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Australia: The Land Where Time Began |
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Dinosaur biology
There was a very wide variety of
dinosaur heads, from delicate to structures that were massively built. A
feature they all shared, that was common to all archosaurs, was that the
nasal passages or sinuses, or both, were very well developed. A large
opening was retained immediately in front of the orbits of many of the
dinosaurs. In other dinosaurs this opening was closed off almost
immediately. A feature which differed from mammals and was similar to
reptiles and birds, is the lack of facial muscls, the skin being appressed
directly to the skull. One result of this feature is that dinosaur
skulls are easier to restore with confidence that the appearance of the
restores skull is as the dinosaur appeared in life.
Another feature is that in the
dinosaurs the external nasal nares are always positioned far forward in
the nasal depression, regardless of the distance on the skull the nasal
openings extend. In some sauropods the nasal openings were set above the
eye sockets. According to the author3 it was believed in the
past that this feature indicated that these dinosaurs could snorkel
while submerged, but in more recent time it has been suggested that the
positioning of the nasal openings are set on top of the head to avoid
being irritated by the needles as they fed on the conifers. The problem
with this suggestion is that at that time most conifers had soft leaves.
It has since been realised that as the fleshy nostrils extend far
forward that places the external nares in the normal position at the tip
of the snout. No evidence has been found indicting that any dinosaur had
a proboscis.
The author3 suggests
that on many dinosaurs the skin covering the large openings in front of
the orbits probably bulged outward gently. He also suggests that jaw
muscles probably bulged gently out of the openings in the skull behind
the eye sockets.
The teeth of amphibians,
tuataras, lizards and snakes have a tendency be close to one another
in jaws that are fairly sharp-rimmed, and when the mouth is closed it is
sealed and he teeth covered by lips that lack muscles. This seems to be
the case in most theropods and sauropods, though the spinosaur
theropods are believed to be an exception, as they have an arrangement
that is more similar to crocodilians in which at least the front teeth
are widely spaced and each is in a separate socket, indicating they
possibly the had no lips, their teeth being exposed when the mouth was
closed.
Beaks were evolved by some
theropods and ornithischians, and there is a suggestion that incipient
beaks were present in prosauropods. The beak formed the front part of
the mouth in ornithischians and therizinosaurs, though in many birds, as
well as in some theropods, all the teeth were displaced by the beak. In
most beaked birds there are no cheeks and they all lack lips. Condors
are unusual in that they have short mouths, elastic cheek tissue
covering the sides of their jaws, a different arrangement than occurs in
most mammals where the side teeth are covered
by muscular cheeks.
In the first sauropods, the
herbivorous prosauropods, as well as ornithischians, there is a tendency
for the side teeth to be inset from the side of the mouth; the
surrounding spaces being smooth surfaced and the foramina feeding the
soft tissue of the mouth are fewer but larger, which the author3
suggests indicates that some, and possibly all of the side teeth were
covered by elastic cheeks that were well developed. The ornithischians
were the dinosaurs with the best developed such system, and in some
ankylosaurs the cheek tissues are actually ossified, in which the cheek
extends all the way to the beak.
The teeth of all dinosaurs were
set in sockets and were constantly replaced throughout life as occurs in
reptiles. Dinosaurs had a wide range of tooth types from blunt,
leaf-shaped teeth that were suitable for crushing plant material to the
serrated blades that have been described as ‘steak knives’ as in
carnivorous dinosaurs such as T. rex that were designed
to pierce flesh. As with the carnivorous mammals of the present the
teeth of carnivorous theropods were not as sharp as razors as they are
often portrayed. According to the author3 a finger can be
safely run over these serrated teeth while pressing hard without fear of
being cut. Iguanodont teeth were concentrated in compact dental
batteries comprised of hundreds of teeth, and even more so in hadrosaurs
and ceratopsids, though these plant-processing pavement were formed in a
minority at a given time.
