ORIGIN AND RADIATION 



shell, and it is usually only the external form of the shell that is 

 preserved. One notable exception is a series of 14 specimens 

 described in 1876 by Brogniart. These were found in a silicified 

 fruit from the Coal Measures of St. Etienne, and show a degree of 

 preservation comparable with that of Lepidocaris. Ostracod shells 

 have been described from the Upper Cambrian (about 400 million 

 years ago), and over 3,000 species have been described from 

 Palaeozoic rocks. They are also abundant to-day, both in fresh- 

 water and the sea. 



It is clear that we do not know enough about the early fossils 

 to make a coherent story of the origin and primary radiation of 



Fig. 7. Copepoda. A, a member of the Cyclopoida. B, a 



member of the Calanoida. C, a member of the Harpacticoida. 



A and B have an actual length of about 2.5 mm., while 



C is only 1 mm. long. 



the Crustacea. We are confronted with the ostracods as abundant 

 fossils early in the record, with Lepidocaris as a possible ancestral 

 branchiopod, and with fossil Conchostraca occurring at about the 

 same time. The situation is not helped when we look at other 

 groups of Crustacea; the copepods (fig. 7) are practically unknown 

 as fossils, and the cirripedes are so very distinct that relating them 

 to any other group of Crustacea seems impossible. All that can be 

 done is to study the radiation that has occurred within each of the 

 groups. 



There has been radiation of a spectacular nature within the 



