VERTEBRATE PALAEONTOLOGY 141 



relative temporal positions? To what extent can one assume that 

 the climatic conditions in different parts of the world would not 

 have affected the distribution of animals so that a local change in 

 climatic conditions lasting some millions of years did not lead to a 

 migration of specimens into different parts of the world? For 

 then, if one decided that strata with similar fossils were of similar 

 age, one could be in serious error. What is the maximum error 

 that can be allowed in the estimation of the date of any given 

 series of rocks? All these questions have to be answered before 

 one can decide that fish A is earlier than fish B in time. 



There is another and in this case a minor difficulty. This 

 concerns the nature and naming of the strata. There is often some 

 discussion as to whether a given series of rocks should be placed 

 at the bottom of one level or the top of another. Thus the 

 Tremadoc has been placed both at the top of the Cambrian and 

 also at the bottom of the Ordovician. Another problem concerns 

 the Downtonian; are these Late Silurian or Early Devonian rocks? 

 This point is only of importance when the fossils are classified as 

 Late Silurian or Early Devonian instead of Downtonian. It is 

 indeed a great tribute to the work of the geologists and palaeonto- 

 logists that so much agreement has been reached concerning the 

 dating of the various strata. But it is unfortunate that the 

 difficulties are often glossed over and only the most simple story 

 presented. When one is dealing with the evolution of the basic 

 vertebrate types all within a comparatively short time of each 

 other, the problem of accurate dating becomes one of critical 

 importance. Many of the conclusions that we have today are only 

 tentative ones. 



We can state with certainty that the earliest bony fragments are 

 those of Agnathan fish. These are separated by some 300 million 

 years from the earliest mammalian fragments. But it is much more 

 difficult to decide just how much earlier the Agnatha were than the 

 very first Placoderms; or how much earlier the first Osteichthyes 

 were than the first elasmobranches, or when the first Amphibia 

 arose relative to the time of origin of the reptiles. We can believe 

 that one group arose before the other and there is good evidence 

 that one group of fossils may be commonly found before another, 

 but when it comes down to giving a precise date, or even a reason- 

 able estimation of the time of origin of the groups, it is quite 