Dental arrays that were replaced
rapidly were evolved by a few sauropods, at the front edge of the jaws
in their case where the teeth would be expected to wear out more quickly
as they were used to crop plant material. Dinosaurs didn’t have
flickering tongues as occur in lizards and snakes. As the dinosaurs had
hyoids that were well-developed it is suggested that their tongues were
also well developed. It is suggested by the author3 that the
tongues of theropods were probably simple and inflexible. He also
suggests the tongues of herbivorous dinosaurs may have been more supple
and complex as they would have been used to manipulate food, and in the
case of ornithischians, to chew food.
The eyes were in the upper part
of the orbits in some of the larger dinosaurs. The actual size of the
eye is often indicated by the sclerotic ring (bony ring), the diameter
of the inner ring indicating the size of the part of the eye that is
visible when the eye is open. Though eyes were large in dinosaurs the
relative size of the eye tended to decrease as the size of the animal
increased. In the giant theropods the eyes were large, though they
appeared small in comparison with the head size.
Ostriches have the largest eyes
of extant terrestrial animals but don’t appear to be particularly large in the
living ostrich. There is a bony bar above the eyeball in daylight
raptors that is the root of the fierce ‘eagle look’. This bar is present
in some of the smaller ornithischians giving them a more intimidating
appearance than their herbivorous mammalian equivalents of the present
that have a doe-eyed look, though it was lacking in the carnivorous
theropods. The function of this bar is uncertain, the author3
suggesting its function may be to strengthen the skull during feeding
and chewing or protect the eye from glare. Other suggestions are that it
possibly protected the eyes of burrowing ornithopods from dirt and dust.
It is not known if the dinosaur pupils were round or slits, slits being
most common in nocturnal animals. It is possible that some dinosaurs had
circular pupils while others had slit type pupils. Nictitating membranes
protect the eyes of reptiles and birds. The author3 suggests
the same may have been true for dinosaurs.
In dinosaurs the ear is a small
depression that goes deep between the quadrate and the muscles that
close the jaw that are located at the back of the head. The inner ear was
connected to the ear drum that was set in the depression by a simple
stapes rod. Researchers working on the posture of the different dinosaur
types are using the orientation of the semicircular canals of the inner
ear to determine the posture of the heads. One example is the head of
the diplodocoid dinosaurs that had short necks has been found to point
straight down according to the orientation of the semicircular canals
which implies that they grazed ground cover. This method has been found
to be not as reliable as the workers would have liked, the situation
appearing to be more complicated when living animals are studied. The
posture of the heads of living animals is not always the same on all
occasions, the posture changing when the animals do different things.
An example is seen in giraffes in
which the head points straight down when they are feeding on low shrubs,
but when they are reaching high up to feed on acacia trees the head is
held from horizontal to straight up, that makes the orientation of the
semicircular canals a reliable indication of the normal manner in which
they carry their heads, such as when not feeding. The broad-beaked
hadrosaurs are widely believed to have been grazers which would suggest
that their heads should have often been held directed straight down, but
the semicircular canals suggest a horizontal posture for the head.
In the case of at least some of the prosauropods the semicircular
canals appear to indicate that the heads were normally held with their
noses tilted up to some degree, a posture that is not seen in living
large herbivores. It is now believed the posture of the semicircular
canals is determined in part, at least, by the orientation of the
braincase with the rest of the skull, so does not represent the
orientation of the head as well as had previously been believed.
In many dinosaurs the necks tend
to articulate in an S-curve, as is seen in many birds, as well as in
many theropods and ornithopods. In some theropods the beveling of the
vertebrae is especially strong. According to the author3
there is a tendency for animals to hold their necks more erect than is
indicated by the articulation. The necks were straighter in groups such
as ankylosaurs and ceratopsids. The author3 suggests there has been a
tendency to place the shoulder girdle too far forward that makes the
necks shorter than they actually were. He suggests that in the case of
ankylosaurs the necks were long enough to accommodate 2 or 3 rings of
armour that were well-spaced. The range of flexibility of dinosaur necks
went from low, in which the first few vertebrae of the ceratopsids were
fused together, that
were short-necked, to fairly high in dinosaurs with longer necks. The
necks of all dinosaurs lacked the special adaptations that allow the
necks of birds to be exceptionally mobile.
