540 Comparative Morphology of Chordates 



backward. The tail must have presented a great expanse of flat surface 

 which served as an important factor in the aerial activities of the 

 animal. There are also indications of well-developed feathers on the 

 body. 



The nature of the locomotion in these animals is a matter of specu- 

 lation. The relatively small expanse of the wing and the probably weak 

 condition of the sternum forbid regarding them as strong fliers. It is 

 possible that they merely glided or sailed from tree to tree in the 

 manner of "flying" squirrels. 



The animal represented by the fossil described by Owen was named 

 Archaeopteryx lithographica. The three fossils (the single feather 

 being one) were found in limestone strata belonging to the Jurassic 

 (mid-Mesozoic) Period, a time when reptiles were at the apex of their 

 career as a Class. Archaeopteryx, so far as may be judged from the 

 skeleton and feathers, may be regarded as either a feathered reptile or 

 a very reptilian bird. The unique importance of feathers in the life of 

 the modern bird favors placing the animal on the avian side of the 

 purely imaginary line between reptiles and birds. It is, in a sense, a 

 "connecting link," but at present it is merely an isolated link and not 

 a part of any continuous chain of known transitional animals. So far 

 as feathers are concerned, it is not even a connecting link, for they 

 seem to have attained the level of differentiation of the average feather 

 of modern birds. A really satisfactory connecting link would be a 

 reptilian creature having elongated overlapping horny scales with 

 their free distal ends split into filaments suggesting the barbs of a 

 feather. If Archaeopteryx was literally "the first bird," we would be 

 forced to accept Geoffroy Saint-Hilaire's suggestion (p. 342) that the 

 first bird hatched from a reptile's egg. But our present knowledge of 

 inheritance forbids the assumption that so complex a structure as a 

 quill-feather could be the product of a single mutation or even several 

 successive mutations. 



Archaeopteryx is the earliest bird at present known, but between it 

 and typical scaly reptiles must be a very long history which is un- 

 written in the geologic record — or, at least, has not yet been unearthed. 

 Our knowledge of the birds that must have existed during long ages 

 following the days of Archaeopteryx is not much better than the total 

 blank of the earlier period. At all geologic levels, fossil birds are few 

 and far between. 



In rocks of the later Mesozoic (Cretaceous) occur fossils represent- 

 ing birds whose skeletons had gone far toward acquiring the specialized 

 features characteristic of modern flying birds. The tail was short and 

 its series of several free vertebrae terminated in a pygostyle. The large 

 sternum usually bore a strong carina. However, teeth similar to those 



