Australia: The Land Where Time Began
Dinosaur or Bird
Birds, or avian dinosaurs, are technically reptiles, and there is a lot of evidence that they have descended from maniraptoran dinosaurs.
Since the discovery that many of the theropod dinosaurs that had been known for decades actually had feathers, and many new fossils have been found in China and Mongolia, it has become difficult to know exactly when birds crossed the line from dinosaur to bird. The disagreement now is not if birds evolved from dinosaurs, rather precisely when did this occur. The first hint at just how blurred the boundary was came with the discovery in Argentina of Alvarezsaurus, and from Mongolia, Mononychus. They were believed by some to be large flightless birds, until the postcranial skeletons of a related animal was found, Shuvuuia in Mongolia, when it became clear they were actually very bird-like dinosaurs. They had delicate curved necks with small heads and very small teeth, and long legs, short, powerful arms with 1 very large claw and 2 smaller claws. As research continued and more finds were made they were placed at the beginning of bird evolution, then finally just outside the beginnings of birds.
Some of the features that characterise birds have been present in theropod dinosaurs since some time in the Jurassic, and possibly in the Late Triassic. The pre-adaptations leading to birds had been accumulating since that time. The line, when feathered dinosaurs became birds, has been arbitrarily set at Archaeopteryx. The feature that is apparent in bones that has been used to distinguish between birds and dinosaurs is structure that would indicate the ability to achieve powered flight. If the presence of feathers was the deciding feature all the theropod dinosaurs would probably be considered to be early birds.
Archaeopteryx straddled the line between bird and dinosaur, having features found only in modern birds, such as wings capable of powered flight and feathers specialised for flight, and some skeletal features found only in modern birds. It also had some reptilian features such as separate wrist and finger bones as well as several tail bones.
See Xiaotingia zhengi for information about the possible move of Archaeopteryx from the base of the line leading to birds to a similar position at the base of the line leading to the Deinonychosauria, from an avian dinosaur to a non-avian dinosaur.
A few primitive birds have been known for some tome, such as Hesperornis and Baptornis, both seabirds, from Late Cretaceous sites in North America, and specimens such as Iberomesornis from the Las Hoyas site in Spain. A pygostyle (a bone in the tail of birds) and a coracoid (a strut-like shoulder bone) are prerequisites for the manoeuvrability seen in the flying of modern birds. The wrist structure that allows modern birds to fly with such maneuverability was seen in Eoalulavis, a later discovery.
The discovery of many bird fossils at the Liaoning Site in China greatly increased the knowledge of the forms near the dinosaur-bird boundary. At least 1000 good specimens of what is now the best known Cretaceous bird, Confuciusornis, have been found. Some skeletons have long tail feathers attached, while others don't, which suggests sexual dimorphism may have been a characteristic of the living birds from early in their evolution. Confuciusornis differed from Archaeopteryx in a number of ways, such as a pygidium and a toothless beak. A close relative of Confuciusornis, Changchengornis, had a more delicate skull and a smaller curved beak. The most significant feature was a longer first toe, leading to the condition of the feet that are adapted for perching on branches.
Because the feathered dinosaurs were sharing their habitat with many undoubted birds they are not ancestral to birds, which must have evolved much earlier. Archaeopteryx is from the Jurassic, but the fossils from Liaoning are from the early Cretaceous. This has caused what was being called a temporal paradox, all these apparently transitional forms coming after the species that is accepted as the first bird, archaeopteryx lithographica.
A paper published in Nature, September 2009, describes a troodontid, a theropod dinosaur, Anchiornis huxleyi that lived between 161 and 151 million years ago, in the earliest part of the Late Jurassic in Tiaojishan Formation of western Liaoning, China. It had an extensive covering of feathers, with long pennaceous feathers (contour feathers) attached to the pes. This specimen adds to other evidence, such as Microraptor gui, from the Early Cretaceous, that the earliest birds probably had 4 wings, as do both of these species.
Fossils recently found in deposits from 130 million years ago in the Strzelecki Ranges of southeastern Australia show a strong similarity to those of the Liaoning region of China, fish, plant and insect fossils have also been found that correspond to the Chinese species. 6 feathers have been found in the Koonwarra fossil deposit in Victoria. Because of the difficulty of funding a major operation requiring heavy machinery to get to the fossils the potential of this site may take a some time to be evaluated.
A new species of dinosaur, that is 98 million years old, found at Winton in Queensland in 2008 shows a remarkable resemblance to the skeleton of living emus.
