The Sharktooth Hill Bone Bed is the richest and most extensive marine deposit of bones in the world, averaging roughly 200 bones per square yard. Twelve to fifteen million years ago during the time period geologists call the Miocene Epoch most of Kern County was an ocean bay. The waters lapped against rolling hills that were soon to be pushed up to form the Sierra Nevada Mountains. Northeast of Bakersfield, where the modern Kern River leaves the Sierra Nevadas, a river flowed into the bay.
The river carried sediments and the remains of plants and animals into the bay. These materials, along with the plentiful remains of marine organisms, sank to the bottom and much of the organic remains was fossilized. Subsequent geologic events pushed up the sediments, and they then eroded to form the rolling hills that include Sharktooth Hill. Exposed in these hills is the bone bed that formed from those fossil-rich sediments. The Sharktooth Hill bone bed encompasses more than 110 square miles, but most of it is deep underground. Only east of the Bakersfield area is it exposed.
Isurus is a genus of mackerel sharks in the family Lamnidae, commonly known as the mako sharks. Several extinct species are known from fossils found in sediments from Cretaceous to Quaternary (age range: 99.7 to 0.781 million years ago). The family Lamnidae also includes the great white shark and the porbeagle. Mako sharks are capable of swimming at speeds up to 60 km/h (37 mph), and jumping up to 7 m (23 ft) into the air. The great white shark is also closely related to an ancient mako shark, Isurus hastalis. However, fossil evidence suggests I. hastalis belonged to the genus Carcharodon. Unique among members of the Lamniformes order, Mako sharks occasionally exhibit homosexual behavior outside of their usual mating season.