Australia: The Land Where Time Began
Bird Evolution – Bone Growth
The authors1 began this study by examining the bone tissue of Confusiornis, a crow sized animal from the Early Cretaceous of China, 125 Ma, that is appears to be on the avian family tree, appearing in the fossil record a short time after Archaeopteryx, which has been generally accepted as the first known bird. They found that the inner bone tissue of Confuciusornis was of the fibro-lamellar type which indicates rapid growth, as occurs in other dinosaurs, though in this case it changes to the slower growing bone type in the outside, indicating that following a short, youthful growth spurt the growth rate dropped. When these tissues were compared with those of Troodon, a small dinosaur that was raptor-like, they grew to about 1.5 m in length, which has been studied by David J. Varricchio. The bone tissue of Troodon is of the type that indicates more rapid growth overall.
It is shown by Confuciusornis that these ancient birds became small by truncating their juvenile growth spurt that was most rapid in other dinosaurs, which resulting in the birds, in effect, becoming miniaturised. This miniaturisation influenced their locomotion as the feathers that were also present on the forelimbs of its closest dinosaurian relatives would, according to the author1, have made it more likely that these smaller animals could become airborne. The wings of smaller animals can be flapped more rapidly than large ones, and wing loading (ratio of weight to area of the wing, i.e. the mass a given unit of area has to carry) will be proportionally smaller, and therefore more aerodynamically advantageous.
Birds of the present reach full size rapidly, usually in weeks to months. The author1 suggests that following a slowing of growth rate in the early part of their evolution the growth rate appears to have become more rapid again, these rates often being even higher than rates in extinct dinosaurs. The bone tissue of early birds that were a bit further along in the evolutionary line than Archaeopteryx and Confuciusornis had been studied (Anusuya Chinsamy-Turan et al.). Included among these birds from the Late Cretaceous was a primitive enantiornithine, Patagopteryx, a flightless bird, Hesperornis, a diving bird, and Ichthyornis, a ternlike bird. It was found that they also grew more slowly than dinosaurs, though the forms that were closer to the extant birds were indicated by their bone tissue to have grown somewhat faster than the earlier birds.
Growth rates have been found to have increased substantially about 66 Ma, close to the boundary of the Cretaceous-Palaeogene, to the point that all extant birds grow to full size in less than 1 year, and in sparrows it takes only 7 days. It is not known if the extant birds acquired their rapid grow rates relatively suddenly or gradually, the author1 suggesting this will not be known until further study of birds from the Early Palaeogene have been carried out..
The evolutionary line of birds begins with the appearance of the Archosauria and continuous through a number of stages of the Dinosauria, which eventually develop feathers and a line of these feathered dinosaurs evolved into birds. There is not full agreement at exactly which stage of the evolution of dinosaurs the birds branched. A difference from other reptiles that was already present in earliest known stage of dinosaur evolution is a great difference in the bone tissue. As occurs in birds and mammals of the present, dinosaur bones grew more rapidly than those of other reptiles. When the earliest birds (Avialae) arose their bone growth rate slowed compared with that of other dinosaurs, though it was still more rapid than that of reptiles, and this resulted in a substantial size reduction. At the point when the Aves, the living birds began to diverge from their ancestral forms the growth rate of their bone increased again with the result that birds of the size of a pigeon then matured in weeks instead of months. At the present all birds grow to adult size in less than a year, and do so much more quickly, an example given in a sparrow that matures in 7 days. When the earliest birds arose their growth rate reduced at the juvenile stage of their development, the stage when their ancestral dinosaurs were growing at the peak of their growth rate, which in effect miniaturised the newly evolving birds.
Lamellar-zonal bone tissue; vascularisation is low; growth is slow; basal metabolic rates slow e.g. crocodiles; 3-5 m.
Fibro-lamellar bone tissue; vascularisation high; growth rate rapid; basal metabolic rates higher; e.g. Brachiosaurus 25 m.
e.g. Coelophysis 3 m.
e.g. Troodon 2 m.
Body size reduced by truncation of rapid early growth stage; e.g. Archaeopteryx 60 cm.
Somewhat fewer vascular canals, which is typical for small animals; e.g. Confuciusornis 60 cm.
Enantiornithes 15 cm
Patagopteryx 50 cm
Hesperornis 1.75 m
Ichthyornis 35 cm
Diornis 3-4 m
Sparrow 10 cm
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|