viii. 4 HOLOSTEANS 235 



pletely during the Triassic and flourished greatly in the Jurassic. We 

 may group together the fishes of this type as Holostei but the term 

 is used variously by different authors and includes several lines, whose 

 relationships are not clear. The earliest holostean, *Acentrophorus 

 from the upper Permian, is much like a paleoniscid but with a small 

 mouth, shorter, deeper body and slightly upturned tail. This 'abbrevi- 

 ated heterocercal' tail was presumably made possible by the changed 

 swimming habits resulting from the use of the air-bladder as a hydro- 

 static organ. If the fish floats passively there is no need for a hetero- 

 cercal tail to direct the head downwards (p. 140). Similarly, the head 

 does not need to be flattened to produce an upward lift. The develop- 

 ment of the air-bladder has thus made possible the lateral flattening 

 and shortening of the body so characteristic of later Actinopterygii. 

 The body of holosteans was at first covered with thick ganoid scales, 

 but these became thinner in later types. The jaw suspension is 

 characteristic, the maxilla being freed from the pre-opercular. As a 

 result the lower jaw could now be protruded forwards in front of the 

 upper and a 'sucking' action, characteristic of teleosts was evolved, 

 the prey being drawn into the mouth from a distance (Gardiner, 

 i960). By a change in the insertion of the adductor mandibulae 

 muscle a more powerful jaw action then became possible. Some of the 

 holosteans achieved crushing teeth and replaced the dipnoans in the 

 early Mesozoic. There are various smaller distinctive holostean 

 features, such as the loss of the clavicle. 



We do not know whether fishes of this type arose from a single 

 palaeoniscid stock; it is very likely that the change occurred several 

 times, and that throughout the Triassic and Jurassic there were 

 several lines with these holostean characteristics, evolving separately. 

 During the Cretaceous they became fewer, being replaced by their 

 teleostean descendants, but two holosteans survive today, Lepisosteus 

 the gar-pike (often written Lepidosteus) and Amia the bow-fin. These 

 are freshwater fishes, living in the American Great Lakes and other 

 parts of eastern North America, but the group is mainly a marine one, 

 having taken to the sea in the Trias at a time when other groups were 

 doing the same (palaeoniscids, coelacanths, elasmobranchs). The 

 basic cause of this movement is not known, but perhaps there was an 

 increase of planktonic and invertebrate life on which the fish depended. 



Lepisosteus shows a rather primitive structure and must have 

 remained at approximately the Triassic stage. With its complete 

 armour of thick scales (Fig. 146) it presents all the appearance of 

 a primitive fish. The air-bladder opens to the pharynx and the 



