|Ratfish - Chimera
Ischyodus cf. bifurcatus Case
|Description: The fossil remains of this ancestral ratfish are represented by its bony toothless jaws and jaw fragments, dorsal fin spines, and cephalic hooks.
The bony jaw plates are marked with a few rows of tabular, spongy looking channels called tritors. It is along these tritors where most of the chewing and crushing occurred. The lower jaw was comprised of two roughly trapezoidal shaped plates around 4.5 cm (about 1.75 in) long which were fused with cartilage in life. The vomerine plates did not posses tritors and are rarely recovered.
The elongated fin spines have very fine longitudinal striations and growth cracks on the anterior (leading) edge and are marked with two rows of curved projections not unlike a saw on the posterior (trailing) edge. This is unlike the spines from the hybodont shark, which display a single row of alternating curved projections. Also the surface is relatively unornamented while the dorsal fin spine of the hybodont shark is ornamented with small bumps, so little confusion in identification should occur.
Pelvic claspers occur but are very uncommon and were only present on the males. The hook part resembles a fishhook without the barb but unlike the hybodont, has a greater angle and is smaller in relation to the root. The root is divided into two main flattened lobes on either side of a third smaller central lobe. These may be broken off due to stream action. These hooks served as claspers to help hold the male to the female during mating. All modern sharks possessed sexual claspers on their pelvic fins. Except for the ratfish and the hybodont shark the sexual claspers where only stiffened cartilage and did not fossilize. The modern ratfish possess two sets of sexual claspers. One set is on the head above the eyes (the cephalic set) and the other set on the pelvic fins. It is not known how much different either set was to each other.
Commonality: Pieces of the bony jaw plate from the ratfish are somewhat common while the complete jaw is uncommon. Dorsal fin spine parts and complete pelvic claspers are uncommon.
Similar fossils: The lack of the ornamentation and the double row of projections on the trailing edge can distinguish the fin spine of the ratfish from the spines of the hybodont shark. The pelvic clasper resembles the pelvic clasper from the hybodont shark but they have some differences. Unlike the hooks of the hybodont shark, the pelvic claspers of the ratfish have an angled hook. The root lobes are also unequal in size.
Size: The ratfish probably only reached a length of 1 meter (about 3 feet).
Notes: This ratfish was widespread and fairly common in the Cretaceous but only occupies a small ecological niche today in deep waters. The term "fish" in ratfish in misleading. The chimeras are primitive cartilage fish and are thought to be the evolutionary link between sharks and bony fish.
Top and Bottom Jaws
|Dorsal Fin Spine|
Dorsal Fin Spine
a. Side veiw b. Rear view