Vertebrates are chordates. Therefore, considering 

 them in a separate chapter isolates them from other 

 members of their phylum, the Chordata. Perhaps it 

 is difficult to accept the close relationship between 

 vertebrates and such things as sea blubber. How can 

 animals with backbones and some semblance of a 

 skull — fishes, amphibians, reptiles, birds, and mam- 

 mals — be related to the chordates mentioned in the 

 preceding chapter? The answer already was given: 

 all possess a notochord, dorsal hollow nerve cord, 

 and gill slits during some stage of their life cycles. 



Before considering the individual groups, let us 

 trace the probable ancestry of all of them. 



VERTEBRATE AFFINITIES: 

 SUBPHYLUM VERTEBRATA 



We will start our discussion by reviewing the 

 echinoderm-chordate line (Figure 16.1). Again, 

 although qualifying words or phrases are not always 

 used, phylogeny is probable but often hypothetical. 

 Recall that the ancestor of this line likely was a 

 stalked, attached organism that had a dipleurula 

 larval stage. This ancestor perhaps gave rise to the 

 echinoderms. After a time, chance hereditary 

 changes probably modified both larval and adult 

 echinoderm stages; the adult, although still attached, 

 was a gill filter-feeder rather than an arm-feeder like 



the original adult ancestor. This hypothetical gill 

 feeding, "second step" ancestor was an infinitesimal 

 step below the hermichordate and tunicate ancestor. 

 The actual tunicate ancestors, larval and adult, are 

 believed to have been quite similar to certain modern 

 tunicates. This assumed larva was essentially a 

 tunicate and gave rise to an advanced chordate with- 

 out a sessile adult stage; again, a small amount of 

 change would have been involved. The advanced 

 chordate probably was a direct step to the cephalo- 

 chordates and also to primitive filter-feeding verte- 

 brates. From these primitive vertebrates, in short 

 order, there arose the ancestral jawless Cyclostomata, 

 a group that exists today in the form of hagfishes and 

 lampreys. The cyclostomes contained the now extinct 

 order of archaic, jawless, armored ostracoderms 

 (Figure 16.2), from which, in turn, evolved another 

 extinct group, a class of jawed, armored placoderms 

 (Figure 16.3). It is believed that placoderms evolved 

 into two main lines, one leading to the cartilaginous 

 fishes, Chondrichthyes (Figure 16.4), and the other 

 to the bony fishes, Osteichthyes. The ancestral bony 

 fishes (Figure 16.5) were the source of two living sub- 

 classes, the ray-finned Actinopterygii (Figure 16.6) 

 and the lobe-finned Choanichthyes (Figure 16.7). 

 The latter group still has two living orders, the lung- 

 fishes and the lobe-finned fishes. Ancestral lobe-fins 

 most likely were also ancestors of land animals in the 

 form of amphibians. Some of the earliest amphibians 



266 



