FOSSIL SHARKS

Do fossil sharks support the evolution, creation, or intelligent design model?

EVOLUTIONARY 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.

CREATIONISM MODEL

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).

INTELLIGENT DESIGN

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.


Sharks have not always existed and modern groups of sharks evolved late in the history of sharks. The fossil record does not indicate that all groups of sharks appeared suddenly nor that extinct forms all died in the same cataclysmic event. Instead, the fossil record indicates that diverse lineages of sharks have appeared and disappeared at different points throughout their long history which has lasted more than 400 million years.


In strata dated at about 450 million years ago, there are the remains of tiny scales which are so similar to modern shark scales that they are attributed to primitive sharks. The first shark teeth are not known until strata which are 40 to 70 million years younger. More than 30 shark species existed by the Late Devonian. The first sharks may have originated around the southern supercontinent of Gondwana (Long, 1995).


The first remains of a shark body (other than scales and teeth) date to 380 million years ago in Antarctica, Antactilamna. Around 370 million years ago several groups of sharks existed including ctenacanths (which may have been ancestral to modern sharks and the dominant Mesozoic sharks), cladodonts (which possessed multi-cusped teeth and included Cladoselache), and, shortly afterwards, the freshwater xenacanthid sharks (Perrinne, 1999). Xenacanth sharks were a dominant of primarily freshwater sharks for more than 200 million years (Stephens, 1989).

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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).

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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).
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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 Tertiary.
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A number of Mesozoic sharks reached lengths of 5-6 meters. Mako sharks from the Miocene could reach lengths of 8 meters and gave rise to modern great white sharks. The fossil shark Carcharocles megalodon lived 23 million years ago could reach lengths of 15 meters with teeth 18 cm long (twice the size of the largest great white shark ever caught). These giant sharks appeared around the time that large filter-feeding whales evolved which may have been their prey (Long, 1995).

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Great white shark fossils are often associated with areas which contain the fossils of primitive marine mammals (Stephens, 1989).
One modern group of cartilaginous fish includes chimeras, elephant sharks, and rabbitfish. This group diverged from shark lineages in the Devonian. The earliest fossil rays are known from the Jurassic (Long, 1995).

NESTED HIERARCHY
Anatomical and genetic studies indicate a nested hierarchy of relationships between the diverse lineages of sharks. All sharks form a monophyletic clade and within sharks, there are a number of smaller sharks which have been identified (Carroll, 1988). Individual studies have demonstrated a nested hierarchy of relationships within groups of sharks.
For example, the Order Carcharhiniformes is the largest of the modern orders of sharks and includes 228 species, more than half of modern sharks. Shared anatomical features and molecular comparisons indicate that the order is monophyletic clade. Within this order Scyliorhinidae is the largest family, including about 25% of modern sharks. Molecular studies have indicated that the family is paraphyletic since other families of order arose within this family (Iglesias, 2005).