According to the author3
the posture and function of sauropods necks have become controversial,
with some proposals of a simplistic model in which nearly all sauropods
held their necks almost straight and horizontal, some not being capable
of being raised much above shoulder height. The diplodocoids were a
group in which this was true. In the other sauropods the situation is
more complex and there is not enough understanding in many areas to
develop a proposal that satisfies all workers in the area. The author3
suggests there are obvious misarticulations in many of the restorations
in which the necks are assembled in a horizontal straight line, and
there is a also a problem of restorations being based on badly distorted
vertebrae or vertebrae that are too incomplete to be articulated in a
reliable manner.
In the necks of different
individual giraffes there is variation of the articulation with some
being strongly arced downward to some that are arced strongly erect.
This results from the variation of the thickness of the cartilage pads
between the vertebrae that illustrates that without the cartilage the
necks cannot be articulated properly. As the cartilage is only rarely
preserved in fossils it creates a problem for the correct articulation
of fossils in which there is no cartilage preserved. When dinosaur
skeletons are discovered many are found with the vertebrae jammed
together tightly. The author3 suggests
this is probably the result of
the cartilage drying out after death which pulls the vertebrae tightly
together.
In some dinosaur fossils that
have been found the vertebrae are separated by the substantial gap that
would have been occupied by the cartilage in life. The single example of
the cartilage between the vertebrae being preserved is in a sauropod is
a fossil of an old camarasaur that had 2 vertebrae fused together
before death. In spite of the prediction that all sauropods had straight
horizontal necks the vertebrae are flexed upward indicating that in life
the neck was held above shoulder level. According to the author3
if there was 10o of upward flexion between each pair of
vertebrae most sauropods could have held their necks almost vertically
in which case the head would have been well above shoulder level.
The necks of ostriches and
giraffes are held at different angles, and the author3
suggests that it is at least possible that sauropods also held their
necks are different angles. He also suggests that there is no evidence
indicating that sauropods didn’t hold their necks higher than is
indicated by the bones minus the cartilage, and increasing number of
researcher are said by the author3 to now believe that many
sauropods held their heads high.
The necks of giraffes need to
support a large head and their vertebrae are solid, but they are not
heavily muscled. According to the author3 the necks of
sauropods should not have been heavily muscled as they held a small head
and the vertebrae were highly pneumatic. A fairly deep set of nuchal
tendons helped support the neck of some sauropods, as indicated by tall
shoulder spines. To improve neck support the number of neural spines
were doubled in some other sauropods. In pachycephalosaurs and
ceratopsians, that both had big heads, the muscles of the upper neck
should have been powerfully built. Tall shoulder withers indicative of
deep nuchal tendons in some ceratopsians.
Mummies that have been found of hadrosaurs indicate that they had deep
nuchal tendons to assist in holding up their heads that were large
relative to their slender neck vertebrae. The author3
suggests that the strongest neck muscles are probably found in predatory
theropods to help drive the teeth deeper when they bit their prey.
In dinosaurs the trunk vertebrae
articulated in 1 of 2 ways, either in a straight line or in a dorsally
convex arch that varies in various dinosaur types from subtle to very
strong. The dinosaurs tended to have stiffer backs than lizards,
crocodilians or mammals, though the trunk vertebrae were not normally
fused as they are in birds. This is indicated by the nature of the
vertebral articulations, and in many cases ossified interspinal tendons.
In articulated dinosaur skeletons
of all types the front ribs are swept back strongly, as they are in
lizards, crocodilians and birds, while in many mammals they are
vertical. In dinosaurs the ribs of the belly region tend to be vertical,
though the degree tends to be variable. In the carnivorous theropods the
belly and hips were narrow, partly because of their smaller digestive
tract and also because they had an athletic form. When they are hunting
the bellies of big-game-hunting predators of the present, such as lions,
are hollow because they gorge after a kill then fast until the next
kill. The author3 suggests the same should be true for
carnivorous theropods, though some of the space that would otherwise
have been available has been filled by abdominal air sacs, even
when they were hungry. The digestive tracts of herbivorous dinosaurs
were larger so the hips and abdomens were also larger to accommodate it.
