The Pull of the Recent describes a phenomenon in the fossil record, that causes past biodiversity estimates to be skewed towards the modern taxa, modern biodiversity being the best sampled. Diversity estimates, since Sepkoski's, have consistently shown a global increase in biodiversity since the Cambrian. The cause of this, according to the Pull of the Recent is due to favourable sampling by taphonomic processes of more recent fossils (time proportional to destruction of all geological records), as well as the ease of studying extant taxa.
The effect of the Pull of the Recent has recently been called into question, after analysis of Cenozoic bivalves, which showed 95% of living genera have fossil representatives dating back to the Pliocene.
References
Pull of the recent Wikipedia(Text) CC BY-SA