The K–Pg mass extinction event occurred at the end of the Cretaceous System (K, for kreta or chalk, a common Cretaceous rock type) and the beginning of the Paleogene System (Pg). During the last million years of the Cretaceous, just prior to the K–Pg boundary, between 40% and 75% of marine invertebrate and terrestrial vertebrate species disappear from the fossil record. Included in this extinction are such well-known groups as dinosaurs (but not birds), pterosaurs, mosasaurs, ammonites and many species of marine plankton. Although there is broad consensus that these extinctions were driven by climate/oceanographic change, the potential causes of these changes remain topics of controversy. What is known is that the Earth was subjected to a substantial fall in sea-level (c. 100 m), widespread volcanism, and a large comet/meteor impact during the extinction interval.

Key Concepts:

• Mass extinctions occur when atypically high numbers of organismal extinctions take place within geologically short intervals of time.
  
• Stratigraphic successions are characterised by stacked intervals of net sediment accumulation and discontinuities that mark intervals of no sediment accumulation or active erosion.
  
• Molecular phylogenetic data indicate that many groups originally thought to have radiated after the K–Pg extinction (e.g. mammals and birds) have deeper histories than is indicated by the fossil record and should be regarded as K–Pg survivors.
  
• Sea-level changes across the K–Pg boundary indicate that more complete records of the biotic change will be found in shallow marine environments rather than in terrestrial or deep-marine settings.
  
• Patterns of species extinction in complete K–Pg successions indicate that this event occurred over a prolonged interval (e.g. tens to hundreds of thousands of years) prior to the K–Pg boundary rather than suddenly and coincident with the K–Pg boundary.
  
• Of the three main K–Pg extinction mechanisms – sea-level change, volcanism, asteroid/comet impact – all affect the environment in remarkably similar ways.
  
• Flood basalt volcanism is the only extinction mechanism that exhibits a consistent association with intervals of mass extinction.
  
• The multiple-cause model – in which a geologically unusual juxtaposition of sea-level change, volcanism and asteroid/comet impact seems the most likely candidate for the cause of this extinction event.