136 



VERTEBRATE PALAEONTOLOGY 



Fig. 4 1 . Diagram to show a more complex view of the level of origin 



of the various vertebrate fossils. Note that the precise time of 



origin is often not clear and that the jawed fish, amphibia and 



reptilia all arose within a comparatively short time of each other. 



(1942), that suggests that the modern amphibia are diphyletic, 

 the anurans coming from one stock of bony fish whilst the 

 urodeles came from another; the two lines being separate in the 

 Early Devonian.) 



The reptiles arose in the Carboniferous. There are certain 

 forms such as Seymouria that are of interest in that they have body 

 characters that are reptilian and head characters that are amphibian. 

 Seymouria is sometimes thought of as a link between the Amphibia 

 and reptiles. Unfortunately Seymouria is found in the Permian 

 whilst the first reptiles arose in the Pennsylvanian, some 20 or so 

 million years earlier. The situation concerning the origin of the 

 mammals is not very much more clear, though the mammals 

 certainly evolved at a later date than the first reptiles. Just how the 

 major groups of the mammals evolved is not very clear. Thus we 

 have three distinct mammalian lines, the Monotremes, the 

 Marsupials and the Placentals, and there is no good evidence that 

 all three came from the same reptilian stock. It is quite possible 

 that many of the mammalian-like characters such as warm- 

 bloodedness, double circulation through the heart, development 

 of the neopallium, development of hair and secretion by milk 

 glands, could be homoiologous and that the mammals resemble the 

 arthropods in that they are not a phylum but a grade of organisa- 

 tion that has been achieved many times from a basic stock. 



There are other points of interest that arise when we consider 

 the time of origin of these various groups. On embryological 



