Estimation of number of species at an earlier time
according to the National Science Foundation press release"Species Have Come and Gone at Different Rates than Previously Believed" indeed show what Atl LED suggests, that the progression is not linear, with the study showing that:


  The previously thought exponential increase in diversity is not there.


Further, there is the "Palaeobiology Database, that


  seeks to provide researchers and the public with information about the entire fossil record. 


The site also has links to further papers of the topic.