x. 5 FOSSIL DIPNOI 273 



A similar sac occurs in fossil coelacanths back to the Devonian but its 

 function is quite unknown. The eye, inner-ear, and lateral line system 

 are well developed. 



It is hard to see what features have enabled Latimeria to survive 

 with little change since the Jurassic or earlier (see p. 771). It clearly 

 cannot be by special development of the brain or receptors. Its 

 habitat is isolated, but not especially protected and its population 

 seems to be small since even by exceptional efforts so few specimens 

 have been found. Perhaps they are more numerous in deeper waters. 

 In some of its features it shows developments parallel to those of the 

 Teleostei rather than to the Dipnoi, whose remote ancestry it shares. 

 Several of its characteristics are paedomorphic. These can hardly be 

 alone responsible for such a long survival, but some of them also 

 appear in the other survivors from the Paleozoic, Polypterus, stur- 

 geons, and Dipnoi. 



5. Fossil Dipnoi 



The Devonian Dipnoi were more like their osteolepid relatives than 

 are the surviving modern forms (Fig. 163). The early members of this 

 group, such as *Dipterns (Fig. 211), showed the typical elongated 

 body, thick cosmoid scales, heterocercal tail, lobed fins, and well- 

 ossified skull. The pattern of the bones was obscured by a seasonal 

 deposit of cosmine, this being periodically absorbed to allow of growth. 



The individual bones have a certain similarity to those of osteolepids, 

 but there are extra bones that are difficult to name. There was no 

 premaxilla or maxilla, nor any teeth along the edge of the jaw; instead 

 broad, ridged tooth-plates were developed on the palate and inside 

 of the lower jaw, presumably as an adaptation for eating molluscs 

 and other invertebrates. These crushing-plates are characteristic of 

 the Dipnoi and preclude even the earliest of them from being the 

 actual ancestors of the amphibia. By the end of the Devonian the 

 Dipnoi were showing changes similar to those of the osteolepids and 

 palaeoniscids. The body became shorter, the first dorsal fin dis- 

 appeared, the tail became diphycercal, and the scales lost their shiny 

 surface-layer and became thin. The teeth of *Ceratodus appear in the 

 Triassic and were known to geologists long before the related living 

 animal was discovered. There has been very little change in this 

 animal in more than 150 million years, though the recent members are 

 placed in a distinct genus Neoceratodns. 



The evolution of Dipnoi is especially interesting because the rate 

 of change has actually been measured (Westoll, 1949). Twenty-six 



