478 



VERTEBRATE LIFE AND ORGANIZATION 



is very important, and the eyes and optic regions of the brain are 

 unusually well developed. The eyes oi birds occupy a large portion of 

 the head and both eyes together are often heavier than the brain. The 

 visual acuity of birds, that is, their ability to distinguish objects as they 

 become smaller and closer together, is several times as great as that of 

 man. The ability to accommodate rapidly is also well developed in birds' 

 eyes, lor birds must change quickly from distant to near vision as they 

 maneuver among the branches of a tree or swoop down to the ground 

 from a considerable height. Muscular coordination is also very important 

 in the bird way of life, and the cerebellum is correspondingly well 

 developed. The cerebral hemispheres are large and are an important 

 association center. 



209. The Origin and Evolution of Birds 



One might infer simply from the structure of modern birds that 

 they have evolved from archosaurian reptiles, but we need not stretch 

 our inferences, for two specimens of a fossil bird are known that are 

 clearly intermediate between archosaurs and modern birds. The fossils 

 are preserved with remarkable detail in a fine-grained, lithographic lime- 

 stone from Jurassic deposits in Bavaria. 



Arcliaeopteryx lithographica (Fig. 24.7 A) was about the size of a 

 crow. Its skeleton is reptilian in having toothed jaws, no fusion of trunk 

 or sacral vertebrae, a long tail, and a poorly developed sternum. Birdlike 

 tendencies are evident in the enlarged orbits, some expansion of the 

 brain case, and in the winglike structure of the hand. As in modern 

 birds, the "hand" is elongated and only three "fingers" are present; 



Figure 24.7. Extinct birds. A, A restoration of Archaeopteryx, the earliest known 

 bird; B, a restoration of Hesperornis, a large diving bird of the Cretaceous. (A, Heilmann; 

 B, courtesy of the American Museum of Natural History.) 



