The Morrison Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 154

P ALEOECOLOGY OF THE
M ORRISON F ORMATION
The Morrison Formation was deposited
in a terrestrial basin near the western
margin of Laurasia (following the break-
up of Pangea), situated in the low mid-
latitudes between 30° and 40° N. The
climate is interpreted as having been arid
to semi-arid, but with some seasonal
rainfall (Demko and Parrish, 1998).
A mountainous region to the west
probably had a rain-shadowing effect
and low annual rainfall is supported
by the presence of evaporites, aeolian
sandstones, and saline lake facies.
However, the presence of various
freshwater invertebrates and fish suggests
that there were perennial streams and
lakes present on the wide, open plains of
the Morrison Basin, and the flora of
horsetails, ferns, cycads, ginkgos, and
various gymnosperms suggests at least
short periods of a more humid, tropical
climate (Ash and Tidwell, 1998). It seems
that this fluvial-lacustrine environment
was strongly influenced by repeated
cycles of drought and flood.
The lush lake margins and swampy
river courses were home to huge herds of
herbivorous dinosaurs, which roamed
the plains in search of food. Smaller
quadrupedal herbivores, such as
stegosaurs, browsed on low-level
horsetails, ferns, cycads, and small
conifers, while the giant sauropods with
their long necks were eating the tops of
the tallest trees, mainly conifers, ginkgos,
and tree ferns. Meat-eating carnivores
(such as Allosaurus ) followed the
herbivores and by pack-hunting were
able to overcome and kill even the
largest sauropod.
Frogs, sphenodons and lizards made
their home in and around the lakes and
streams which were also inhabited by
turtles and crocodiles, the latter being the
top predator of these aquatic habitats.
Pterosaurs, probably living on lake

Other reptiles and amphibians
These are never common, but do include
some rare records of frogs (the oldest
anuran is Lower Jurassic), the lizard-
like sphenodons, some true lizards,
crocodiles and turtles, and a few
records of pterosaurs, including
pterodactyloids and rhamphorhynchoids.
Records of birds have all been later
refuted (Padian, 1998).

Mammals
The Morrison Formation mammals
comprise one of the most important
Jurassic mammal faunas ever discovered
as they provide a rare window on the long
early history of mammals in the Mesozoic.
Known mainly from isolated jaw bones
and teeth are the primitive triconodonts,
docodonts, symmetrodonts, and
dryolestoids, while the multituberculates
are a more developed group of rodent-like
omnivores which survived into the Eocene
(Engelmann and Callison, 1998).

Fish
Lungfish (sarcopterygians) were first
reported by Marsh and for many years
were the only known Morrison fish.
More recently a variety of actinopterygians
(ray-finned fish) have been reported
including a primitive teleostean (modern
bony fish), a variety of holosteans (bony
ganoid fish), and a new chondrostean
paleoniscid, Morrolepis , the 'Morrison
fish' (Kirkland, 1998).

Invertebrates
These include freshwater mollusks
(gastropods and bivalves), ostracods,
conchostracans, crayfish, and caddisfly
cases.

Plants
Flora from the Brushy Basin Member
includes bryophytes, horsetails, ferns,
cycads, ginkgos, and conifers (Ash and
Tidwell, 1998).