68 



THE OOLOGIST 



THE EARLIEST KNOWN BIRD 



(Archaeopteryx) 



The oldest bird of which tlaere is a 

 geologic records is represented by two 

 well preserved individuals; one head- 

 less in the collections of the British 

 Museum, the other which is complete, 

 in Berlin. Both were found in the 

 lithographic limestone at Solenhofen, 

 Bavaria, a' treasure storehouse from 

 which many of the most perfect Juras- 

 sic fossils have come. 



The first suggestion of the existence 

 of birds in the Jurassic was the find- 

 ing of the imprint of a feather, in Aug- 

 ust, 1861. A month later the bird it- 

 self was found, and in 1877 a second 

 specimen appeared. These birds dif- 

 fer from those existing today in many 

 ways; a'nd while not of the same 

 species, enough of their unique char- 

 acteristics are common to both indi- 

 viduals to warrant their assignment to 

 a single genus. 



Archaeopteryx shows an advanced 

 state of evolution, and at the same 

 time unmistaka'ble indications of a rep- 

 tilian ancestry. Prom this ancestry it 

 retained teeth set in sockets, rep- 

 tilian finger-like claws upon its fore 

 lim'bs, a long vertibrated tail, aniphi- 

 coelous vertibrae, and divided pelvic 

 girdle. Its bird-like characteristics 

 were equally distinct. Its head and 

 bra'in were aviarian; its sternum, 

 while not as well developed as in mod- 

 ern forms, was unmistakably for fly- 

 ing in bird fashion, and not in ptero- 

 saurian fashion; its posterior limbs 

 were adapted for bird-like walking; 

 and it possessed feathers. The rec- 

 trices were not arranged in the fan- 

 like ma'nner characteristic of modern 

 birds, but were grouped in pairs upon 

 either side of the vertibrated tail. In 

 subsequent birds the tail is shortened, 

 and the fan-like arrangement is gradu- 

 ally assumed. The reptilian charac- 



teristics are so distinct that had the 

 feathers not been found, the speclmenh 

 would have doubtless been classified 

 as reptiles. 



Various estimates of the time which 

 lias elapsed since the Jurassic perioa 

 have been made. These estimates 

 vary from that made in 1893, by Wil- 

 liams, of 10,000,000 years, based upon 

 the rate of erosion of the Cenozoic 

 and Mesozoic sediments; to that ot 

 Barren, in 1917, of 195,000,000 yearb, 

 based upon the rate of disintegration 

 of radioactive minerals, contained in 

 tile rocks, to lead. Prom these esti- 

 mates, of which Barren's figure of 

 195,000,000 years is more generally ac- 

 cepted by geologists, some realization 

 may be had of the time which has 

 ela'psed since the earliest known bird 

 lived. The acquisition of the ability 

 to fly was not an instantaneous ac- 

 complishment. The evolutionary de- 

 velopment of the bird-like character- 

 istics of Arcliaeopteryx were well ad- 

 vanced, and so, to this estimate or 

 195,000,000 must be added at least 

 half again as much time, in which the 

 evolution from a true reptile took 

 place. This estimate of 282,500,000 

 years as the age of the birds may ap- 

 pear far too great to those unfamiliar 

 with tlie reckoning or geologic time. 

 It is, nevertheless, as close an approx- 

 imation of the time which has ela'psed 

 since the reptilian ancestor began to 

 assume aviarian characteristics as 

 the present state of the science of 

 geology renders possible. 



William D. Johnston, 

 The University of Chicago. 



Bibliography 



1. J. Barren : Rhythms and the iVIeas- 

 urement of Geologic Time, Bulle- 

 tin of the Geological Society of 

 America, vol. 28, 1917. 



2 R. S. Lull: Organic Evolution, The 



Macmillan Co., 1917. 



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