The hip and belly broadening is
seen in extreme form in some of the herbivorous dinosaurs such as
therizinosaur theropods, titanosauriform sauropods, most stegosaurs and
especially ankylosaurs. The author3 describes the degree of
widening present in some of the fattest armoured dinosaurs as absurd. In
ankylosaurs the shoulder blades were twisted along their long axis to
allow them to fit onto the rapid shift from the narrow shoulders to the
fat abdomen. The trunk musculature of the dinosaurs was rather light, as
in birds, because the trunk vertebrae and ribs of dinosaurs formed a
short body that was fairly rigid in which the shoulder and hip girdles
were close together. Gastralia, a series of flexible bony rods present
in the belly skin, were retained in
prosauropods and theropods. Each
section of these gastralia were composed of multiple pieces so they were
usually flexible. The author3 suggests they may have been
used in prosauropods as they flexed their trunks while galloping. He
suggests they were needed on Theropods because of the large difference
of belly size that changed regularly as they were in states of feast or
famine. He also suggests the presence of gastralia in
Therizinosaurs may
have been the result of these dinosaurs that had rigid trunks keeping
the abdomens full of fermenting plant material. Sauropods and
ornithopods didn’t have gastralia.
In most stegosaurs, theropods,
and Sauropodomorphs the tails were highly flexible. This is especially
true of titanosaurs, the ball-and-socket joints of which are suggested
to have possibly indicated they were capable of arcing their tails over
their backs. All or part of the tails of the
dromaeosaurid
theropods, with sickle claws, ankylosaurs that had clubbed tails, and
ornithopods, were
stiffened by ossification of the tendons. In hadrosaurs and
iguanodonts
the tails were especially inflexible.
The hip vertebrae were in mostly
the same line as the trunk vertebrae in most dinosaurs. A feature that
is not commonly found in the many known dinosaur trackways for all major
dinosaur types is tail drag marks indicating that the old style
convention of dinosaurs that consistently drag their tails cannot
according to the author3 be correct, even in dinosaurs having
tail bases that swept downwards. The hip and tail were flexed upwards
relative to the trunk vertebrae in therizinosaurs and some of the
sauropods. This arrangement allowed the trunk to be held pitched up
strongly while hips and tail remained horizontal, the result being that
the dinosaur could reach high Into trees to feed while moving on its
hind legs. A feature that was common to all dinosaurs was the ability to
rear up on their hind legs, including the few with arms longer than
their legs. This could be done because they all bore most of their
weight on their hind legs and usually had long tails as counterweights
for the front part of their bodies.
A consistent difference between
dinosaurs and mammals was that none of them had all 4 legs ending in
similar hands/feet, the hands can always be distinguished from the
feet. Even among the really giant sauropods the hands never had a heavy
central pad as did the feet. The short fingers were bound into a hand
that resembled a hoof, the fingers being encased in a single tight pad,
in forms such as Stegosaurs, sauropods, iguanodonts and hadrosaurs. A
thumb weapon, that was inwardly directed, bearing big claws, was a very
distinctive character in theropods, prosauropods, and some of both
ornithischians and sauropods, that could be held clear of the ground
even when walking on arms and legs. Another feature common to all
dinosaurs was that when they walked on their arms the palms always faced
at least partly inwards, and often strongly inwards, especially in
bipeds. The hind feet of larger dinosaurs such as iguanodonts,
hadrosaurs, armoured dinosaurs, ceratopsids and sauropods, had a large
central pad under their feet, as do rhinos and elephants of the present.
To accommodate the shoulder
girdle of dinosaurs the front of the rib cage was narrow from side to
side, both sides meeting one another on the chest, and the shoulder was
immediately in front of the rib cage. The shoulder joint of mammals is
on the side of the chest. The shoulder girdle is fixed in place in
theropods, including birds, partly as a fused furcula as a brace for
both scapula blades. The stride length of the arms of many reptiles and
mammals is helped to increase by having mobile shoulder girdles. The
author3 suggests this is probably also true of quadrupedal
dinosaurs because their clavicles are either not fused together, are not
in contact with each other, or have been lost completely. The scapula
blade of most dinosaurs was subvertical, as is the case in most
tetrapods, and not horizontal. The birds and bird-like theropods are
exceptions that have scapula blades that are horizontal.
