VERTEBRATE PALAEONTOLOGY 137 



grounds it had been considered that cartilage was more primitive 

 than bone. Thus cartilage appeared in the embryo in most cases 

 before bone did, and the elasmobranch fishes in many ways 

 appeared simpler than the bony fish. The elasmobranches were 

 in fact considered to be more primitive than the bony fish and it 

 was not till more attention was paid to the palaeontological dating 

 that it become clear that the elasmobranches were more recent 

 than the Osteichthyes. The Osteichthyes arose in the Early and 

 Middle Devonian whilst the elasmobranches arose in the Middle- 

 Late Devonian. Furthermore most of the fossil groups were bony 

 when first found but there was a tendency to reduce the ossifica- 

 tion so that the later forms are less bony and more cartilaginous. 

 The palaeontological evidence thus indicates that bone is more 

 primitive than cartilage and in this respect conflicts with ideas 

 that are derived from embryological studies. 



The fact that the groups Agnatha, Placoderms, Osteichthyes, 

 Chrondrichthyes, Amphibia and Reptiles all arose within a 

 relatively short time of each other (possibly by explosive evolution, 

 the explosion lasting over 50 million years) means that one has to 

 be much more accurate in dating the fossils than if it had taken, 

 say, 300 million years. 



There are two main ways of dating rocks: an objective method 

 of using radioactive data and a subjective method by which one 

 analyses the relative position of the rocks and their included 

 fossils and then comes to conclusions concerning the contempor- 

 aneity and priority of the different strata. Neither of these 

 methods is completely free from objection, as we shall now see. 



Radioactive dating of rocks 



There are various methods by which it is possible to use the 

 ratio of various radioactive materials to determine the absolute 

 age of rocks. Some of these will briefly be mentioned here. 



The first method is the uranium-lead method, or the so-called 

 radiogenic lead method. Nier (1939) published a review of this 

 method as applied to various samples and later reviews of the 

 subject have been written by Knopf (1948), Kulp (1955, 1956) and 

 Ahrens (1956). There is also the book by Zeuner (1958) which 

 discusses the various methods of dating material. The common 

 lead isotope has the atomic weight of 204, However, other 



