Ostracodes are abundant in early Paleozoic rocks where foraminifera and 

 plant spores are absent and, therefore, are especially useful in correlating these 

 early sediments. 



Other Arthropoda 



The disarticulate exoskeletal remains of other arthropods are found in sedi- 

 ments dating to the pre-Cambrian. Many arthropod fossils encountered in amber 

 are so well preserved that a detailed study of the spines and hairs is possible. 

 Insect wings and other body elements have been found in perfect condition in 

 shales. An extinct group, the trilobites (PL 11-IV, figs. 6, 7) are exclusively 

 marine, and are especially abundant in the early Paleozoic rocks. 



Scolecodonts (PI. 11-IV, fig. 12) 



The scolecodonts are worm jaws composed of a chitinous, horny substance. 

 They are found in brackish to marine sediments and have a geologic range from 

 Ordovician to Recent. These minute fossils (50 microns-5 mm.) are not used 

 as extensively by the economic paleontologist as are the conodonts, partly be- 

 cause the forms show little change through geologic time. Although they re- 

 semble the conodonts, they are different in their chemical composition and prob- 

 ably have different biological affinities. 



Conodonts (PI. 11-IV, figs. 8-11) 



Branson and Mehl (1944) described the order Conodontophoridia as "mi- 

 nute toothlike objects ranging in shape from simple recurved cones, through 

 denticulate bars and blades, to highly specialized platforms ; having either fibrous 

 or laminated internal structure; comprised of calcium phosphate; and attached 

 to fragments of similar composition assumed to have been jaws. (L. Ord. — 

 Perm; Mesozoic?)." 



Conodonts are extremely valuable as index markers in Paleozoic rocks. 

 These peculiar, minute, toothlike microfossils range from Ordovician to Triassic. 

 They vary in size from 100 microns to over 2 mm. Conodonts differ chemically 

 from scolecodonts, in that they are composed of calcium phosphate rather than 

 chitin and silica. Often conodonts can be differentiated from the black scole- 

 codonts by their amber to light-brown color. Because of their chemical compo- 

 sition, conodonts are destroyed by hydrochloric acid, but they can be liberated 

 from carbonate rocks by treatment with acetic or citric acid. 



The wide distribution of conodonts leads some workers to believe that they 

 are remains of unknown pelagic organisms, and if this is so, they should be valu- 

 able for long-distance correlations. 



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