Hypural Bone Description, Body Location and Function

The hypural is a fan-shaped series of bones constituting an important part of the framework of ray-finned fish. The hypural bones are sometimes fused into one or two plate-like bones, such as with the case from my fossil collection, shown above. The hypural boney-plates join the last few vertebrae of the fish’s spine to the slender bony rays of the fish’s caudal fin. Caudal fin is a fancy scientific term for the tailfin. The caudal fin is attached to the end of the fish’s caudal peduncle by the hypural bone. The caudal peduncle is the narrow part of the fish’s body. The tailfin helps the fish steer, balance and propel. (See illustration below)

My hypural fossil was unburied in the Pungo River Formation, Aurora District, North Carolina, USA, dated from the early to middle Miocene Epoch around 23 to 14 million-years-ago. It is a large sample. I found a close match to the fossil of a tuna fish, Thynnus sp. posted by the Fossil Guy from the same location.

Tuna Facts

Tuna are a type of ray-finned bony fish belonging to the class or subclass of Actinopterygii. Ray-finned bony fish comprise over 50% of all living vertebrate species. The ray-finned fishes are so-called because their fins compromise webs of skin supported by slender bony spines (rays), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish).

Tunas are teleost fish identified by their symmetrical forked tails with the upper and lower halves both exhibiting identical size.

Check out the list of a few common east Atlantic USA tuna species beginning with the largest and possible matches to the hypural fossil. You can check out the links from NOAA, National Oceanic and Atmospheric Administration, for information about their range, life cycle, population status, appearance etc.

• Western Atlantic Bluefin reaching up to13 feet (4 meters) long, weighing 2000 lbs (900 kg)
• Atlantic Big Eye reaching up to 8 feet (2.5 meters) long, weighing 400 lbs (181 kg) or more
• Atlantic Yellowfin reaching up to 7 feet (2 meters) long, weighing 300 lbs (181 kg) or more
• North Atlantic Albacore reaching up to 4 feet (1.2 meters) long, weighing 88 lbs (40 kg) or more
• Atlantic Skipjack reaching up to 3 feet (.9 meteres) long, weighing 40 lbs (18 kg) or more

Tuna Interesting Facts

In addition to the caudal fin, these high performance tuna fishes are equipped with finlets and keels. The finlets are small fins along the trunk that help the fish cut through the water. The keels can be described as a pair of lateral structures that rise slightly forward along the caudal peduncle which are a remarkable specialization in tunas, which, by the way, have also arisen in other fast-swimming marine animals.

Tuna are formidable predators with the ability to outmaneuver, outswim, and eat just about anything they can fit into their mouths.

Tuna can not only swim fast, but can reach remarkable distances as they migrate.

Origins of the Bluefin Tuna and Evolution Development

The bluefin tuna originated from an exothermic ancestor. Exothermic means acquired heat source from the environment to stay warm. Earliest tuna fossils have been found in the Late Paleocene (65-55 mya) and Early Eocene (55-50 mya) epochs in the Tethys Sea deposits from the Middle East, southern Europe and the London clay formation. A close relative of the tuna, the earliest bonito fish, identified as Sarda palaeocenica were found in the region from the Early Paleocene. The extinct tuna-like fish, Paleothunnus parvidentatus, displayed characteristics common to both the tuna and the bonito, suggesting that the split of the tuna and bonito did not occur until the Early Eocene Epoch. Within 8-10 million years the two species diverged, forming the tuna genus Thunnus (Dickson and Graham).

Development of Endothermy in Tuna

As the oceans began to cool, warm waters began to compress into the tropics. In the Cenozoic Period, waters were the warmest at the end of Paleocene Epoch. The Tethys Seaway, a large tropical sea, began to shrink due to tectonic events and contributed to ocean cooling across the planet. Endothermy (body mechanisms other than shivering that generate heat internally) possibly evolved in tunas as a result of the need for migration and diving into colder waters for plentiful hunting grounds.

Because of the evolution of endothermy (internal heat sources) within the bluefin and other tuna, the fish are able to migrate across large distances. Dickson and Graham state that this has enabled the Atlantic Northern Bluefin Tuna to greatly expand its range and take advantage of the rich feeding areas in northern waters and warm spawning areas in the tropics, effectively expanding its thermal niche.

About Pungo River and Yorktown Formations

The Lee Creek Mine in Aurora, NC, is an open-pit phosphate mine operated by the Potash Corporation. The mine exposes two fossiliferous geological marine layers: the Pungo River Limestone (middle Miocene, Langhian age), and the Yorktown (lower Pliocene, Zanclean age) formations. The Pungo River Limestone and base of the Yorktown Formation are richly phosphatic and are commercially mined. These two units have yielded one of the most important assemblages of Neogene (between 23 and 2.6 mya) marine vertebrates in the world, including hundreds of species of sharks, rays, skates, bony fish, birds, sea turtles, estuarine crocodiles, seals, walruses, dolphins, and baleen whales. These fossils are found on-site as well as in reworked sediments in the spoils leftover from the mining. Unfortunately, the mine has been closed to the public since 2009.

Partial list of other Miocene bony fish fossils discovered in and around the U.S. Eastern Atlantic Coastline of South Carolina and North Carolina, as well as Maryland and Virginia.

• Burrfish, Filefish, Tilefish, Pufferfish
• Marlin, Sailfish, Boxfish, Sturgeon
• Anglerfish, Bonita, Tuna, Bowfin
• Catfish, Hake, Toadfish, Needlefish
• Sea Robin, Bass, Bluefish, Barracuda
• Boxfish, Goosefish, Toadfish, Pinfish