What Are Dermal Denticles?
Chondrichthyans (Class of Cartilage Fish) including sharks, rays and skates, interestingly enough, have tooth-like scales called dermal denticles embedded in the skin. But in older texts, the term, placoid scales, literally meaning (plate-like) is customarily used. Today, most scientists prefer the more descriptive expression, dermal denticles, meaning (tiny skin teeth).
Denticles are similar to scales, but are actually just modified teeth with hard enamel coverings. These structures are often packed tightly together and grow with their tips facing to the rear of the fish. If you were to run your fingers from tail to head, it would feel very rough, but from head to tail, it would feel smooth. Dermal denticles provide protection for the fish, and in most cases, less resistance in the water.
Dermal denticles enfolding sharks are tiny and closely packed together resembling diamond shapes. My fossil is a large dermal denticle and is from a large ray fish such as a Roughtail Stingray which can grow 7 feet (2.1 m) across from wing tip to wing tip and 14 feet (4.2 m) long, including the tail. It can weigh as much as 660 lbs.
Dermal denticles on ray fish are typically located along the mid-body lines and tails, plus around the eyes or on the wing tips, but also can be placed more sporadically. Shape and size of dermal denticles can vary greatly. Some may be quite thick, and some may be quite bumpy with furrowed edges, while others compose tiny sharp claw-like spines etc.
It is debated in the scientific community whether oral teeth evolved from dermal denticles that migrated into the mouth or the other way around. The earliest vertebrates, such as placoderm fish, possessed boney blades rather than teeth.
Special Credit: The thefossilforum.com is a great site to help identify and learn about fossils. The above photo was provided by seasoned member “digit”.
Difficulty In Identifying
Dermal denticles are difficult to pin a specific ray family/genera/specie unless they are found in association with other identifiable ray fossils such as teeth or dental plates. Also, dermal denticles can vary widely on a specific individual ray depending on the location over the body. And, there are a lot of similarities between the dermal denticles of various extant ray fish which also make it very difficult to determine a specific genera/specie, let alone extinct species.
Possible Match with my Fossil
Below, I listed a few extinct ray fish possibly a match with my fossil shown at the beginning of this article according to age and location. These extinct species range in age from the Oligocene Epoch beginning 33 mya through the Miocene beginning 23 mya to the Pliocene Epoch beginning 5.3 mya. Two localities from where these extinct species fossils have been found include the Chandler Bridge Formation, Dorchester County, Southern Carolina and/or the Pungo River Formation, Beaufort County, North Carolina. Here are some possibilities:
- Mobula, pectinata or Mobula loupianensis “Giant Devil Rays” also called “Flying Rays”
- Paramobula, fragilis “Eagle Ray”
- Dasyastis cavernosa or Dasyastis rugos “Stingrays”
- Plinthicus stenodon “Eagle Ray”
- Raja dux “Giant Skate”
Additionally, stingrays first appeared in the fossil record around 60 million years ago during the Paleocene Epoch and the large roughtail stingray, Dasyatis centroura, mentioned above, also occurred in the range where my fossil was unearthed making it another possibility.
Ray Fish Interesting Facts
Ray fish belong to the superorder Batoidea with about 600 species and 26 families. Rays are the largest group of cartilage fishes. They are distinguished by their flattened disc-like bodies, very large pectoral, wing-like fins that are fused to the head, and by how the mouth and gill slits are situated on the fishes’ underside. A few extant and extinct families include electric rays, sting rays, skates, sawfish, spotted eagle ray, fiddler rays, or manta rays.
Rather than breathing through the mouth, ray fish breathe by taking water in through openings near the eyes called spiracles and passing it outward through the gills. More Facts below . . .
Ray Fish Interesting Facts Cont…
The ray’s tail is generally long and slender and many species bear one or more sharp, saw-edged, venomous spines that can be used to inflict painful wounds. But, rays are docile creatures and very rarely have caused human death. If threatened, their base reaction is to quickly swim away. However, a defensive reaction, such as being stepped on or a predator attack, will cause them to whip up their stingers.
Rays are predominantly marine and are found in all oceans. Many are slow-moving bottom dwellers. The well-known manta rays feed on plankton and small animals; others feed on various fishes and invertebrates. Other than skates, all rays bear living young.
Large rays live about 70 years, some live for more than 100 years.