Hexanchiformes – Frilled & cow sharks

Ancient sharks with extra gill slits and a primitive skeleton, roaming the depths of the ocean

This group of sharks represents a fascinating chapter in the evolutionary history of these ancient creatures. They are often considered the most primitive type of shark, showcasing features that offer a glimpse into their ancient origins.

One distinctive characteristic is the presence of a single dorsal fin, a trait harkening back to their ancient lineage. They possess six or seven-gill slits, which is another indicator of their primitive nature, as most modern sharks typically have five-gill slits. Notably, they lack a nictitating membrane, the protective third eyelid that covers the eye in many other shark species.

Their caudal fin, or tail fin, is equally noteworthy. It features a long and slender shape with a small lower lobe, a unique configuration that sets it apart from more modern shark species. Additionally, their vertebral column extends into the upper lobe of the caudal fin, further reflecting their ancient heritage.

In terms of their physical structure, these sharks have large mouths equipped with sharp teeth that are similar in both jaws. This dental symmetry is a characteristic feature consistent with their primitive lineage.

What distinguishes these sharks is their cartilaginous skeleton, which contrasts the strong and calcified skeletons of many modern shark species. This flexible cartilaginous framework enhances their maneuverability and agility in the water, allowing them to navigate their deep-sea habitats easily.