230 



THE ORIGIN AND EVOLUTION OF LIFE 



thiomimus and the modern cursorial flightless forms of birds, 

 such as the ostriches, rheas, and cassowaries. 



In the opposite extreme to these purely terrestrial forms, 

 the flying arboreal birds also gave off the water-living birds, 

 one phase in the evolution of which is represented in the loon- 

 like Hesperornis, the companion of the pterosaurs and mosa- 

 saurs in the Upper Cretaceous seas. It was on the jaws of the 



Fig. iio. Reversed Aquatic Evoli;ti(l\ ok Wim, axd Body Form. 



Wing of a penguin (.4) transformed into a fin externally resembling the fin of a shark (B). 

 Skeleton of Hesperornis (C) in the American Museum of Natural History and restora- 

 tion of Hesperornis (D) by Heilman, both showing the transformation of the flying bird 

 into a swimming, aquatic type, and its convergent evolution toward the body shape of 

 the shark, ichthyosaur, and dolphin (compare Fig. 41). 



Hesperornis and smaller Ichthyornis that Marsh made his sen- 

 sational announcement of the discovery of birds with teeth, 

 a discovery confirmed by his renewed studies of the classic 

 fossil bird type, the Jurassic Archceopteryx. These divers of 

 the Cretaceous seas {Hesperornis) are analogous to the modern 

 loons, and represent one of the many instances in which the 

 tempting food of the aquatic habitat has been sought by ani- 

 mals venturing out from the shore-lines. As in the most highly 

 specialized modern swimming birds, the Antarctic penguins, 

 the wing secondarily evolves into a Im or paddle, while the 