Another similarity between feathered dinosaurs and birds is that they both seem to have been affected by the same diseases. The skulls of a number of T. rex skulls have pits that are believed to have been caused by trichomonosis. The 2 best known T. rex skulls, Sue, the best preserved skeleton, and the type skull used in the description of the species. As about 60 % of T. rex skulls show evidence of facial biting, this could explain the spread of the disease, as with the squabbling over carcasses by Tasmanian devils, which has been spreading the facial tumours.
In a Letter to Nature in 2006 the authors describe why bird flight is much easier that that of other flying vertebrates, such as bats and pterosaurs. In non-bird flying animals flight is powered by muscles, but in birds it is powered by the highly specialised mechanism utilising the acrocorahumeral ligament (ALH) that transmits the force of the breast muscles through the coracoid bone in the shoulder that becomes a compressive strut. The result is that the bird halves the muscle action needed to move the wings, the muscles pulling them down, then the ALH and coracoid system spring them back up without the need for muscle contraction. Kangaroos use a similar means of reducing muscle usage by having a springy mechanism in their hind legs when hopping. Each bound compressing the spring that helps launch the next bound.
The position of this ligament in fossils is being used to trace the rise of birds. In crocodiles and dinosaurs the force used in the limbs is balanced by muscle contraction systems, whereas in forms such as Sinornithosaurus and Sinornithoides, advanced feathered dinosaurs, the attachment of the shoulder ligaments have begun to move to higher up on the shoulder girdle, towards the position it occupies in birds. The beginning of the ALH system replacing the muscle-based system used to operate the shoulder is seen in the first birds, such as Confuciusornis. The result of the change in position of the ALH attachment can be clearly seen in the position of the articulation between the arm bones and the shoulder, the facet moving higher in relation to the coracoid bone as the position in birds is approached. The ALH runs parallel to the direction of the wish bone in modern flying birds, representing an advanced stage in which the ALH is doing most of the work. To make maximum use of this system to power flight birds have other features, such a sensory system that can process information faster to allow them to manoeuvre at high speed and also have evolved a very efficient breathing system.
A number of features that would be combined in birds to allow them to fly were evolving in various dinosaurs for millions of years before the birds arrived on the scene.
Sinosauropteryx is the first dinosaur discovered with downy type feathers. It had typical theropod arms, short and with 3-fingered hands. But it also had a scapula similar to the blade-like form seen in birds.
Dromaeosaurs such as Velociraptor had a bone in the wrist that was the shape of a half moon that allowed them to swivel their arms to the side and fold their arms in the way birds do. This structure was later incorporated into the mechanism used by birds in the flying stroke. A close relative, Sinornithosaurus, has been found covered with downy feathers.
Unenlagia was an ostrich-sized dinosaur from Argentina. It had developed shoulder bones that allowed its arms to be raised above its head. It has been suggested it may have done this to help with balance when running. It has since been found that other dromaeosaurs could also raise their arms above their head. Another feature that has been incorporated into the flight mechanism of birds.
By the Archaeopteryx stage asymmetrical flight feathers were present, though it had a small sternum and the wings were still formed by long arms with claws on the fingers, and it had a long reptilian tail. It has been suggested that it probably wasn't a good flier. Probably only flying over short distances.
Eoalulavis was a more advanced flier than Archaeopteryx, though still a primitive bird. The thumb had evolved into a structure that supported the alula. This is a tuft of feathers that modern birds use to control air flow over the wings at low speed, such as take off and landing.
Birds share more than 100 features with some theropod groups such as dromaeosaurs. Some of these when compared to the dromaeosaur Velociraptor are:
Structural similarities seen in birds and theropods
When birds are producing eggs they develop medullary bone in their limbs. This endosteally-derived bone tissue, that is calcium rich, lines the interior of the marrow cavity, and is used as a source of calcium for egg shell. Medullary bone has been found in the bones of the theropods T. rex and Allosaurus and the ornithopod Tenontasaurus. The theropods and ornithopods are on different lines, so it appears the medullary bone may have been a feature common to all dinosaurs.
Skull bone construction
Like birds, the skulls of dromaeosaurs and troodonts are constructed in such a way they the appear more like a series of holes held together with bone struts, rather than a solid bone with holes. There are also air sacs in some of the skull bones. This light weight construction probably evolved to make these predators more manoeuvrable and fast. It was a preadaptation that was carried over into birds, helping keep down the weight for flight.
