"The Netherlands was in the Pliocene still largely covered by the Northsea, in which a rich population of many species could be found including large animals like wales and giant sharks". A Wikipedia note with its references tells us the following about the temperature of the Pliocene: 'The global average temperature in the mid-Pliocene (3.3 Ma-3-Ma) was 2-3 °C higher than today, global sea level 25 m higher...'. For more on those sea levels in the period see also a recent paper:

'Departures from eustacy in Pliocene sea-level records', M.E. Raymo et al., Nature Geoscience 4, 328 (2011)