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SPHYRNA (Rafinesque, 1810)

The Hammerhead Sharks


Shark Info Diving With Hammerheads Tooth Morphology Fossil Examples

Information

There are 3 species of hammerhead found along the Atlantic Coastal Plain, S. lewini (scalloped hammerhead), S. cf. S. media (scoophead), and S. zygaena (smooth hammerhead) (Purdy et al., p.77). The teeth of all three species look very similar, however S. zygaena teeth are generally more robust than the other species. One may have heard or read about a species called S. laevissimus. Purdy et al. states that S. zygaena is synonymous with S. laevissimus (Purdy et al., p.158).

All three of these species hammerhead are extant, meaning they are still with us today. The number of hammerhead species alive today seem to vary from publication to publication. The most recent publication used says, “There are two genera and at least eight species” (Hamlett, p.28).

Hammerheads are probably the easiest sharks to identify due to their scalloped-shaped extensions of their heads. These extensions are sometimes called cephalofoil (Michael, p. 68). The shape of the cephalofoil changes slightly from juveniles to adults. However, in adults, the shape of the cephalofoil is consistent and varies from species to species. This makes identification of adults among species somewhat easy. Probably one of the most common questions about hammerheads is “Why do hammerheads have a scalloped-shaped head?” No one is exactly sure. However, there are some unique advantages to having this peculiar head structure. The scalloped-shaped head spreads out the shark’s senses, including its sight, smell, and electric field receptors. This enhances the shark’s senses. The head may also aid in maneuverability, acting as a bow plane in the water (Michael, p. 68). Finally, hammerheads have been known to use their heads to bat and pin stingrays to the ocean bottom for easier feeding (Michael, p. 68). Another distinctive feature of hammerheads is their large and tall dorsal fins, and elongated upper portion of the caudal (tail) fin. The scalloped head along with the unmistakable dorsal fin and tail make these sharks unmistakable in identification.

Probably, the most well known hammerhead is S. mokarran, the Great hammerhead. The Great hammerhead has a nearly straight cephalofoil with an indentation at the center anterior margin. Specimen sizes of around 14 feet (425 cm) are common, and they may reach lengths of up to 20 feet (610 cm) (Castro, p. 155). These sharks are found world wide in tropical to semi-tropical waters, including the Atlantic from the coast of North Carolina to Florida and the Gulf of Mexico (Castro, p. 155). The Great hammerhead preys heavily on rays, with other sharks, bony fishes, squid, and crustaceans in its diet (Michael, p. 69). Based on this varied diet, it is not surprising that some attacks on humans have been attributed to Great hammerheads. However, they are usually not aggressive toward divers (Michael, p. 69). This shark has little commercial value, and is not directly targeted by commercial fisheries. However, it is often caught on fisheries’ longlines and drift nets. Some countries use the shark for its fins. It is also classified as a game fish and loved by sports fisherman. The International Union for Conservation of Nature and Natural Resources (IUCN) has the Great hammerhead listed as data deficient. This means there is not enough data on the sharks’ populations to make a conclusion about the status of this shark.

Images - Diving with Hammerheads in the Galapagos

The Galapagos islands are famously known for Darwin and the amazing endemic land fauna. However, because of severe fishing restrictions around these remote islands, the waters around the Galapagos are completely undisturbed. These waters have become one of the worlds premier dive sites to see large pelagic fauna, specifically sharks and rays.

I was fortunate enough to dive in the Galapagos Islands and experience the fauna first hand.

Below are some images of ~10 foot Scalloped Hammerhead sharks I routinely dove with. I borrowed a Dive buddys camera for a few dives and snapped these shots.

Click on an image to see it full sized.

This is the 1st hammerhead I encountered. As soon as I dove in this large Scalloped Hammerhead began inquisitively circling.
This Scalloped hamerhead is a female. Females often have scars behind their gills and on their fins from males attempting to mate.

The hammerhead is very easy to identify. Besides the obvious scalloped shaped head, they have a very tall dorsal fin, and the upper portion of the caudal (tail) fin is elongated.
To take this shot, I crouched behind a rock, waiting for it to swim by. As soon as it was almost over me, I jumped out and snapped the shot. Floss anyone?

Here is a close-up of the teeth. Pretty cool.
Scalloped hammerheads are unique in that females tend to school together during the daytime. In the Galapagos, schools of hundreds of hammerheads can readily be seen.

Notice the large dorsal fin on hammerheads.

Teeth

Sphyrna teeth have smooth cutting edges, and have a distinct notch on the distal side separating the crown and enameloid shoulder. This is often referred to as a “hammerhead notch” (Cocke, p.76). Their distal enameloid shoulder is another distinguishing characteristic. This shoulder is convex and often not serrated. Larger Sphyrna teeth can have week serrations on this shoulder. Hammerhead teeth also have a deep nutrient groove. All of these characteristics can be seen in the image below which shows a lingual and labial view of a hammerhead tooth. As for size, their teeth usually range from ¼” to around ¾” (6 mm to 19mm).

This image shows a Sphyrna zygaena tooth. It labels some common tooth terminology associated with hammerhead teeth. The tooth is 1.9 cm (3/4”) in width.

This image shows a comparison between a worn Sphyrna, a worn lower Carcharhinus, and a Rhizoprionodon tooth.

Hammerhead teeth can be confused with gray shark teeth, especially worn gray shark teeth. They can also be confused with a smaller genus of shark, Rhizoprionodon sp., the sharpnose sharks. The image below shows a comparison between a worn hammerhead (Sphyrna sp.) tooth, a worn lower Carcharhinus sp. tooth, and a Rhizoprionodon sp tooth.

The Fossils

Sphyrna zygaena aka laevissimus
(Hammerhead Shark)
The Common Hammerhead (Sphyrna zygaena) still populates the Chesapeake bay today, and is one of the largest sharks in the bay.
The two leftmost teeth are labial views, all others are lingual views.

These are much less common than grey shark teeth, and, if you're not careful, they can easily be confused with them.
However, notice the deep notch in the roots, also there are no serrations present on the teeth.

Formation:
  • Calvert
    Age:
  • Early - Middle Miocene ~ 18-15 m.y.
    Location:
  • Along the Calvert Cliffs, Calvert Co., MD
    Size:
  • usually between 1/2-3/4" (13-19mm)


    • Sphyrna sp.(Rafinesque, 1810)
    Hammerhead Shark
    There are three species of Hammerhead found at Aurora, S. lewini (Scalloped Hammerhead) , S. cf. S. media (Scoophead), and S. zygaena (Smooth Hammerhead).

    Formation:
  • ?Pungo River
    Age:
  • ?~18-22 m.y.
    Location:
  • PCS Mine, Aurora, NC
    Size:
  • ~1/2" slant height (12mm)
    Date:
  • March, 2004 Trip


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