Some other characteristics common to the members of the Maniraptora are, feathers, the neck, a fused clavicle, sternum, a downward pointing pubis instead of forwards pointing (as in typical saurischians), a shortened tail that is stiffened distally, long arms with a hand (manus) that is larger than the foot (pes). There is evidence in the bones of large theropods that they had a respiratory system similar to birds, with air pockets in their bones. Another feature that was predated in bird ancestors. There is evidence that at least some theropods slept with their heads under their arms as modern birds do. T. rex would have a problem with this feature.
There is evidence that at least some dinosaurs brooded and cared for their young. A number of specimens of Citipati have been found that were on top of eggs, as modern birds brood their eggs. Maiasaura, and many other dinosaurs have been found that were apparently moving in herds comprising the very young as well as the adults. A dinosaur embryo has been found that lacked the teeth of the adult, indicating that it needed to be fed for some time after hatching.
Gizard stones, gastroliths, are common to birds and dinosaurs.
An immature Scipionyx samniticus was found in Italy that retained parts of the intestine, colon, liver, muscles and trachea.
Tissue resembling soft tissue was found in the leg bone of a 68-million-year old T. rex from the Hell Creek Formation in Montana. After rehydration of the tissue the 7 collagen types recovered from bone fragments were compared with collagen from chickens, revealing that theropods and birds are indeed closely related. Demineralisation of the fossilised bone marrow cavity over several weeks revealed evidence of intact structures, blood vessels, bone matrix and connective tissue (bone fibres). Microscopic examination revealed microstructures had been retained down to cellular level.
Several groups of maniraptor have been suggested as the group giving rise to birds.
It has also been suggested that these groups were sister groups to birds, all having a common ancestor that gave rise to them and birds.
According to the author3 there is now enough detailed information on the relationship between dinosaurs and birds that it is possible to reconstruct the stages which could have changed a dromosaurian theropod to an early bird. Early small theropods such as Compsognathus have a bird-like appearance, a long neck, long spindly legs, a smallish head with eyes that were rather large and pointed forward, though they had retained jaws with teeth, hands with claws and a tail that was long and bulky, all dinosaurian features.
These dinosaurs had a bird-like appearance and had made a number of anatomical changes from the basic dinosaur plan, some that were subtle and others that were less subtle. The reduction of the thickness of the tail was one such change, resulting in a very narrow tail that was stiffened by bundles of long, thin bones and was flexible only at the hips. It has been suggested that this tail was used as a dynamic stabiliser when the animal was chasing fast-moving prey that rapidly changed direction. As the tail was no longer a heavy, muscular cantilever for the front part of the body the posture of the animal necessarily changed, making it unstable without other changes. One of these changes was a subtle move of the pubic bone, that marks the rearmost part of the gut. In theropods it normally points forward and downward from each hip socket.
In dromaeosaurs it had rotated backwards to lay parallel to the ischium, the other lower hip bone. The change allowed the gut to be swung backwards until it was beneath the hips. One result of this was to shift the weight further back which would have gone some way to replace the counterbalancing weight of the tail that was then much lighter. As well as in maniraptoran theropods this arrangement is seen in both living and extinct birds.
It has been suggested that shortening the chest in front of the hips was another way of compensating for the much reduced tail mass, and this is a feature seen in these bird-like theropods. The predatory nature of these animals is seen as probably being the reason for the signs of stiffening the chest. The long arms with their 3-clawed hands would need to be powerful as they were used for catching and subduing prey. It is suggested by the author3 that the chest needed strengthening to arms and shoulders as they needed to withstand large forces involved in grappling and subduing prey. In birds the chest has been greatly strengthened to anchor the powerful flight muscles.
There is a V-shaped bone in the front of the chest between the shoulders, the fused clavicles, that functions as a spring-like spacer separating the shoulders, that also helped anchor the shoulders in place while the animals were subduing prey. In birds the fused collar bones form the elongated furcula, wishbone, that functions as a mechanical spring separating the shoulder ones during flapping flight.
These theropods had adaptations of the joints between the arm bones and the hands that allowed them to swing outward and downward very rapidly and with considerable force to strike at prey in a 'raking' action. There was also great advantage gained from the leverage of the system. The powerful arms could be folded against the body when not being used to secure prey. The arm muscles powering this mechanism were placed close to the chest while long tendons attached to the muscles allowed the hands to be operated remotely, also a way of keeping the weight close to the hips which would help with balance. There is close similarity between the mechanisms of arm-striking and arm-folding of these theropods and the opening and closing mechanisms birds employ when opening and closing their wings during and after flight.
Birds from dinosaurs
It has been confirmed by the discoveries from Liaoning that many of the highly active bird-like dinosaurs were small animals. According to the author3 their small size put great physiological stress on them if they were, as is believed, endotherms, as they would lose a large amount of body heat through their skin. It is precisely these small, active endothermic dinosaurs that would benefit from the addition of an insulating layer on their skin. It has been suggested that they evolved feathers to protect them from heat loss rather than to allow them to fly. The author3 suggests that the flight feathers that were genuinely bird-like may not have evolved for flight.
In the deposits at Liaoning there are a number of 'dinobirds' that appear to have feather tufts on the ends of their tail that the author3 described as similar in shape to a geisha's fan, and along the arms a fringe of feathers, as well as on the head and running down the spine. It is necessary to be careful to take account of the biases that can be introduced by the preservational process that may determine which parts of the body feathers are indicated to be present. It has been suggested that feathers possibly evolved as structures associated with behaviour for such functions as recognition signals as is seen in birds of the present, or possibly used in mating rituals, probably a long time before they were used for flight.
Gliding and flight would have been add-on benefits that came later rather than being the sine qua non of avian origins. Once the feathers had evolved, for whatever purpose, they had the potential for aerodynamic uses, and as occurs with modern birds the ability to jump and flutter could have possibly been used in 'dinobird' courtship displays. Microraptor could be seen as an example with its combination of feathers fringing its arms, legs and tail that would allow it to launch itself from high points such as branches. This may well have been the path that lead to the evolutionary line to flight.
A crucial point concerning the Liaoning quarries of Early Cretaceous age is that the fossils are at least 30 million years younger than Archaeopteryx, the earliest known feathered dinosaur that was well preserved and had wings that were highly developed and complex. The fossils at Liaoning were not of the first flying dinosaurs, and ultimately true birds, they are more a snapshot of the evolutionary diversification of avian theropods, as well as some true birds, though are not connected to bird origins. The origins of birds are still unknown but are believed to be in the Middle Jurassic, or possibly the Early Jurassic, some time before archaeopteryx. All the known evidence indicates that the theropods are very closely related to early birds, though the early ancestors of Archaeopteryx have still to be discovered.
It was suggested above that during the Mesozoic the dinosaurs lived in a world that had a climate that favoured very active animals with large bodies that were capable of maintaining high body temperatures that were stable without needing to be genuinely endothermic with the associated costs of that type of physiology.
The findings at Liaoning suggest that this view is wrong, as small theropods that had evolved insulation, according to the author3 simply had to be endothermic, and the point is reinforced by the closeness of their relationship with birds, known endotherms.
The author3 says his response to this is 'well, yeas and no'. That bird-like theropods were true endotherms, there is now little doubt. He suggests that of the more traditional dinosaurs, the argument that they were inertial homeotherms still holds, citing some evidence in support of his view among living endotherms. The metabolic rate of elephants is much lower that than that of mice - for exactly these reasons. Mice lose heat rapidly to the environment as they are small so have to maintain a high metabolic rate to counter this loss of heat. Elephants have a stable internal body temperature as a result of their size, being large, generally dinosaur-sized, though they are endothermic. He suggests there are also physiological challenges to being a large endotherm, such as the problems suffered by elephants if they move around too quickly. A large amount of extra chemical heat is produced by their postural and leg muscles, requiring them to lose heat by 'flapping' their large ears to assist in dissipating the extra heat to the environment. If they can't lose this heat rapidly enough there is the possibility of overheating that can be fatal.
He suggests that as on the whole dinosaurs were super-sized animals, so they would have been able to they would have capable of maintaining a constant temperature; extrapolating from the situation in elephants it would have been a disadvantage for dinosaurs to be genuine endotherms, especially as at that time global temperatures were warm. Having evolved as mass-homeotherms, that could maintain a constant body temperature due to their size alone, the dromaeosaurian theropods were the only group to go against this trend towards large size, evolving instead into a small-bodied group.
The anatomy of dromaeosaurians indicates they were very active, and being on the small side they could benefit from being true endotherms. and they would have required a constant supply of oxygen and nutrients to support their relatively large brains. The problems resulting from small body size in homeotherms is that they need some form of insulatory covering to avoid losing too much heat, as the area to volume ratio of their bodies is such that heat is lost rapidly.
The author3 proposes that among the dinosaurs the issue of homeothermy is not a case of 'all or nothing', most dinosaurs being able to sustain high activity levels without all the costs of true homeothermy, as is found in mammals and birds, by virtue of their large size, being mass-homeotherms. On the other hand small dinosaurs, dromaeosaurs in particular, theropods, as well as their true descendants the true birds, needed to develop full endothermy and the associated insulatory coverings.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|