Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur biology - General anatomy

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. See Dinosaurs - Sauropod Necks

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 10^o 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.

Sources & Further reading

1. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
2. Norman, David, 2005, Dinosaurs: A Very Short Introduction, Oxford University Press
3. Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.

Links

DinoData

Dinosaurs