ganoin (enomel) layers 



Figure 8-33. Section through a scale of Cheiroiepis an early palaeonrscoid. (After Aldinger, 1937) 



generations of denticles 



ganoin layers 



Sharpey's fibei 



B 



vascular canal 



Figure 8-34. Sections through the scales of paloeoniscoids. A, Orvikwna vardiaensis (Upper 

 Middle Devonian) in which the ocellulor base has many branching canals of Williamson; B, 

 £/onicht/iys punctafus with a cellular base and a laminated ganoin cover. (A, after 0rvig, 1957; B, 

 after Aldinger, 1937) 



ganoin layers 



dentinal tubules 



Figure 8-35 



Williamson's canals. 



canals of Williamson 

 Scale of a perleidid, the iepisosteid type having 



type, occurring already in the Lower Devonian genus Chei- 

 roiepis, and it is generally assumed that those with the 

 canals (the lepidosteid type) were derived from those with- 

 out. However, Orvikutna (Figure 8-34) of the Middle De- 

 vonian already had a scale suggesting the lepidosteid tyjse 

 (Figure 8-37). The base of the scale has numerous ascending 

 canals which show many side branches. These canals do not 

 reach the pulp cavities of the dentine and appear to house 

 only the osteoblasts of the acellular bony base. The early 

 appearance of such a modified scale suggests an early radi- 

 ation of scale types among the palaeoniscoid fishes. 



It is unlikely that the scale of Orvikuina is related to that 

 of Lepisosleiis. The lepidosteid type may have been achieved 

 independently by many kinds of fishes at different times 

 (i.e. a case of parallel or convergent evolution). It is also 

 possible that both types of scales (with and without Wil- 

 liamson's canals) arose at the same time and have continued 

 down to the present, each radiating into a number of sub- 

 types. 



SCALES 



235 



