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Binosaurs - Bipedal Dinosaurs and relatives
Binosaurs is a name I call bipedal thecodonts,
dinosaurs, pterosaurs, and birds. Their bipedal nature freed
up their hands for other purposes. However, since many of
the later, well known dinosaurs were quadrupedal, the
significance of this bipedal stage of development is often
overlooked or not even known. Presumably, all the dinosaur
branches, as well as birds, pterosaurs and some of the
thecodont branches got their start as bipedal creatures!
The bipedal gait I believe resulted when thecodonts
evolved an alternate approach for an erect gait. Instead of
using hip based muscles and a flexible back (possibly a tree
jumping adaptation) like mammals, the binosaurs continued to
use the Caudi-femoralis muscles to retract their hind legs.
Heel like ankles (crocs or reverse crocs) or the Mesotarsal
Ankle allowed the foot to rotate under the body which, when
combined with hip changes, resulted in an erect gait. Since
they still used the tail to pull the femur back at the fourth
trochanter, the muscular weight of the tail placed their
body's center of gravity over the rear legs, allowing the
The hands freed by the bipedal gait was a major
evolutionary step. The free hands for the predatory
paleodinosaurs would have allowed them to reach for and catch
small animals or large insects or to hold prey as they bit
with the teeth.
The pterosaurs were the binosaur branch which took
advantage of the hands freed due to bipedal locomotion to
occupy a niche previously denied to backboned animals - they
flew. The ability to fly was a remarkable feat when you
think of it. One of the rewards for the pterosaurs was that
flying insects were unaccustomed to being prey in the air.
Probably more importantly, small fish were not used to
watching above them and hence probably didn't have modern
camouflage markings. The pterosaurs then did not need to
perfect their bipedal running motion and as a result are
often not considered as true dinosaurs because they lack an
open hip socket. Once flight was perfected, it would be
harder for any other branch to occupy this role unless it
could find an open, unfilled segment.
Theropods, the early predatory dinosaurs had pretty
well perfected their means of bipedal locomotion and filled
the medium and large predator roles. They changed little
from the basic bipedal design in their 160 million year
reign, unequaled by any other land predator type. They were
so successful that no other medium, large or extra large
predators challenged them once they took over. Some
theropods probably used the hands to grab or kill food.
Those that did such as Baryonyx walkeri tended to have more
shallow heads. Others lost their teeth altogether.
Ancestral birds appear to be bipedal theropod
dinosaurs (or closely related) that also used their free
hands to fly, even though the niche was already occupied..
In fact, if birds had not lost their hand claws, teeth, and
tails, before the dinosaurs died out, the predominant
predators today might well look like a theropod.
The prosauropods, often called anchisaurs, probably
developed from the early bipedal paleodinosaur meat eaters.
Since other plant herbivores walked on four feet, leaves high
on tree branches were an untapped source of food. This
abundant food source offered a new niche for animals that
could take advantage of it, even if it meant converting from
a meat eater to a plant eater. Prosauropods, being bipedal
and having long necks, could reach up to eat leaves in the
trees. Their divergent thumb claws could have been used to
steady themselves (and conserve energy) on tree trunks or to
bring down branches.
Bipedal ornithopods may have used their hands freed
up by the bipedal gait to reach up for food or to manipulate
branches. The bipedal gait may have allowed for more speed
or agility. However, this branch may also have been
successful because other ground foraging competitor died out
or ornithopods developed such an efficient chewing action
with their replaceable teeth. If the bipedal gait was as
fast as I think it was, then the predentates would have used
this gait to escape predators. A fast moving herbivore should
have been successful, especially if previous herbivores were
Several branches eventually lost the bipedal gait
to large size, sauropods and some ornithopods, body armor, or
heavy head frills and horns.