Cretoxyrhina

Cretoxyrhina (/ˌkrˈɪtɔːksiːrhaɪnə/; meaning 'Cretaceous sharp-nose') is an extinct genus of large mackerel shark that lived about 107 to 70 million years ago during the late Albian to early Maastrichtian of the Late Cretaceous period. The type species, C. mantelli, is more commonly referred to as the Ginsu shark, first popularized in reference to its theoretical feeding methods being comparable to that of the rapid slicing and dicing when using a Ginsu knife. Cretoxyrhina is traditionally classified as the likely sole member of the family Cretoxyrhinidae but other taxonomic placements have been proposed, such as within the Alopiidae and Lamnidae.

Measuring up to 8 meters (26 ft) in length and weighting up to 3,400 kilograms (3.3 long tons; 3.7 short tons), Cretoxyrhina was one of the largest sharks of its time. Thanks to the discoveries of many exceptionally well-preserved skeletons of Cretoxyrhina, it is the anatomically best understood extinct shark to date. These fossilssuggest that it may have had a similar appearance and build to the modern great white shark. The possession of the most extreme hydrodynamic features suggests that Cretoxyrhina may have been capable of burst speeds up to 70 kilometers per hour (43 mph), making it one the fastest swimming sharks. Its teeth were razor-like and had thick enamel, built for stabbing and slicing prey with powerfully kinetic jaws. Cretoxyrhina had very large eyes that took up as much as one-third of the skull, and thus acute vision, suggesting that it used mainly eyesight to hunt prey. In a 2008 study, the lifespan of Cretoxyrhina was estimated to be 38 years, though a 2013 analysis reduced that to 21 years. Both studies found that Cretoxyrhina grew extremely rapidly during the first few years of its life, doubling in size by three years of age and reaching sexual maturity at around four–five years of age.

Cretoxyrhina has been found worldwide, being most frequent in the Western Interior Seaway area of North America. It was an apex predator in its ecosystem, preying on a variety of marine animals including mosasaurs and plesiosaurs, large fishes including other sharks, and occasionally dinosaurs. Nevertheless, it may have faced competition from other contemporaneous predators such as pliosaurs, giant mosasaurs like Tylosaurus, and large sharks like Cardabiodon. Cretoxyrhina may have possessed the earliest forms of regional endothermy in mackerel sharks, allowing it to live in waters as cold as 5 °C (41 °F).