
(see section on Taphonomy).
The upper unit of the Petrified
Forest Formation often indicates a
somewhat different aquatic environment.
Metoposaurs are rare, all vertebrates are
less common than in the lower unit, but
phytosaurs are the most common apart
from one genus of aetosaur. Freshwater
bivalve and gastropods are, however, often
very common along with archosaurs,
dinosaurs, fish, and amphibians.
Elsewhere the upper unit is dominated
by large numbers of the small theropod
dinosaur,
Coelophysis
, which roamed the
floodplains, gathering around stream
channels and pools for food. Insect larvae
fed off detritus in the pools and were
themselves food for the numerous crayfish
which were in turn eaten by a variety of
fish. These formed the main diet of the
carnivorous
Coelophysis
. Stream margins
were vegetated by lycopods, horsetails,
ferns, seed-ferns, cycads, bennettitaleans,
ginkgos, and conifers, while on the higher
ground outside the enclosed basin were
vast forests dominated by tall monkey-
puzzle trees.
C
OMPARISON OF THE
C
HINLE
G
ROUP WITH OTHER
P
ERMO
-
T
RIAS TERRESTRIAL SITES
Dockum Group, Texas and New
Mexico
The Upper Triassic Dockum Group
outcrops in western Texas in a band
extending north from San Angelo to
Amarillo, and in eastern New Mexico over
large areas in Quay and Guadalupe
counties. The Group comprises a lower
Tecovas Formation (mainly mudstones)
and an upper Trujillo Formation (sand-
stones and conglomerates), but exact
correlation over such a vast area is difficult
and often the terms 'lower' and 'upper'
are used informally.
The Dockum is the lateral equivalent
of the Chinle and not surprisingly the biotas
are very similar, although the Dockum
fauna has a greater diversity (Murray, 1986).
Fish include lungfish, coelacanths, sharks,
palaeoniscids, redfieldiids, semionotids, as
in the Chinle, and amphibians include the
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