ARMORED 



FISHES- 



{Continued from page 2) 



be easily capable of biting other fishes in 

 two. The body and tail of this fish are 

 unknown, but his total length is esti- 

 mated to be as much as 1 5 feet. If he 

 was a strong swimmer, and we have no 

 way of knowing surely, he must have 

 been a formidable predator. If, on the 

 other hand, he was a poor swimmer, he 

 may have been a scavenger or a feeder 

 on larger invertebrates. The original of 

 the Dunkleosteus in our exhibit is one of 

 many placoderms that have been found 

 in the black shales that underlie the city 

 of Cleveland. 



o> 



"ne of the most peculiar placoderms, 

 and, for that matter, of all fishes, is Both- 

 riolepis, the best known genus of a group 

 that was common in middle and late 

 Devonian streams (see Fig. 1). Their 

 distribution was world-wide, and, ex- 

 cept for South America, they are known 

 on all continents, including Antarctica 

 and Greenland. Their armor is an ex- 

 aggerated form of the usual placoderm 

 jointed shield. Instead of proper fins, 

 they had developed a pair of peculiar 

 flippers, usually jointed, with which they 

 propelled themselves around on stream 

 bottoms. The form of the tail is known 

 from impressions that have been pre- 

 served in one famous locality on the 

 Gaspe peninsula of Quebec. Their 

 mouth was a small opening bounded by 

 strange jaws on the flat lower surface of 

 the head. 



Some years ago, when I was working 

 at Dartmouth College, I had an oppor- 

 tunity to study material collected on the 

 Gaspe" by the late Professor William Pat- 

 ten. Serial sections sawed through his 

 specimens of Bothriolepis showed a variety 

 of sediments filling the shield. Most of 

 the fill was the same coarse-grained sand- 

 stone in which the fossils were buried, 

 yet in the trunk shield of many speci- 

 mens was a mass of fine mudstone. I 

 soon became convinced that this was a 

 filling of the intestine, and almost cer- 

 tainly was a remnant of some mud that 

 the Bolhriolepis had eaten shortly before 



Page 8 MARCH 



its death. This indicated that this fish 

 fed on mud of the stream bottoms and 

 extracted nourishing material from it in 

 its digestive tract. 



A third sediment, this one a fine- 

 grained sandstone, filled certain parts of 

 the Bothriolepis that communicated with 

 the exterior. This filling may have hap- 

 pened when a flood killed a number of 

 these fishes by burying them on a stream 

 bottom. The fine sand preserved in part 

 the shape of the mouth cavity and gills. 

 In addition, I could recognize the filling 

 of a pair of elongated, bladder-like or- 



Fig. 2. Drawing by the author. 



gans that connected with the pharynx. 

 These sacs could be identified only as 

 lungs, though lungs are air-breathing or- 

 gans that had previously been thought 

 to occur only in two groups of fishes: 

 lungfishes, and the crossopterygian an- 

 cestors of land vertebrates. These sedi- 

 mentary fillings made it possible for me 

 to reconstruct some of the soft anatomy 

 of Bothriolepis, as is shown in the accom- 

 panying illustration (Fig. 2). 



A third group of placoderms is repre- 

 sented in the featured exhibit by Gemu- 

 endina, which has a flat body, much en- 

 larged pectoral fins, and a relatively 

 narrow tail. Superficially it looks much 

 like some modern skates and rays, and it 

 must have had similar habits. But in 

 the details of its structure, Gemuendina 

 shows its relationship to placoderms, and 



its similarities to modern skates and rays 

 have resulted from convergent evolution. 



Jl lacoderms are generally believed to 

 have died out at the end of the Devonian 

 period, which lasted for about 60 million 

 years. It is possible, however, that some 

 members of the group survived until late 

 Paleozoic times and perhaps even until 

 today. Among the fishes found by Dr. 

 Zangerl in Pennsylvanian rocks at the 

 Mecca Quarry (see his articles in this 

 and last month's Bulletin), are a few 

 that cannot be assigned to any familiar 

 group of fishes; it is not impossible that 

 they will be shown to have a relationship 

 to placoderms. In today's oceans are a 

 number of peculiar fishes with a skeleton 

 of cartilage, known as chimaeroids or 

 ratfishes. Ordinarily they are classified 

 with the sharks and rays, which also 

 have a cartilaginous skeleton. But a 

 number of characteristics suggest that 

 their real relationship may be to a 

 group of placoderms of the Devonian 

 period. Current research may well 

 solve the problems of these questionable 

 relationships and determine whether 

 placoderms, like the lungfishes and cros- 

 sopterygians, may have survived to mod- 

 ern times, (end) 



MECCA- 



{Continued from page 4) 



spanned a similar period of time as it 

 does under like conditions today, we may 

 calculate the rate of deposition of the 

 Mecca Quarry shale as 1 millimeter in 

 about 5 days. For the entire foot of 

 shale at this place (308 millimeters), the 

 period of deposition would thus come to 

 1,540 days, a value that suggests the or- 

 der of magnitude of 4 years. 



It remains to be seen to what extent 

 this simple and rather accurate method 

 of determining the passage of time in the 

 distant past is applicable to other rocks 

 than the Mecca and Logan Quarry 

 shales. Henceforth, students of sedimen- 

 tary rocks are bound to look over their 

 fossils with care for evidence of the kind 

 displayed by the lowly gastric residue 

 pellet from the Logan Quarry, (end) 



PRINTED BY CHICAGO NATURAL HISTORY MUSEUM PRESS 



