Posts Tagged ‘Posture’

Advantages of an Erect Posture

Saturday, December 19th, 2009


The erect posture of dinosaurs is often said to be the key to their success. Why is this? An important reason is that an erect posture is mechanically more satisfactory than a sprawling one. The weight of the body is supported entirely from below. In a sprawler, the weight of the body is supported from the sides. While gravity effectively pulls straight down from the center of the body mass, in a sprawler this force has to be converted into a sideways component along the femur or humerus (upper arm bone), and then a vertical component down the tibia and fibula, and the radius and ulna (forearm bones), which causes great stresses are avoided if the gravitational force of the animal’s mass is transferred down through a straight, erect limb.

This mechanical advantage is important. Firstly, erect animals can run in a more sustained way: not necessarily faster, but with more stamina, because the effort of supporting the body weight is much less than in a sprawler. This would have been an immediate advantage to an archosaur chasing sprawling prey animals or escaping from a sprawling carnivore. Interestingly, the main plant-eating groups of the Middle Triassic, the pig-like rhynchosaurs and dicynodonts, were evolving semi-erect gaits at the same time. Indeed, the ancestors of the mammals, the cynodonts – which were moderate-sized carnivores at that stage – also showed similar advances.

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The Origin of the Dinosaurs

Tuesday, December 15th, 2009


Most of the synapomorphies of the leg that appear in Ornithosuchus, advance in Lagosuchus, and come to full development in the dinosaurs are concerned with the acquisition of an erect gait – or the fully upright posture. It is important to note that erect or upright gait does not necessarily mean bipedal. Cows and horses have the erect gait and posture, just as much as humans do.

The first archosaurs were sprawlers, like modern lizards and salamanders. The limbs stuck out sideways from the body, and the elbows and knees form right angles at all times as the animal walks. Even at speed, a lizard generally swings its limbs far out to the side of its body, and it is assumed that the Early Triassic archosaurs moved in a similar way. During the Middle Triassic, most archosaurs adopted a semi-erect posture in which the body could be lifted clear of the ground, with the arms and legs tucked partly underneath for rapid locomotion. Finally, in the Middle and Late Triassic, the two archosaur lineages noted above – the crocodilian and dinosaur lines – adopted an erect posture in which the limbs were tucked underneath the body at all times. This seems to have happened independently in each line.

The aetosaurs, rauisuchians and early crocodilians evolved an erect posture in which the acetabula shifted beneath the hip bones and the heads of the femurs fitted straight up into them, like straight columns beneath a building. The members of the dinosaur line used the approach seen in mammals, in which the acetabula remain on the side of the hip bones but the femurs develop right-angled heads that fit in from the sides. In this design the relationship of hip girdle and leg is more like a buttress on the side of a church building, rather than a column beneath its roof, but the result is the same. The legs of dinosaurs, and of mammals, come together in a slightly knock-kneed fashion beneath the body, and this is a crucial feature.

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What are the Dinosaurs?

Saturday, December 12th, 2009


The inturned head of the femur (the beginnings of the full right-angled femur head seen in dinosaurs and in a different from in mammals); the straight knee joint; the reduced hinge-like ankle joint (technically termed the advanced mesotarsal, or AM, ankle); the long toes and the digitigrade posture of the foot, in which only the toes touch the ground, not the sole of the foot as in earlier archosaurs – and in humans today.

Most of the dinosaur-like characters are also seen in the flying pterosaurs. Certain paleontologists argue that Lagosuchus, the pterosaurs, and the dinosaurs together form a major clade that arose in the Middle to Late Triassic, some 230 million years ago.

The dinosaur-like synapomorphies of this clade, and their further modification in the dinosaurs proper, are part of a major series of related anatomical changes that took place among the archosaurs during the Triassic, and which may have been the key to the origin of the dinosaurs.

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