PRE-CAMBRIAN AND PALEOZOIC ERAS 147 



representing in all probability a lapse of many millions of years. In view 

 of these lost chapters in the record we need not be surprised that the story 

 of animal evolution does not commence in the Cambrian where it left off 

 at the end of the Proterozoic era. In place of the paucity of fossils char- 

 acteristic of Proterozoic rocks we find in Cambrian deposits abundant fos- 

 sils, particularly in the later deposits of the period. Cambrian oceans 

 teemed with a wide variety of invertebrates. 



One reason for the increase in completeness of record may lie in the fact 

 that in the interim between Protero- 

 zoic and Cambrian, animals possess- 

 ing hard parts (shells and exoskele- 

 tons) increased greatly in numbers. 

 Brachiopods constituted an impor- 

 tant portion of the Cambrian fauna. 

 These animals are enclosed within 

 shells consisting of two portions or 

 valves. Unlike the shells of bivalve 

 molluscs, such as clams, the two 

 valves are unequal in size (Fig. 8.1). 

 Molluscs themselves are represented 

 in Cambrian seas by a few snails and, 

 near the close of the period, rare 

 cephalopods (p. 151). 



Worm burrows, recalling those of 

 the Proterozoic, are abundant in 

 some Cambrian deposits. 



The dominant animals in the Cam- 

 brian seas were the trilobites, consti- 

 tuting some 60 percent of the known 



inhabitants of those seas. They were small animals for the most part, 

 ranging between one and four inches in length, the giant among them being 

 18 inches long. The examples presented in Fig. 8.2 are typical. Fig. 8.3 

 shows a trilobite with such structures as antennae and appendages restored 

 to the appearance presented in life. 



The first thing to note about trilobites is that they are members of Phylum 

 Arthropoda, to which such creatures as lobsters, crayfishes, spiders, and 

 insects also belong. The general similarity to a lobster, for instance, is evi- 

 dent in the shell-like exoskeleton, the segmented body, and the jointed ap- 

 pendages. Arthropods form the "highest" (most complex and specialized) 

 phylum of invertebrates. Thus in the first geologic period that we know 



FIG. 8.1. Brachiopods, attached to 

 rocks by their pedicles. Note the larger 

 size and difPering shape of the half- 

 shell pierced by the pedicle. (Mainly 

 after Dunbar, Historical Geology, John 

 Wiley & Sons, Inc., 1949.) 



