Do fossil sharks support the evolution, creation, or intelligent design model?
If evolution has occurred, then modern sharks have not always existed. The first sharks should possess primitive traits not found in modern sharks. Anatomical and genetic evidence should support a nested hierarchy relationship rather than distinct, unrelated kinds.
If the creationism model is correct, modern shark kinds have always existed. There is no expectation that fossil sharks would possess anatomical features which would identify them as being ancestral, transitional, or primitive in any way. Distinct, unrelated kinds of sharks should be evident (unless they represent one single kind).
If intelligent design has occurred, then the complex features of modern sharks should appear in an "irreducibly complex" fashion. It is not expected to find fossil sharks which have some, but not all, of the features of modern sharks.
There were at least 7 major groups of Paleozoic sharks, most of that became extinct during the Permian Period. The most primitive forms lacked calcified ribs. Some had spines in their fins and others had multiple dorsal fins. All modern sharks descended form one ancestral group during the second great shark radiation in the Jurassic. In these Mesozoic sharks, the vertebral centrum was calcified and the fin spikes were reduced. They had larger nasal capsules and probably a better sense of smell (Carroll, 1988).
After the extinction of the placoderms, a major group of
sharks diversified known as the hybodont sharks. They possessed multicusped
teeth in the front of their mouths and crushing teeth in the backs of
their mouths. They also possessed horn-like cephalic spines on the tops
of their heads. Hybodonts are thought to be related, or perhaps even ancestral,
to the first modern sharks or neoselachians which appeared at about 200
million years ago (Perrinne, 1999).
In the Early Mesozoic, one group of sharks (of which Paleospinax is a good early example) developed shorter jaws, a more ventrally positioned mouth, and a stronger vertebral column. As the mouth moved, the feeding strategy of sharks changed. Instead of swallowing prey whole in one bite, they more frequently used their teeth to cut and chop prey. Paleospinax is the earliest neoselachian with its calcified cartilage backbone (Perrinne, 1999; Moss, 1984).
An estimated 84% of neoselachian shark species became extinct
at the Cretaceous/Tertiary boundary, including 7 entire families. Skates,
rays, and the large sharks at the top of the food chain suffered the greatest
rate of extinction (Kriwet, 2004).Hybodont sharks, the dominant sharks
of the Paleozoic, had been declining throughout the Mesozoic and became
finally extinct at the K/T extinction (Kriwet, 2004).
Most modern groups of sharks are about 100 million years
old with the oldest dating about 180 million years. A few rare sharks
(such as the megamouth shark and the goblin shark) are known since the
Great white shark fossils are often associated with areas
which contain the fossils of primitive marine mammals (Stephens, 1989).