page_title: Mosasaurus
section_title: Extinction
hierarchical_section_title: Mosasaurus / Extinction
caption_attribution_description: English: Badlands near Drumheller, Alberta, where erosion has exposed the K-Pg boundary.
context_page_description: Mosasaurus is the type genus of the mosasaurs, an extinct group of aquatic squamate reptiles. The genus lived from about 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cretaceous. The earliest fossils known to science were found as skulls in a chalk quarry near the Dutch city of Maastricht in the late 1700s, which were initially thought to have been the bones of crocodiles or whales. One particular skull discovered at around 1780, and which was seized during the French Revolutionary Wars for its scientific value and transported to Paris, was famously nicknamed the "great animal of Maastricht". In 1808, naturalist Georges Cuvier concluded that it belonged to a giant marine lizard with similarities to monitor lizards but otherwise unlike any animal known today. This concept was revolutionary at the time and helped support the then-developing ideas of extinction. However, Cuvier did not designate a scientific name for the new animal; this task was completed by William Daniel Conybeare in 1822 when he named it Mosasaurus in reference to its origin in fossil deposits near the Meuse River.
context_section_description: By the end of the Cretaceous, mosasaurs like Mosasaurus were at a height of radiation, and their extinction was a sudden and unexpected event. The youngest fossils of Mosasaurus occur up to the Cretaceous-Paleogene boundary (K-Pg boundary) and its demise was a result of the Cretaceous-Paleogene extinction event that also wiped out the dinosaurs. Localities where Mosasaurus fossils have been found at least 15 meters (49 ft) below the boundary or less include but are not limited to the Maastricht Formation, the Davutlar Formation in Turkey, the Jagüel Formation in Argentina, Stevns Klint in Denmark, Seymour Island, and Missouri. Prior to the boundary event, global sea levels dropped, which drained the continents of their nutrient-rich seaways and altered circulation and nutrient patterns, reducing the number of available habitats for Mosasaurus. The genus endured by accessing new habitats in more open waters.
  Mosasaurus fossils have been found within the K-Pg boundary itself between the Paleocene Clayton Formation and Cretaceous Owl Creek Formation in southeastern Missouri. Vertebra fossils from the layer were found with post-mortem fractures and the deposition of the layer itself was likely the result of a tsunamite, alternatively nicknamed the "Cretaceous cocktail deposit"; it was formed as a result of a combination of catastrophic seismic and geological disturbances, mega-hurricanes, and giant tsunamis as direct consequences of the impact of the Chicxulub asteroid. Aside from the physical kinetic destruction of these events, the impact also had subsequent environmental reverberations like blocking out sunlight that led to a collapse of marine food webs. Any Mosasaurus that may have survived the immediate cataclysms by taking refuge in deeper waters would have eventually died out due to starvation from a total loss of prey structure.
  One enigmatic occurrence is of Mosasaurus fossils found in the Hornerstown Formation, a deposit that is typically dated to be from the Paleocene Danian age, which was immediately after the Maastrichtian age. The discoveries have been found with an association of fossils of Squalicorax, Enchodus, and various ammonites within a uniquely fossil-rich bed at the base of the Hornerstown Formation known as the Main Fossiliferous Layer. This does not mean that Mosasaurus and its associated fauna may have survived the K-Pg extinction; there are several possible explanations as to why these animals have been found in nominally Cenozoic deposits. One argument proposes the fossils actually originated from an earlier Cretaceous deposit that was reworked into the Paleocene formation during its early deposition. Evidence of reworking typically comes from fossils that are worn down due to further erosion during its exposure at the time of its redeposition. Many of the Mosasaurus fossils from the Main Fossiliferous Layer consist of isolated bones that are commonly abraded and worn; however, the layer also yielded fresher Mosasaurus remains. Another explanation suggests that the Main Fossiliferous Layer is a Maastrichtian time-averaged remanié deposit, which means that it originated from a Cretaceous deposit with little sedimentation and was gradually winnowed into the overlying deposits. A third hypothesis proposes a representation of a lag deposit of Cretaceous sediments that were forced out by a strong impact event such as a tsunami and was subsequently refilled with Cenozoic fossils.
caption: Mosasaurus went extinct as a result of the K-Pg extinction event; its last fossils were found at or close to the boundary, which is represented by the thick dark band separating the lighter and darker layers of this cliff.
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