Chapter 6 

 FOSSIL FISHES 



Colorado is fortunate in possessing the principal locality 

 where the earliest traces of vertebrate life have been found. At 

 Harding Quarry, west of Canon City, the Ordovician rocks con- 

 tain innumerable fragments which were described by Dr. C. D. 

 Wolcott as belonging to fishes or fish-like vertebrates. These 

 determinations have naturally not escaped criticism, doubts 

 being expressed both as to the nature of the remains and the age 

 of the rocks. With regard to the first objection, I think it is 

 certain that Dictyorhabdus, supposed to belong to the Chimaer- 

 idae, is really an invertebrate. We have excellent material of 

 it in the Museum of the University of Colorado, showing the 

 surface sculpture. The other forms are not so easily disposed of. 

 Astraspis (the word meaning star-shield) is represented by thick 

 plates exhibiting a reticulated or embossed surface (according to 

 the impression shown), certainly similar in type to those of the 

 primitive fish-like animals of Devonian strata. This conclusion 

 is fortified by the microscopic examination of thin sections, and 

 Dr. C. R. Eastman in 1917 felt able to define a new family Astra- 

 spidae, typified by this genus. It is not a true fish,— Jordan 

 places it in a class Ostracophori, but it is a vertebrate of a fish- 

 like type, related to numerous genera found in Paleozoic rocks 

 of later age. Its interest and importance can therefore hardly 

 be exaggerated, for it suggests that the actual origin of the verte- 

 brate phylum must have been considerably earlier, though it may 

 be that no traces of these beginnings will ever be discovered. 

 This, however, is not the whole story. Wolcott's Eriptychius is 

 doubtfully regarded as representing a Crossopterygian fish; but 

 other fossils have been found by Professor P. G. Worcester near 

 Ohio City, Colorado, which show strong affinity with Devonian 

 species. A fragment of a plate closely resembles that of Cocco- 

 steus disjectus from the Old Red Sandstone; another piece is like 

 that of Rhizodus; while striated spines agree with those of the 

 Devonian Diplacanthus. The first of these represents the Class 

 Arthrodira, the second is Crossoterygian (thus tending to confirm 

 the reference of Eriptychius,) the third is a shark. Thus it seems 

 that the vertebrates were not only in existence, but had become 



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