276 



SUBPHYLUM VERTEBRATA: 



.anterior dorsal fin 



posterior dorsal fin 



operculum 



caudal fin 



nasal tube 



LATIMERIA 



pectoral fin 



pelvic fin 



Figure 16.11 The living fossil Lotimeria, a marine crossopterygian. 

 Morphology, Reinhold Publishing Corp., New York, 1962.) 



anal fin 



{From Malcolm Jollie, Chordate 



ships. Their zoological interest stems from a general 

 belief that they were the ancestors of land vertebrates. 

 This premise is made because of the very short devel- 

 opmental step necessary from their skeleton to that of 

 a limbed, land vertebrate. The "step" is outlined in 

 some detail in the fossil record, strongly indicating a 

 gradual transition from fresh-water lobe-fins to fresh- 

 water amphibians. 



Although there are frequent reports of weird crea- 

 tures in the sea and such reports usually can be dis- 

 counted, in 1939 a commercial fisherman captured a 

 living lobe-fin ofT East London, South Africa. Up to 

 that time zoologists were sure that lobe-fins had been 

 extinct for about 70 million years. Since World War 

 II additional specimens have been collected. At the 

 present time the fish is known to inhabit deep waters 

 in the vicinity of the Comoro Islands, north of Mada- 

 gascar. 



Finding this animal was truly remarkable. Who 

 was to expect a deep-sea representative of an artcient 

 fresh-water group? Also, who was to expect a living 

 animal very similar to its ancient ancestor? The 

 living animal would not have been out of place if it 

 had been found with its fossil relatives. 



ORDER DIPNOI (Lungfishes) 



Diagnosis: slender-bodied; fresh-water. 



Three living genera and five species now represent 

 the lungfishes. These five Dipnoi might be considered 

 "living fossils" because they are not very different 

 from their ancestors which were early oflTshoots from 



the ancestral lobe-fins. Of the living lungfish genera, 

 }J eoceratodus (one species) is found in the rivers of in- 

 terior Queensland, Australia, Protopterus (three spe- 

 cies) in the upper Nile River Basin of Africa, and 

 Lepidosiren (one species) in the swampy region of the 

 Paraguay River and adjacent Gran Chaco River in 

 South America (Figure 16.12). 



The early development of lungs in bony fishes is 

 of interest. These structures evolved in fresh-water 

 forms and probably resulted from selection of chance 

 hereditary changes in areas of periodic stagnation or 

 drying of fresh water. The selection for lungs, then, 

 was first associated with periods during which many 

 fresh-water species had difficulty in surviving in their 

 fresh-water environment. Under such conditions, 

 lungs would allow survival. The effectiveness of lungs 

 can be seen in modern species. Lungs enable the 

 Australian species to survive in the stagnant waters 

 of drying pools; however, if ponds dry completely the 

 animals die. The other two living genera can survive 

 the complete drying of ponds. As their ponds dry, 

 they burrow into the bottom mud, and, as the mud 

 dries hard, the fish assume a coma-like, dormant state 

 until the rains restore their ponds. This behavior 

 is known to have been an ancestral condition, because 

 fossil lungfish burrows with the fish remains in them 

 have been found. 



Subclass ACTINOPTERYGII (Ray-flnned Fishes) 



Diagnosis: includes all North American bony 

 fishes; nostrils do not connect to the mouth cavity; 



