The Cretaceous Sharp Nose

 During the Cretaceous, when the crushing Ptychodus sharks were breaking open shells, sharks were often not the greatest predators in the neighborhood. They competed with plesiosaurs, tylosaurs, and even Xiphactinus, a very large fish with impressive dental hardware. Despite being outclassed in many of the world's oceans, sharks were still capable of becoming apex predators and competing directly with their fish and marine reptile neighbors. One of the sharks most capable of direct competition in the Cretaceous seas and oceans was the rather large (up to 8 m, 26 ft, and 3400 kg, 3.3 tons) mackerel shark, Cretoxyrhina. Mackerel sharks are sharks in the Order Lamniformes and are fast, powerful, ocean swimming sharks. The most notable living member of this group is the Great White Shark, which grows to approximately the same size as Cretoxyrhina is thought to have grown to. The genus Cretoxyrhina consists of four recognized species with C. mantelli being the first described; initially teeth of the shark were described in 1822 by Gideon Mantell as a modern shark's teeth and a second formally published description, honoring Mantell, was written in 1835 by Louis Agassiz. These teeth were all collected from England, but a number of exceptionally well-preserved vertebral columns (and some associated other elements including teeth) are known from the work of George F. Sternberg, and his father Charles H. Sternberg, that was conducted in the state of Kansas in the mid-western United States of America. 

 Charles Sternberg found a skeleton and 250 teeth in 1890 that were sold to a German museum (it was destroyed during WWII), but George Sternberg found many skeletons of Cretoxyrhina. The first (1891) contained portions of the jaw as well as a last meal of Xiphactinus and 150 teeth. In 1950 and 1965 he found even more of the shark, including specimens with preserved gills, portions of the skull, complete jaws, and pectoral fins. These are exceptional discoveries because, as many of us may know, sharks are cartilaginous fish, and much of their skeleton is difficult to preserve. Teeth are typical remains that we possess for sharks because of their durability, continuous replacement (allowing for many more teeth to be potentially preserved compared to whole individuals), and the number each animal possesses; remember that Charles Sternberg found 250 teeth in his 1890 fossil.

These large sharks, and their teeth, are associated with numerous other fossils, indicating what kinds of animals they fed on regularly. These include fish like, and including, Xiphactinus, plesiosaurs, mosasaurs, turtles, other sharks, and even pterosaurs and dinosaurs. Pterosaurs are thought to have sometimes fished over ocean waters, and it is thought, as shown in the Mark P. Witton illustration below, taht the sharks may have actively hunted low flying pterosaurs such as Pteranodon. Dinosaurs are often considered a scavanged meal in relation to Cretoxyrhina, showing that the shark was an opportunistic hunter as well. One hypothesis, involving a Claosaurus (shown below also), is that dinosaurs sometimes died on the shore, during a flooding event, or in some other way that enabled their bodies to float out into the ocean where they were eaten by sea creatures of all sorts. Evidence for Cretoxyrhina feeding on dinosaurs includes not only the hadrosaur Claosaurus, but also the nodosaurian Niobrarasaurus. The evidence for larger prey (fish have been found in the stomach areas of Cretoxyrhina) such as plesiosaurs, pterosaurs, and the dinosaur mentioned is mostly based on Cretoxyrhina teeth embedded in bones, where they were broken off during biting and feeding. Finding shark teeth in bone is very interesting. A plesiosaur vertebral column I worked with had shark teeth embedded. I did not diagnose the teeth, but the animal was from a time and area populated by Cretoxyrhina, so it is a likely culprit to include in the list of suspects.