356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



an occurrence could well put "paid" to a weakling species. Anyway, 

 it was thought until 1938 that this group of fishes had gone with the 

 dinosaurs, and their sudden reappearance after an interval of 70 mil- 

 lion years gave the scientific world — and other people too — something 

 to talk about. 



It is this long absence from the geological record that makes the 

 reappearance of the coelacanths so interesting, for in the animal 

 kingdom as a whole so-called "living fossils" are not uncommon. 

 Indeed the coelacanth's own, if distant and rather degenerate, cous- 

 ins — the lungfishes — still exist on three continents. The whole native 

 fauna of Australia is an archaic survival that has been saved from 

 extinction by that continent's isolation. We also have the textbook 

 case of the tuatara, the sole survivor of a group of lizardlike reptiles, 

 the Rhynchocephalia, that flourished in Triassic times, more than 150 

 million years ago. 



However, not all living fossils are by any means the struggling sur- 

 vivors of once important groups — the all too successful cockroaches 

 represent a type of insect that goes back to the time of the great coal 

 forests, while some of the living sharks and skates have a respectable 

 ancestry of more than 100 million years. 



There is, however, one important difference between the history of 

 most of the animals that have been mentioned and that of the coela- 

 canths. Whereas most of the former became adapted to a certain type 

 of environment and stuck to it, the coelacanths have continually 

 changed their habitat. The first primitive types of the Devonian 

 rocks (the diplocercids) were marine, but the succeeding forms of the 

 Carboniferous period were mostly inhabitants of the fresh waters. 

 Later, in Triassic times, they went into the shallow seas, venturing 

 into rather deeper waters as time went on. So one of their claims to 

 zoological fame is ada'ptability without obvious stnwtural change. 



How common coelacanths were in the past it is not easy to say, for 

 fossil numbers do not always by any means reflect the rarity or other- 

 wise of the living animal owing to the varying chances of preservation 

 and discovery. 



On the whole, coelacanths are rather rare as fossils, although occa- 

 sionally in a particular locality they have proved to be not uncommon ; 

 in one instance, many hundreds of fossil coelacanths were found in 

 Triassic rock that was being excavated for the foundations of a new 

 building at Princeton University. 



The accompanying diagram, which also gives a useful time scale, 

 shows the relative numbers of known different kinds at various peri- 

 ods, and the rise and decline in actual numbers of individuals probably 

 follows the same pattern. The absence of the coelacanths from the 

 geological record for the last 70 million years is most intriguing, and 



