CHARACTERISTICS OF MARINE FISHES 149 



Some students are of the opinion that sharks are the most primitive of existing 

 fishes. Indeed, some fragments, particularly spines, occur in rocks at an early 

 period. However, the first remains that are definitely identifiable as those of a 

 shark occur in the Carboniferous period. These oldest primitive sharks w^ere small, 

 probably only 2 to 6 feet long. The skin was covered with hard bony points 

 ( denticles ) , and the teeth had a large central cusp and a broad base with smaller 

 cusps, almost exactly as in some modern sharks. Other sharks succeeded those of 

 the Carboniferous period, but nearly all the old forms perished. One family, the 

 bullhead sharks (Heterodontidae) , however, survived. At least 4 species of 

 bullhead sharks exist today; they are small, generally only a few feet long, and 

 like other primitive sharks have a spine in front of each of the two dorsal fins they 

 bear. 



During the Eocene and Miocene periods the warmer oceans teemed with 

 sharks if one may judge from the abundance of teeth that have been found. Some 

 of these more recent sharks, too, were small, but others were giants. It has been 

 estimated, chiefly from the size of their teeth, some of which are 3, 4, and even 

 5 inches long, that certain species among these primitive sharks attained a length 

 of about 120 feet. By comparison the largest living one, the whale shark ( Rhinco- 

 don typus), which attains a length of about 45 feet, is almost a pigmy. 



Another group of primitive fishes that has survived are the chimaeroids 

 (Chimaeridae) , which are sharklike in character. Remains of these animals appear 

 early in geologic time, certainly as early as the middle Silurian period, and there- 

 fore much earlier than the remains definitely identified with the sharks. The 

 several living forms, generally only about 1 to 2 feet long, are variously known as 

 spookfish, ratfish, and elephant fish. These animals, too, have a cartilaginous 

 skeleton, but they have a strong bony spine in front of the dorsal fin and heavy 

 platelike teeth. The remains in the rocks, of course, consist of these hard parts. 



However, remains of more primitive fishes than the ancestors of the living 

 forms already mentioned are known. In fact the earliest ones date back to the 

 Paleozoic period or almost to the beginning of life on earth. The time in years, 

 of course, is not known. One writer has estimated it at 400 million years. These 

 very old remains unfortunately are so fragmentary that it has not been possible 

 to determine what the fish were like. 



The earliest recognizable fish remains are those of the ostracoderms, so-called 

 because of the dorsal and ventral dermal plates that covered the head and anterior 

 part of the body. The posterior part of the body was usually covered with small 

 plates or scales. The ostracoderms and their similar species first appeared in the 

 Silurian, apparently became abundant in the Devonian, and shortly became ex- 

 tinct. 



The small ostracoderms were followed by the much larger arthrodires, which 

 attained a length of about 10 to 18 feet. These fish also were armored anteriorly, 

 but the armor was not continuous, for there was a hinge at the neck so that the 

 fish could bend their heads. Furthermore, the arthrodires possessed large power- 

 ful jaws and small pelvic limbs. These characteristics may have made them rulers 

 of the sea for a time. However, despite their armor, large mouth, and formidable 

 size they eventually became extinct, and left no near relatives. 



The higher groups, or true fishes, Pisces, also called Teleostomi left more of 

 their remains in the rocks than the cartilaginous fishes because their skeletons 



