THE PHYLUM CHORDATA 



SUPERCLASS PISCES 



Class Agnatha. The ostracoderms, primitive, armored members of the 

 class Agnatha, are the first vertebrates to appear in the fossil record, and 

 they are probably ancestral to the remaining classes. The earliest ostraco- 

 derm fossils are of Ordovician age, but they are neither numerous nor well 

 preserved until the Silurian. They reach a peak of expansion during the 

 Silurian and Devonian, then disappear from the record. Yet they did not 

 actually become extinct, for unarmored representatives of the class have 

 survived to the present as a minor part of our fish fauna, the cyclostomes. 

 But, bereft of their bony armor, these have not been fossilized. 



The Agnatha are extraordinarily primitive vertebrates. Morphologists 

 have long agreed that the Ammocoetes larva of the lamprey ( one of the 

 living agnaths ) approximates the archetypal vertebrate more closely than 

 does any other living form. While there is no doubt that the living lam- 

 preys could not have been ancestral to the other vertebrate classes, none- 

 theless, a lamprey may be regarded as morphologically just an ostraco- 

 derm stripped of armor. Agnaths have no paired fins or limbs, in contrast 

 to all other vertebrates. The mouth is suctorial and without jaws (hence 

 the name of the class ) . The gills are well developed, but unlike those of 

 higher vertebrates. The vertebrae are extremely simple, consisting only 

 of the dorsal arcualia, the notochord still being the major element of the 

 axial skeleton. There are only one or two semicircular canals in the ear. 

 The kidney is pronephric in some (hagfishes), but it is mesonephric in 

 others ( lampreys ) . This brief sketch should suffice to show that the group 

 is extremely primitive, and could potentially be a source for the higher 

 vertebrates. This, coupled with the facts that the ostracoderms are the 

 first vertebrates to appear in the fossil record and that the next class of 

 fishes, the Placodermi, appeared soon afterward, lend weight to the hy- 

 pothesis that the ostracoderms actually were ancestral to the higher 

 vertebrates. 



Class Placodermi. The class Placodermi shows significant advances 

 over its ostracoderm progenitors. It first appeared late in the Silurian, then 

 rose rapidly to dominance during the Devonian. During the Devonian, 

 it gave rise to the remaining two classes of Pisces, and as these increased 

 in importance, the Placodermi dwindled. By the beginning of the Missis- 

 sippian they had been reduced to a minor place in the fauna, and they 

 finally became extinct in the Permian. Like their predecessors, the ostraco- 

 derms, the placoderms were mainly a fresh-water group, but some of them 

 did invade the seas. 



Perhaps the most important advance of the placoderms was the acqui- 

 sition of jaws, making possible a predatory mode of life. These were quite 

 unlike those of modern fishes, and in some the lower jaw was fixed while 

 the upper jaw and the entire head were movable. There was extensive 

 bony armor, and the head armor was movably jointed to that of the thorax. 

 The remainder of the skeleton was largely cartilaginous. Paired fins were 

 also present, and these were quite variable. In some species, they resem- 

 bled those of modem fishes, while others had bizarre fins. In some, the 



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