CAMBRIAN INVERTEBRATES 



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Series of fragmentary remains of that problematic fossil, Bcl- 

 tina danai, which he refers to the Merostomata and near to the 

 eurypterids, thus making it probable that either eurypterids, or 

 forms ancestral both to trilobites and eurypterids existed in pre- 

 Cambrian times. More extensive adaptive radiations are found 

 in the Lower Cambrian life period of Olenelliis. This trilobite 

 is not primitive but a compound phase of evolution, and rep- 

 resents the highest trilobite 

 development. Trilobites 

 are beautifully preserved as 

 fossils because of their dense 

 chitinous armature, which 

 protected them and at the 

 same time admitted of con- 

 siderable freedom of mo- 

 tion. The relationships of 

 the trilobites to other in- 

 vertebrates have long been 

 in dispute, but the dis- 

 covery of the ventral sur- 

 face and appendages in the mid-Cambrian Ncolcnus serratus 

 (Fig. 20) seems to place the trilobites definitely as a subclass 

 of the Crustacea, with affinities to the freely swimming phyl- 

 lopods, which swarm on the surface of the existing oceans. 



A most significant biological fact is that certain of the 

 primitively armored and sessile brachiopods of the Cambrian 

 seas have remained almost unchanged generically for a period 

 of nearly thirty million years, down to the present time. These 

 animals afford a classic illustration of the rather exceptional 

 condition known to evolutionists as "balance," resulting in 

 absolute stability of type. One example is found in Lingulella 

 (Lingula), of which the fossil form, Lingulella acuminata, char- 



FiG. 20. A Mid-Cambrian Trilobite. 

 N^coloius serratus (Rominger). After Walcott. 



