The Chinle Group - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 132

up to 10.6 m (35 ft) thick, which includes
the mass concentration of petrified logs,
mostly of Araucarioxylon arizonicum ,
which the Petrified Forest National Park
was founded to conserve.
The Owl Rock Formation, overlying the
Petrified Forest Formation, consists of at
least 30 m (100 ft) of pale red sandstones
and siltstones with intercalated beds
of pisolitic limestone. These are the
youngest Triassic rocks exposed in the
National Park area, and are overlain
unconformably by rocks of upper
Cenozoic age.
Taphonomically, the Chinle Group
can be regarded as a Concentration
Lagerstätte with bone-beds and concen-
trations of petrified wood recorded at
different levels. For example, the lower
unit of the Petrified Forest Formation
contains a multispecific bone-bed,
dominated by Placerias , near St Johns in
Arizona, while the upper unit of the
formation contains an almost monotypic
bone-bed, dominated by Coelophysis ,at
Ghost Ranch, New Mexico, plus the well-
known accumulations of petrified wood in
the National Park itself. The taphonomy
of the bone-beds and the petrified wood
shows some obvious differences.
There is little question that the
petrified trees did not actually grow in
what is now called the Petrified 'Forest'.
Instead, it is generally accepted that the
accumulation of such vast numbers of
fossil trees resulted from flowing water on
a large scale. Many of the Chinle Group
sediments were deposited on a broad
floodplain (see section on Paleoecology),
and the trees are usually interpreted as
having been transported into the area by
huge floods from source areas outside the
sedimentary basin. Signs of water wear on
the petrified logs have been frequently
described and preferred orientation of
logs at several sites may indicate direction
of flow.
Similarly the concentrations of
Coelophysis in the upper unit of the
formation are thought by Schwartz and
Gillette (1994) to have been washed

downstream by fluvial currents, eventually
clogging and filling a small channel, their
skeletons preserved in crude alignment.
The cause of death of so many animals
(probably several thousand) has previously
been attributed to volcanic eruptions,
drinking from poisonous water, or
floundering in a sticky predator trap
similar to the Rancho La Brea tar pits
(Chapter 14), but Schwartz and Gillette
(1994) suggest that the dinosaurs perished
during seasonal droughts - their long
necks are commonly strongly recurved
suggesting desiccation of the carcasses
before burial.
Similar concentrations of Placerias in
the lower unit of the Petrified Forest
Formation have been attributed either
to mass death resulting from an environ-
mental agent, or to serial predation by
other reptiles over time (Camp and
Welles, 1956). Fiorillo et al . (2000) ruled
out death by predation since the bones
show little evidence of tooth marks or
damage by trampling. Mass death need
not be instantaneous (see Discussion, p.
156, Chapter 9), and may occur over
several seasons. These authors proposed a
similar drought-induced model for the
mass death of Placerias , but in this case it
is not thought that the carcasses were
subsequently transported by fluvial
processes as they show little evidence of
post-mortem alteration from prolonged
exposure.
In both cases, the herds seem to have
been concentrated around gradually
diminishing ephemeral water bodies where
they eventually died of dehydration. There
is ample sedimentological evidence from
both sites (mudcracks and calcretes etc.) to
suggest that the late Triassic climate of the
southwestern United States was punc-
tuated by periods of severe drought.
However, while the Placerias herds were
preserved where they lived and died
(autochthony), in a soil with a high water
table, the Coelophysis carcasses were
apparently swept away by a flash flood and
redeposited (allochthony), as were the
huge trees.