EXERCISE XIII 

 PALEONTOLOGY 



In the exercise on Homology it was found that the limbs of vertebrates 

 begin their development in the same way, as a simple outpushing of the 

 body wall, whereas the adult limbs of different vertebrates are quite unlike 

 in the details of structure. These and other facts are believed to show 

 that all vertebrate animals have descended from a common ancestor. If 

 this belief is well founded, vertebrates have changed (" evolved") greatly 

 in the generations subsequent to the common ancestor. 



In the following exercises it is shown in the case of two typical groups 

 of animals, one vertebrate and one invertebrate, that such an evolution 

 has actually taken place. Either or both of these exercises may be used 

 at the option of the instructor. The change is demonstrated by the 

 remains of animals preserved in the rocks as fossils. In general, the 

 deeper rock strata contain the fossils of the more ancient animals, the 

 more superficial rocks the more recent animals. Why? 



In the study of the fossils used, reference should be made to the geo- 

 logical time scale in "Principles of Animal Biology," by Shull, LaRue 

 and Ruthven, Chapter XV, or in Pirsson and Schuchert's " Text-book 

 of Geology." This time scale should be before the student throughout 

 the exercise. 



A. EVOLUTION OF THE CEPHALOPODA 



The cephalopods of the past lived within their shells, like the present 

 day Nautilus, but unlike the squid or cuttlefish, which are also cephalopods. 



Examine a bisected shell of Nautilus, also a shell of Nautilus contain- 

 ing the animal. Note that the shell is divided into a number of chambers 

 by septa (singular, septum} . These were successively produced from the 

 center to the opening of the shell. The animal, as it grew, moved forward 

 in its shell at intervals, and formed new septa behind it. 



The ancient cephalopods lived in shells somewhat similar to that of 

 Nautilus. The line of union of a septum with the outer wall of the shell 

 is called a suture. The sutures of Nautilus are not visible externally 

 because of a pearly layer, the nacre, on the outside. Fossil cephalopod 

 shells, however, usually show these sutures. Examine a fossil Loxoceras, 

 Orthoceras, or other orthocone. 



Notes. No preliminary notes on the cephalopods are required. The 

 questions asked below are intended chiefly to direct attention. The 



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