Why the cow shark is considered the most primitive shark swimming today

Nature · Published 3 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: With skeletons resembling ancient, extinct forms and unspecialized internal systems, cow sharks are living relics. Unlike the standard five-gilled shark, these predators possess six or seven gill slits, a trait that may link them to survivors of the Permian-Triassic extinction. They remain one of the ocean's most mysterious, enduring lineages.

The Hexanchidae family, commonly known as cow sharks, represents a unique evolutionary branch. While most modern sharks are defined by five gill slits, cow sharks are distinguished by an additional pair or, in some genera, two extra pairs. This anatomical anomaly is paired with a skeletal structure that mirrors ancient, extinct forms, lacking the specialized adaptations found in more modern shark species. Their digestive and excretory systems are similarly unspecialized, reinforcing the theory that they closely resemble the ancestors of all sharks.

The history of the cow shark is etched in a sparse fossil record, primarily consisting of isolated teeth. While skeletal remains have been discovered dating back to the Jurassic period, these finds are exceptionally rare, limited to specific sites like the lithographic limestones of South Germany and calcareous sediments in Lebanon. Some researchers suggest that because these records are so thin, the family may actually be more diverse and numerous than currently documented. A potential hexanchid tooth found in Permian-era Japan even hints that the family could be a direct survivor of the Permian–Triassic extinction event.

Today, only five species of cow shark remain extant, including the bluntnose sixgill shark, which can grow up to 7.9 meters, and the broadnose sevengill shark. These cylindrical predators are ovoviviparous, meaning mothers retain egg cases within their bodies until they hatch. Their diet is opportunistic, ranging from crustaceans and carrion to large fish and even other sharks. Despite their ancient lineage, they continue to thrive in tropical and temperate waters across the globe, serving as a living connection to the deep past of marine evolution.

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