which brachiopods had built graceful shell-bank structures. 

 The onrushing waters of the sea inundated the peat swamp, 

 dumping shell debris all over its floor. Worse yet, the salt 

 water killed the trees; being pithy structures, for the most 

 part, they were ill prepared to withstand the waves and soon 

 fell over onto the surface of the peat, where they rotted. 

 Gradually, the marine floodwaters receded and peace re- 

 turned as the dry season of the year 285,000,000 b.c. began. 

 The country had taken on an entirely changed look. The 

 forest had vanished and the lowland on which it had stood 

 was permanently covered with water. Not a single stick of 

 timber remained standing. But the inundated area teemed 

 with life as our insect-observer might have noted while cruis- 

 ing over the Mecca region: most abundant were small, deli- 

 cately-shelled scallops that could be seen in large numbers 

 just about everywhere. Common also were cephalopods, 

 distant relatives of the living Nautilus. And there were palae- 

 oniscoid fishes, all manner of sharks, and acanthodians re- 

 sembling eels, but belonging to that ancient group of verte- 

 brates that were the first to evolve movable jaws. As the 

 dry season progressed, the water level gradually sank on the 

 former coastal plain, and the animals that had ventured into 

 this area could no longer be observed from the air because 

 of a thick, floating, mat-like growth of plants (perhaps algal 

 in character — though our observer won't say) that developed 

 at or under the water surface. 



VV hat happened toward the end of the dry season and 

 during the following three years is written, so to speak, on 

 successive pages of the packet of evenly bedded, black shale 

 that has been the subject of our intensive study. 



The floating mat of vegetation produced a lush growth 

 near the surface, where it had access to light, but the plants 

 beneath died and decayed, their remains forming a dense, 

 very black layer of mud on the bottom. By the end of the 

 dry season the water level had become so low that the float- 

 ing mat (called a "flotant") actually touched bottom, except 

 here and there where shallow depressions had existed on the 

 former forest floor. Even in such places the water was shoal, 

 possibly one or two feet deep. The slow lowering of the 

 water level resulted in the progressive crowding together of 

 the animals that had ventured out into this newly created 

 fringe zone of the sea. Finally, they were almost wholly con- 

 centrated in the residual pools, where most of them perished, 

 and their remains became entombed in the black bottom mud. 



The next rainy season marked the beginning of the year 

 284,999,999 b.c. With the forest gone, water from the higher 

 lands east of the Mecca area was carried in by rivers, flood- 

 ing much of the former coastal plain. Indeed, the volume 

 of incoming fresh water exceeded the amount that could run 

 off into the inland sea at any given moment. The result was 

 a temporary stagnation all along the new fringe of the basin, 

 so that the water cover may have been several feet deep. 

 Most likely this water soon became somewhat brackish and, 

 with its flow reduced, it shed a great deal of its burden of silt. 

 The flotant, however, seems not to have been materially 

 disrupted. 



With the water level high, a new wave of marine crea- 

 tures pioneered into the Mecca region. They were mostly 

 vertebrates of the same kinds that had come the previous 

 year, but now there were greater numbers. The delicate 

 scallops, however, were not among them.' This would tend 

 to indicate that the scallop, of all the members of the buried 

 community of the Mecca Quarry shale, was the only animal 

 that could not tolerate the flooding fresh water. The cepha- 

 lopods, generally thought to have been strictly marine, are 

 present, however, suggesting that these forms had a notable 

 tolerance to fluctuations in the salt content of the water. 



A he dry season of 284,999,999 B.C. again saw the gradual 

 lowering of the water level over the Mecca area, and again 

 the animals that had ventured onto the flooded plain became 

 concentrated in scattered residual ponds. The invaders were 

 mostly palaeoniscoid fishes, numerous species of sharks, and 

 acanthodians. A tremendous number of these fishes perished 

 at this time, and their remains were buried in the black muck 

 that accumulated at a rapid rate on the bottom and which 

 is now the dense, black, sheety shale. 



Under conditions of severe crowding (the ratio of volume 

 of fish to the volume of water at their disposal lay somewhere 

 between 1 :5 and 1 :291) one would expect mass mortality to 

 occur from lack of sufficient oxygen in the water, or through 

 poisoning by the noxious gases that developed from the de- 

 caying carcasses at the bottom, or perhaps from the over- 

 heating of the water in the shallow pools. But, apparently, 

 the death of these animals was not due to causes of this sort; 

 instead, every specimen collected from this deposit shows 

 unmistakable evidence of bite injury, or the fossils represent 

 specimens actually eaten by medium to large-sized sharks 

 that evidently were prone to disgorge hard-to-digest skeletal 

 parts and scales. These "shark pellets" (similar to owl pel- 

 lets) are among the most common and characteristic fossils 

 of the black shales. Further evidence of feeding activity lies 

 in the occurrence of countless coprolites (fossil dung). The 

 large number of sharks and fishes, prey and predators alike, 

 that were not eaten but merely fatally mutilated, probably 

 reflects a frantic behavior arising from the highly over- 

 crowded conditions in the residual ponds. 



-Louring the rainy and dry seasons of the years 284,999,998 

 and 284,999,997 b.c. the described drama occurred twice 

 more, but by the beginnings of the rainy season of 284,999,- 

 996 b.c, the coastal plain had settled to the point where the 

 following dry season no longer left its mark in the sediments 

 of the area. Now all the coastal plain was under sufficiently 

 deep marine water so that residual ponds no longer formed. 

 No concentrations of carcasses have been found on the burial 

 ground of this year, indicating that the animals had much 

 greater freedom of movement. As the subsidence of the 

 Mecca plain continued and the water became deeper, the 

 flotant disappeared and marine invertebrates established 

 themselves as a new bottom community all over the area. 

 {Continued on page 8) 



February Page S 



