Primitive Sharks
As George Orwell might have said, all sharks are primitive, but some are more primitive than others. The sharks that bear the closest resemblance to the extinct species are known collectively as the primitive sharks, and they show certain characteristics that make them unmistakable, and totally different from the more "modern" species. There are about 250 species of sharks, and almost all of them have five gill slits. However, one family, the Hexanchidae, includes three genera of six- and seven-gill sharks; and another family, the Notorynchidae, is comprised of one species of seven-gill. (These are almost the only sharks with more than five gill slits. The only other is a species of sawshark, Pliotrema warreni, with six gill slits, found off the South African coast. It is differentiated from Pristiophorus, the "five-gill sawshark," by the number of gill slits.)
There are now thought to be two species of the genus Hexanchus: the bluntnose six-gill Hexanchus griseus, and the bigeye six-gill, Hexanchus vitulus. H. griseus is the larger of the two, reaching a length of fifteen feet and perhaps even more; H. vitulus rarely exceeds seven feet. They are both deep-water species. The common name of H. griseus is six-gill, but it is also called cow shark, mud shark, comb-toothed shark, and bulldog shark.
The seven-gills are generally similar to the six-gills, with one obvious difference. There are two distinct genera, Heptranchias and Notorynchus, which are primarily differentiated by the shape of the head.
Prominently mentioned in books on sea serpents, the frilled shark can easily qualify for this designation. It is certainly serpentlike in appearance; in fact, it looks more like an eel than a a shark. (Its specific name, anguineus, comes from the word for eel.) Where other sharks have plain gill slits, this creature has a collar of frills, and its mouth, instead of being underslung as the mouth of a shark is supposed to be, is terminal. Its teeth are three-pronged (trident-shaped). Surely this must be the archetypal sea serpent, and it would be -- if it ever grew longer than six feet.
The heterodontids, or hornsharks, are distinguished from all other modern sharks by the presence of two dorsal fins with spines in front of them and an anal fin. (Other species that have two spiny dorsals have no anal fin, e.g., the dogfishes and other squaloids). They are peculiar-looking creatures with oversized, knobby heads and bony arches over each eye. They are bottom dwellers and bottom feeders, with a mouth that is designed for feeding on shell fish. They exhibit the closest thing to parental care known in sharks: After the eggs are laid, the female takes them in her mouth and places them in rock crevices, presumably to protect them. They harden in these fissures, making it almost impossible to get them out.
Two other genera of sharks are included here, not necessarily because they are primitve, but because they do not fit anywhere else. The six-gilled sawshark Pliotrema warreni is mentioned above; the other genus of sawshark is the more widely distributed Pristiophorus, with five gill slits. (The sawsharks are distinguished from the sawfish, Pristis, by the presence of gill slits on the lateral surface. The sawfish, which is a ray, has its gill opening on the ventral surface.) The sawsharks are fairly small, reaching a maximum length of about four feet, including the rostrum, or extended "saw." All the sawsharks have a pair of long barbels located on the underside of the saw, another characteristic that separates them from the sawfishes.
The other family lumped here with the primitive sharks is the Squatinidae, the monk or angel sharks. These flat-bodied sharks are thought to be transitional between sharks and rays, since they are compressed bottom dwellers that, to the uninformed, look much like a ray or a skate. Like the sawsharks, however, the angel sharks' gill slits are partly lateral, while those of the rays or batoid fishes are always on the underside. In addition, the pectoral fins of the rays are fused to the side of the head, while those of the angel sharks (and of the sawsharks as well) are free at the anterior edges.
