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Fossils from Monmouth County, New Jersey

Ratfish (Ischyodus bifurcatus)

Age – Late Cretaceous; Commonality – uncommon, Size – crushing plates: ¼ – 3 ½ inches, claspers: 1-2 inches (slightly larger than Hybodont claspers), fin spines: ~3-6 inches (when complete)

The ratfish belonged to the subclass Holocephali of the class Chondrichthyes. They are cartilaginous fish that are sort of on the border between the sharks and fish. They possess three types of bony crushing plates. They possessed two mandibular crushing plates on the roof of their maxilla. They also possessed two vomerine plates at the front of the maxilla; these plates look similar to the incisors of a rodent. The mandible possessed two palatine plates. Each plate was mostly bone with spots on the crushing surface containing tritors. Most of the crushing occurred on the tritors. Tritors are rows of spongy-looking rows of calcified cartilage. The tritors are almost always fossilized with a yellow color. The mandibular and palatine plates are found much more frequently than the vomerines. The ratfish possesses a single fin spine which is located in front of the anterior dorsal fin. The spine contained venom and was used to ward off predators. The spine is flattened laterally and possesses fine striations and growth cracks. The trailing edge has a double row of denticles and is slightly hollow. The spines are slightly curved toward the posterior edge, although not as much as Hybodont spines. The claspers possess a hook and three root lobes. The root lobes aren’t as even in size (length) as the ones on Hybodont claspers. There is a more of an angle rather than a curve between the hook and the rest of the clasper, unlike Hybodont claspers which have a smooth curve. The apex of the hook is blunter than in Hybodont claspers. The ratfish claspers are also larger on average.

Palatine Mouth Plates

Occlusal View

Basal View

Oblique View

A relatively complete left palatine. There were two palatine mouth plates located on the maxilla.

Occlusal View

Basal View

Oblique View

A right palatine. This one has a bit more of the enameled ledge on the anterior of the mouth plate than the one above.

Occlusal View

Basal View

Oblique View

Some different colors on this left palatine.

Occlusal View

Basal View

Oblique View

A complete right palatine mouth plate. Note the presence of the four tritors and the enameled ledge. Palatine mouth plates are the easiest to find complete.

Occlusal View

Basal View

Oblique View

Ratfish (Ischyodus bifurcatus) Palatine Mouth Plate fossil, New Jersey

A significantly smaller right palatine.

Mandibular Mouth Plates

Occlusal View

Ratfish (Ischyodus bifurcatus) Mandibular Mouth Plate fossil, New Jersey

Basal View

Oblique View

A nearly complete right mandibular. This type of mouth plate was located on the mandible (two on either side).

Occlusal View

Basal View

Oblique View

A large left mandibular.

Vomerine Mouth Plates

Ratfish (Ischyodus bifurcatus) Vomerine Mouth Plate fossil, New Jersey

A nice complete left vomerine. Two of these formed a beak-like structure at the anterior of the maxilla. These are the hardest type of ratfish mouth plate to find in nice shape.

This one is a right vomerine.

Cephalic Clasper

Occlusal View

Basal View

Oblique View

Ratfish (Ischyodus bifurcatus) Cephalic Clasper fossil, New Jersey

Ratfish claspers are much rarer than the mouth plates and differ from the Hybodont ones with more elongated lateral root lobes and a very small central root lobe.

Fin Spine Fragment

Leading Edge

Trailing Edge

Ratfish (Ischyodus bifurcatus) Fin Spine fossil, New Jersey

Side View

Oblique View

Ratfish fin spine sections differ from Hybodont fin spine sections in that they possess a double row of tubercles on the trailing edge and never have striations on the sides.

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