A HISTORY OF VERTEBRATES: FISHES 427 



Records of these types, together with footprints and other indications 

 of the activity of organisms, are known as fossils. 



From the fortuitous ways in which fossils are formed, uncovered 

 (and sometime destroyed) by erosion, and finally discovered, it follows 

 that the fossil record is far from complete. It is also a somewhat biased 

 sample of the life of the past because organisms living in or near water, 

 or on plains where their remains can be covered by wind-blown sand, 

 are more likely to be fossilized and preserved. Forest-dwelling species 

 in particular leave few fossils, for decay is very rapid on the forest 

 floor. Nevertheless, the study of fossils (the science of paleontology) and 

 earth history (geology) can provide us with much information con- 

 cerning the history of organisms. Earth history will be considered more 

 fully in Chapter 35. For our present purposes it is sufficient to realize 

 that geology can tell us the sequence of the fossils, can give us estimates 

 of their age, and can help to tell us something of the environment in 

 which the organisms were living. Geologists clivide earth history into 

 eras, periods and sometimes smaller units of time. Those that pertain to 

 a history of vertebrates, together with an indication of their age, are 

 shown in the diagram of vertebrate evolution (Fig. 22.2). 



195. Vertebrate Beginnings 



Although vertebrate origins are obscure, we have a reasonably com- 

 plete fossil record of their subsequent evolution. The most primitive 

 are jawless types placed in the class Agnatha. This group flourished 

 during the middle Paleozoic era, when it was represented by several 

 orders collectively known as the ostracoderms. These ancestral verte- 

 brates were small, fresh-water, bottom-feeding animals that were fishlike 

 in general proportions, but somewhat flattened dorso-ventrally, espe- 

 cially near the front of the body (Fig. 22.3). They had an extensive 

 armor of thick, bony plates and scales develo^^ed for the most part in 

 the dermis of the skin. 



The ostracoderms had median fins but (with the possible exception 

 of pectoral flaps in a few genera) not paired fins equivalent to the 

 paired appendages of other vertebrates. The upper portion of the caudal 

 fin was larger and more rigid than the lower because it included an 

 extension of the body axis. This heterocercai tail is characteristic of both 

 fossil and living primitive fishes (Fig. 22.6). 



The significance of this type of tail was demonstrated in 1936 by 

 Professor Harris of Cambridge, who studied the role of the fins of 

 fishes by a series of amputation experiments and by measuring the 

 forces at work on models placed in a wind-tunnel. He pointed out that 

 primitive fishes lack lungs or swim bladders and hence their bodies 

 have a relatively high specific gravity. Such a fish tends to sink to the 

 bottom, but a head flattened on the ventral surface, or large pectoral 

 appendages such as those found in many sharks, tend to raise the 

 anterior part of the body off the bottom when the fish moves forward 

 through the water. The lateral motion of the trunk, and the hetero- 

 cercai tail with its rigid upper portion, give a compensatory lift to the 



