ex 



'ur planet Earth has a history span- 

 ning about four and a half billion years. 

 The earliest evidence of organisms living 

 on its surface dates back more than 600 

 million years. 1 



All during these enormous intervals of 

 geologic time, an infinite number of ma- 

 jor and minor processes ran their courses. 

 Chemical compounds were formed and 

 destroyed, volcanic ash was transported 

 by winds, gases were vented from marsh 

 bottoms, the remains of plants and ani- 

 mals were buried in sediment — to men- 

 tion just a very few. Many of these proc- 

 esses required no more than seconds, 

 days, or weeks to be accomplished. Yet 

 all these events, no matter how minute 

 or seemingly insignificant, had some ef- 

 fect upon the environment in which they 

 took place and hence left behind some 

 mark of their occurrence. 



The ancient fish traps of Mecca, de- 

 scribed in the January Bulletin, existed 

 during Pennsylvanian time, a period 

 that ended 280 million years ago. Dr. 

 Eugene S. Richardson, Jr., and I have 

 made a detailed study of the fossil fish 

 graveyard found in the Mecca and Lo- 

 gan Quarry shales of Parke County, In- 

 diana. Perhaps the most startling result 

 of this study is that we were able, some 

 285 million years after the events, to ex- 

 amine and to reconstruct the history of 

 a specific four-year period. 



JTJ.OW was it possible to date this se- 

 quence of events in which vast numbers 

 of animals were destroyed on the fringes 

 of a transgressing (flooding) Pennsylva- 

 nian inland sea? 



The record is "written" in the rocks, 

 our only source of information concern- 

 ing ancient geologic events. Potentially, 

 rocks contain a wealth of evidence re- 

 garding the conditions that prevailed 

 prior to, during, and since their forma- 

 tion. Experience has taught us, how- 

 ever, that most rocks have undergone 

 such severe changes since the time when 

 they were formed that much of this evi- 

 dence has been destroyed or so severely 



1 An article by Dr. Edward OUen, Curator 

 of Mineralogy, explaining the principles in- 

 volved in absolute dating of the geologic past, 

 will appear in a future issue of the Bulletin. 



altered that we can no longer recog- 

 nize it. 



The present dense, tough, finely lami- 

 nated shales of the Mecca and Logan 

 quarries look as if their substance had 

 been severely compressed from the time 

 when they were presumably a smelly, 

 semi-liquid mud. Clearly, these are not 

 the kind of rock that one would expect 

 to contain much evidence of the myriad 

 physical, chemical, and biochemical 

 processes that must have occurred on 

 the site where thousands of animal car- 

 casses were buried. 



stances (for example, pyrite) that be- 

 come a part of the rock. 



Evidence of this sort is very abundant 

 in the Mecca and Logan Quarry shales; 

 it gives testimony to commonplace events 

 of little consequence — or so they would 

 seem, except for the fact that these proc- 

 esses have a small time dimension, meas- 

 urable in seconds, which provides us 

 with a potential clock built right into 

 the shale. 



Suppose it could be demonstrated that 

 the decay process left recognizable marks 

 both in the fossil specimens and the mud 



THE ANCIENT FISH TRAPS 



MECCA 



PART 



RAINER ZANGERL 



CHIEF CURATOR, GEOLOGY 



As it turned out, however, just the op- 

 posite is the case. These shales are full 

 of fascinating phenomena that are, we 

 believe, related to processes that took 

 place in Pennsylvanian time when the 

 ancient shales were accumulating mud 

 on the coastal plains of the Mecca area. 



Most of these phenomena seem to be 

 the result of the disintegration of plant 

 and animal tissues, a process that is al- 

 ways accompanied by the formation of 

 gases (for example, methane and hydro- 

 gen sulfide). Under water such gases 

 form bubbles that, being lighter than 

 water, are vented to the surface. If a 

 large bubble forms beneath mud, or 

 within the carcass of an animal, or with- 

 in a fecal pellet, the escaping bubble dis- 

 arranges the structures in its path, and 

 such disarrangement, if preserved, may 

 leave a fossil record of that event. 



If, on the other hand, the gas bubbles 

 are tiny, they may remain trapped be- 

 neath a layer of mud and the gas may 

 chemically react with other compounds 

 in the environment. The result may be 

 the formation of relatively stable sub- 



in such a way as to indicate the thick- 

 ness of the mud blanket that had accu- 

 mulated on a decomposed carcass. This 

 would mean that we could now measure 

 directly the equivalent thickness on the 

 present shale into which the mud has 

 turned. 



IV. specimen precisely fitting the de- 

 scribed situation is shown on page 4. 

 Fig. 1 represents a section cut through a 

 stomach residue pellet of a shark from 

 the Logan Quarry. The pellet contains 

 mostly scales, the leftovers of a meal that 

 consisted of parts of palaeoniscoid and 

 acanthodian fishes. The palaeoniscoid 

 scales appear on the section as elongated 

 black bars, those of the acanthodian as 

 squarish black spots. The scales are em- 

 bedded in a ground-mass of light brown 

 color. 



It may be noted that the palaeonis- 

 coid scales show a peculiar arrangement, 

 and one of them, on the upper side of the 

 mass, extends vertically into the shale; 

 to the left of its tip there is a little "flag" 

 (Continued on next page) 



MARCH Page 3 



