From Reptile To Bird

earliest-known bird (Archaeopteryx)

birds evolved, so paleontologists believe, trom small carnivorous dinosaurs that ivOuld run upright on their long, slim hindlegs. The skeleton of the dinosaur Campsognathus (top) is strikingly similar to ihe skeleton of the earliest-known bird, Archaeopteryx (middle). Both animals had long running legs, long bony tails, birdlike teet, clawed fingers, and sharp, pointed teeth. However, Archaeopteryx had feathers -unmistakable bird characteristics — which were preserved as impressions in the rocks. Its collar bones also formed a distinct birdlike wishbone.

A modern flying bird (right) has a short, compact body, centered over the legs for balance. The breast bone has developed into a large, keeled sternum, to which the flight muscles attach. The tail is reduced to a stump (pygostyle) and the jaws are toothless.

modern bird of prey (Caracara falcon)

Keeled sternum (breast bone)

early seabirds had teeth in their jaws, to help catch slippery fish prey. Some, such as the loonlike hesperornithids, had lost the powers of flight, while others, such as the ichthyornithids, still had wings, and were probably more like modern terns.

Several types of bird have lost the ability to fly. This seems to happen particularly when there are no active predators in the birds' environment, and the energy-expensive power of flight is no longer necessary. Such was the case at the end of the Cretaceous period, when the predatory dinosaurs had disappeared, and new mammalian carnivores had not yet appeared — or had not yet reached certain areas. Many types of flightless bird, evolved at that time, in the relative safety and isolation of, for example, South America, Australia and New Zealand.

The modern flightless birds (the ostrich, emu, kiwi, cassowary and rhea), together with the extinct moas and elephant birds, are placed in a group called the Ratites. These great flightless birds all share some features of the bony palate and shoulder girdle, which may suggest that they have descended from an original stock of birds that had become flightless.

Living birds and their fossil representatives are grouped in the subclass Neor-nithes. They all share a common, complicated structure of the bony palate, and this fact leads paleontologists to believe that they have descended from a common ancestor.

Throughout their evolutionary history, birds have retained essentially the same skeletal structure. Because of this similarity, together with the fact that the fragile bones of birds are rarely well preserved in the rocks, the fossil record reveals comparatively little about the interrelationships of the 166 families of modern birds. The relationships shown in the evolutionary chart on pp. 170-171 are based on recent studies by Sibley and his colleagues in the USA on the compatibility of DNA strands from the cells of living birds.

Since the fossil record for bird bones is so incomplete, and since the impressions of feathers are rarely fossilized (and feathers, of course, give birds their shape, as well as their plumed accessories, such as a peacock's tail and headdress), no-one can state with any certainty what prehistoric birds looked like. The reconstructions on the following pages are mainly the traditional view of these birds, based on the known fossil names and a degree of educated guesswork from studies of modern, related species.

Was this article helpful?

0 0

Post a comment