EVOLUTION OF EARLY PALEOZOIC FAUNAS 31 



ment and an accumulated tendency to change resulted in the great 

 evolution of life in the lower Ordovician. 



LIFE AT THE BEGINNING OF KNOWN CAMBRIAN TIME 



The traces of pre-Cambrian life, though very meager, are sufficient 

 to indicate that the development of life was well advanced long before 

 Cambrian time began. The characteristic fossil of the known pre- 

 Cambrian fauna is Beltina danai,^ a crustacean probably more highly 

 organized than the trilobite. The associated annelid trails indicate 

 that this phase of the fauna w^as also strongly developed. Strati- 

 graphically, this fragment of what must have been a large fauna occurs 

 over 9,000 feet beneath an unconformity at the base of the upper por- 

 tion of the Lower Cambrian in northern Montana.^ This fact indicates 

 that it is practically hopeless to search for the first forms of life — those 

 that could leave a trace of their existence — in strata now referred to 

 the Cambrian or early Paleozoic. With this thought in mind we shall 

 consider what is known of the life of early Lower Cambrian (Georgian) 

 time. 



The oldest known Cambrian fossils are found deep down in the 

 Lower Cambrian strata of southwestern Nevada and the adjoining 

 Inyo County area of eastern California. In sections 120 miles apart 

 the Lower Cambrian has a thickness of over 5,000 feet, with a great 

 limestone forming the upper 700 to 2,000 feet. Below this limestone 

 calcareous strata occur, but the predominating rocks are sandstones, 

 and arenaceous, siliceous, and calcareous shales. In the lower 400 

 feet of the Waucoba Springs section and the Barrel Spring section 

 south of Silver Peak in western Nevada^ the fauna includes: 



Annelid trails Trematobolus excelsis Walcott^ 



Protopharetra, sp. undt. ObolcUa, sp. undt. 



Archaeocyathus, sp. undt. Orthotheca, sp. undt. 



Ethmophyllum cf. whitneyi Meek^ Holmia rowei, new species 



Mickwitzia occidens Walcotts Nevadia weeksi, new species 



1 Bull. Geol. Soc. Amer., Vol. X, 1899, pp. 238, 239. 



2 C. D. VValcott, Observations of igo8. 



3 Walcott, Smithsonian Miscellaneous Collections, Vol. LIII, No. 5, 1908, 

 pp. 185-89. 



4 See Bull. U. S. Geol. Survey, No. 30, 1886, pp. 81-84. 



5 Smithsonian Miscellaneous Collections, Vol. LIII, No. 3, 1908, p. 143. 

 ^ Ibid., p. 146. 



