PHYLOGENY 



fins were l)n)a(l al the l)a.se and pointed at the tip. In many, smaller ae- 

 eessory fins extended along a line between the main pcetoral and pelvie 

 pairs. These and other data indieate that paired fins may have originated 

 from a pair of longitudinal fin folds. During their period of expansion, the 

 plaeoderms produced a wide variety of adaptive types. Most of these 

 were not sueeesslul for long, and it is probable^ that none of the known 

 plaeoderm lossils was in the direct line of descent leading to the higher 

 vertebrates, lint the Chondrichthyes were almost certainly derived from 

 some plaeoderm line, and it is quite probable that the Osteichthyes were 

 also derived Irom plaeoderms. 



Class Chondrichthyes. At about the same time, in the late Devonian, 

 both the C>'h()ndrichlh\('S and tlu> Osteiehthves ap]")ear in the fossil rcx'ord, 

 each arising Irom a dillerent stock of plaeoderms. I'he Chondrichthyes 

 originated in the sea, while the Osteichthyes originated in fresh water. As 

 the history of the Chondrichthyes, the sharks and their allies, is much 

 simpler than that of tlie Osteiehthves, it may be sketched first. Processes 

 involved in (he origin of the sharks from plaeoderms wcmc many, but a few 

 may be singled out lor discussion. One ol these was the loss of armor, 

 thus permitting a very much more active existence than was possible for 

 their predecessors. Of the many jaw types with which nature experi- 

 mented in the plaeoderms, the one carried over into the new class was 

 (and still is) characterized by an upper jaw which was rather firmly 

 jointed to the skull, but not fused to it (except in the chimacMas); and a 

 lower jaw which was freely movable. Where the plaeoderms gcneralh- had 

 bony plates lor biting, the sharks, like some plaeoderms, developed true 

 teeth with a core of dentine and an enamel surface. These are identical in 

 structure and in mode of development with the placoid scales which 

 cover the shark skin, and it may be that the tec>th of sharks are derived 

 from such scales bv simph^ (MilargemcMit. l<'urth(M\ while their forebears 

 possessed considerable bone, (his was lost in the sharks, leaving a skeleton 

 of cartilage. 



The sharks first appeared in the late Devonian and reached a climax 

 in the Mississippian. Their numbers were reduced in the Permian and 

 Triassic, but they recovered and reached a new climax in the C^rc^taceous. 

 This was accompanied by th(> h)rma(ion of a new adajitixe tvpe, the ra\s. 

 These are essentiallv llalt(Mied sharks, the teeth of which are modificxl to 

 form plates for crushing the shells of the molluscs on which they feed. 

 One more group of allied fishes, the chimaeras, comprising the order llolo- 

 cephali, forms a part of the modern class C'hondrichtlues. The chimaeras 

 are not well known in the fossil record. The class as a whole* has been 

 reduced sonx^what since the (Cretaceous, but it c()n(inuc\s to form a fairlv 

 important part of the marine fauna of the world, and a few sharks have 

 even invaded fresh water. 



Class Osteichthyes. The donn'nant fishes of the world are, and long 

 have been, the Osteichdiyes, or bony fishes. Following Ilaeekel, it was 

 long b(>lieved (hat these took their origin from primKixc sharks. V>\\K this 

 no longer seems likcl)', for the lossil (>\ idence indicates that the Osteich- 

 thyes arose earlier than did the C>hondrichthyes. Further, the bony fishes 



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