To enable flying birds to fly the
shoulder joints face sideways to allow the arms to be held out to flap.
The arms could move in a similar manner in many predatory theropods to
allow them to grapple with prey. According to the author3
there is evidence from the many known dinosaur trackways that running or
walking dinosaurs that their legs did not sprawl sideways as those of lizards do. The
presence of thick pads of cartilage in the joints of the limbs of dinosaurs
in life, that is similar to those that can be seen in chickens, make
it difficult to restore the precise posture of dinosaurs as these
cartilage layers are not preserved, apart from very rare fossils that
have been fossilised under very unusual conditions, such as some
mummified hadrosaurs, though some basics can be determined. It can be
seen that the arm of some quadrupedal dinosaurs could be swung below the
shoulder joints and, as indicated by the shoulder joints of such
dinosaurs facing down and backwards. The cylindrical hip joints of such
dinosaurs forced the legs to work below the hips. A feature that is also
common in many mammals is that the elbows and knees were bowed outward
to some degree to allow the legs to clear the body, so the arms and legs
didn’t have to move entirely in a fore and aft direction.
One feature of the hands of
dinosaurs that differs from those of mammals is that whereas the hands
of mammals are often near the body midline, those of dinosaurs were
nearly always separated by at least 2 hand-widths, the hands being
rarely placed closer to the midline than the feet, often being further
from the midline than the feet. The reason for this was that the arms
were oriented in such a way that the hands were either directly beneath
the shoulder joints, or a bit further apart. Even in some of the largest
quadrupedal dinosaurs the hindfeet fell on the midline, never being
separated by much more than the width of
a single hindfoot print. This also occurred among the
broadest-hipped of the sauropods and armoured dinosaurs.
Among most dinosaurs the hands
and feet were digitigrade, the wrists and ankles being held above the
ground when they walked. The joints at the shoulder, elbow, hip, knee,
and ankle were always flexed strongly, an arrangement that resulted in a
spring-like action for achieving a full run in which at some point in
each complete step cycle the feet were all off the ground at the same
time. Also, the ankle could push the dinosaur into a ballistic stride as
a result of the ankle remaining highly flexible. This also occurred in
even the largest of the theropods, ornithopods, ankylosaurs and
ceratopsids, dinosaurs that reached masses of 5-15 tonnes. If the knee
joint of flexed-limbed dinosaurs were straightened they were not fully
articulated.
The vertical bodies of humans
place the centre of gravity in line with the hip socket resulting in
humans having vertical legs and straight knees. In bipedal dinosaurs the
centre of gravity was ahead of the hip socket, even with the long tail
as a counterbalance, as the head and body were held horizontal and well
forward of the hip socket resulting in the femur needing to slope
strongly forward to place the feet below the centre of gravity. The
birds, that have short tails, take this arrangement to the extreme with
the femur being almost horizontal when walking to place the feet and
knees far enough forward. When birds run their femur swings more
strongly backwards.
According to the author3
among the dinosaurs 2 groups, the stegosaurs and the sauropods,
straighter-jointed limbs that were of an elephantine type, being
columnar were evolved. In this arrangement the configuration of the knee
had been altered to allow it to remain fully articulated when straight.
The hindfeet of these dinosaurs were very short and the ankle was less
mobile. The body of these dinosaurs were prevented from achieving a true
run by the combination of these adaptations. Among elephants even the
juvenile elephants cannot achieve a true run, or run any faster than
adult elephants, at least 1 foot being on the ground at the same time,
even at their highest speed.
The author3 suggests
the dinosaurs with straight limbs should not have been able to run
faster than elephants, reaching speeds of no more than 25 km/hr (15
mph). He also suggests that the small to medium sized dinosaurs that had
long slender legs that were flexed could probably reach speeds that were
similar to those achieved by ground birds and galloping mammals of 40-60
km/hr (25-40 mph). With the dinosaurs that weighed many tonnes that had
flexed limbs it is difficult to estimate the top speeds they could run
at. When considering
Tyrannosaurus it has been
estimated by some computer analyses that they could run at speeds
ranging from that of an elephant up to 40 km/hr, the maximum speed
reached by a sprinting human. The author3 suggests that it is
unlikely Tyrannosaurus was as
slow as elephants as it was adapted better for running than are
elephants. Other estimates of the top speed of
Tyrannosaurus have it
running at almost twice as fast as an elephant, reaching the speed of
rhinos and non-thoroughbred horses.
