The Dinosaurs of Wyoming - Wyoming State Geological Survey ...

24 THE DI OSADRS OF WYOMING
forced to meet and the marvelous degree of adaptation to the
environment which they underwent.
"I imagine the conditions which gave to the dinosaurs their
initial evolutionary trend were such as are thought to have
prevailed, beginning in the Permian, throughout Triassic time:
This is well shown in the region now known as the Connecticut
valley. The older notion of the estuarine origin of these deposits
has been abandoned in favor of the idea that they were
of terrestrial origirl, the climatic conditions being those of semiaridity
with areas here and there which were subject to inundations
occurring in times of torrential rains such as are observed
t.o-day under similar climatic conditions in different
port.ions of our globe. This lends color to t.he view that the
early dinosaurs were truly terrest.rial types, with marked cursorial
adaptation, indicated in the free, bipedal stride and compact,
bird-like foot. which is shown by the fossil footprints.
"Ruene derives the Theropoda and Parasuchia from one
st.em, the supposit.ion being that the distinguishing charact.erist.ics
were developed during the oldest. Trias through adaptation.
Increasing aridity of climate would render it. necessary for
an animal to go farther afield for wat.er and possibly for food
and thereby place a premium on good powers of locomotion,
so that selection would be very act.ive in weeding out t.he unfit
or inadaptable lines. This locomotor adaptation in the quadrupedal
stage is beautifully shown in the Parasuchian genus
Stegomus from the Connecticut valley Trias, evidently a persistent
type which, possibly because of the retention of armor,
remained a quadruped though long of limb and with t.he great.er
portion of the weight borne on the hinder extremities. Stegomus,
I imagine, t.hough belonging, morphologically, to a very
different. race, represents a st.age in the adapt.ation of the dinosaurs
which was reached early in the Trias.
"Many modern lizards are amazingly swift of movement., but
their journeys are brief and the rapidly moving types are small.
It. is a well known fact that a number of lizards, when st.art.led,
r.ise on t.he hinder limbs and run with a t.ruly bipedal gait. It is
significant t.hat t.he bipedal lizards, so far as my knowledge goes,
are all found in semi-arid climates-Aust.ralia, Southwestern
United States. This tendency t.oward bipedalism, with a conseQuent
profound alteration of the hind limbs and pelvis, both
in bone and musculature, seems therefore to have developed to
meet the need of greater range of movement necessit.ated by
increasing aridity, and was t.he prime factor in the early evolution
of the dinosaurian race.
"So strongly \-vas t.his feature impressed, that the main lines
of dinosaurian evolution, whet.her plant or animal feeders, were
cursorial terrest.rial types, though, as new conditions arose, or
were met with during their forced migrations, aberrant. types of
marvelous complexity and range of specialization developed.
These aberrant forms, from the fact that t.heir remains were
more readily preserved, are the ones best known to us and have
colored our whole conception of the dinosaurian race.
"When the plant-feeding Orthopoda arose we do not know.
Nanosaurus is known from the upper Trias of Colorado, while
in the possibly contemporaneous beds of the Connecticut valley
there have been found many footprints which Lull has shown to
belong to plant-feeding types of general proportions not unlike
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