Until recently, the rock sequences spanning the Permian–Triassic boundary were seen as containing too many gaps for a small sample size and so reliable dating couldn’t be generated (Shen 2011). However though the use of Zircon populations in ash layers a much more accurate date can be calculated. These populations used to date still have issues however, for example Mundil (2004) looks at Zircon populations being affected by contamination of indistinguishable older Xenocrysts. Zircon’s are used as a more precise method of calculating the mass extinction but due to contamination, the exact date of the Permian Mass Extinctions found using Zircons is highly debated. One way to combat this contamination is to analyse single Zircon populations in which the problem of lead (Pb) loss has been removed by heating the populations to high temperatures, creating more reliable data from ash layers. However the aggressiveness of this technique often leads to less reliable results. The Zircon data leads to the conclusion that the biotic crisis occurred at 252.6 Ma and so the P-T boundary must be slightly younger (Mundil 2004). The P-T extinction exact date is still heavily debated however due to the reliability of much of the data.