The author3 suggests
that the computer models used at the present to estimate top speeds and
other aspects of dinosaur locomotion are not capable of including all
necessary factors in their analyses. He also suggests that as trackways
show clearly that the largest of the supersauropods walked on dry land
without the need of assistance from water, as their bodies appear no
better proportioned than those of the much small elephants. He poses the
question ‘did the supersauropods not need “super” adaptations beyond
those seen in elephants to move about the Mesozoic landscape, or did
special adaptations such as stronger muscle fibres and pretensed tendons
evolve to solve the problem?' If they did indeed have special
adaptations he suggests the giant running dinosaurs may also have had
such adaptations to allow them to moved faster than the computer models
suggest.
The limb muscle mass of dinosaurs
is an important consideration when trying to assess the speed and power
of a dinosaur, this muscle mass tends to make up a larger proportion of
the total mass of a fast running dinosaur than in slower animals. At
best all that can be done for determining the muscle mass from the
fossilised skeletons of dinosaurs is an approximation. The muscle
patterns of dinosaurs are simpler than the complex limb muscles of
mammals, so living mammals cannot be used to estimate the muscle mass of
dinosaurs. The muscle patterns of dinosaurs are of a spindlier type than
those of mammals having been inherited from their reptile ancestors. In
dinosaurs the tail-based caudofemoralis, a major muscle that helped pull
the hindlimb back during the propulsive stroke, has no equivalent in
birds or mammals.
The relative sizes of dinosaur
muscle groups can be approximated. The ilium in the hips of
reptiles is so short that the thigh muscles are required to be narrow,
which limits their size. A broad, powerful set of thigh muscles are
anchored by the much longer ilium of birds and mammals. In the early
prosauropods and herrerasaurs the ilium is short, indicating that they
had narrow thigh muscles. The ilium is longer and deeper in other
dinosaurs, so is able to anchor a set of thigh muscles that are larger
and capable of producing more sustainable power, this trend being taken
to extremes in some dinosaurs. In the ornithomimids, that were
ostrich-like, and the tyrannosaurids, the presence of exceptionally
large leg muscles is indicated by the pelvis that is oversized,
suggesting that these dinosaurs could move at high speed. The even
longer hips of ceratopsids are suggested by the author3 to
have supported large leg muscles that would have been required to propel
fast charges to fend off tyrannosaurs that were also strong-muscled.
A feature of the very large
sauropods is that they did not have especially large ilia, because as
they didn’t need to move fast they didn’t need the large muscles, as
also occurs in elephants that don’t have large muscles below the knees
as the feet that the shank muscles operate are very short and close to
immobile. In sauropods and stegosaurs the same applied. A large bundle
of shank muscles to operate the long, mobile feet, by the attachment of
long tendons, are found in
faster animals. The large drumstick-shaped collection of muscles below
the knee is anchored to the cnemial crest that projects forward of the
knee joint.
The author3 suggests
the surface contours of dinosaurs are commonly simplified in
restorations, resulting in their legs, tail and neck being simple tubes,
and smoothing over the topography of the body. He also suggests that in
the sauropods the bulging of each neck vertebra was probably visible on the
sides of the neck in life, as can be seen in giraffes. He suggests the
bottoms of sauropod necks should be fairly flat as the oesophagus and
trachea were probably tucked up between the cervical ribs. This doesn’t
apply to giraffes as they lack cervical ribs that were well developed.
The upper end of the arms bulged
out a bit in dinosaurs with large arms, and in many dinosaurs, though
not all, a very large crest of the humerus would have formed a prominent
contour along the upper front edge of the arm. In front view the elbow
formed a large bulge, especially in dinosaurs such as the ceratopsids,
armoured dinosaurs and diplodocoids. The author3 suggests
that in living dinosaurs, especially the herbivores, the upper edge of
the ilium would have been visible in a similar manner to the pelvic
bones of cows can be seen under the skin.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |