SHARK VARIATION

Does the variation that exists within lineages of sharks support evolution, creationism, or intelligent design?

EVOLUTIONARY MODEL

If evolution has occurred, then significant variations could evolve within specific lineages of sharks.

CREATIONISM MODEL

If the creationism model is correct, the major features of any kind are expected to appear since creation and not as variation within a kind which evolves over time. It is not expected that the degree of variation within a kind would exceed that which separates the kind from potential ancestors.

INTELLIGENT DESIGN

If intelligent design is correct, then it isn't expected that major reworking of an ancestral design would be required for survival.

Modern sharks are known from freshwater and saltwater, shallow water and extreme depths of almost 9,000 feet. While the smallest measure 6 inches in length, the largest can measure more than 50 feet. They vary in color, some are patterned, some are bioluminescent, and others can camouflage themselves by changing color to match their background. Aristotle discovered that female maiden sharks possess a hymen-like membrane. Many lay eggs (oviparity), some females retain their eggs inside their body until after hatching (ovovivparity), and others give live birth (vivparity) and may even nourish the young with a placenta. Some are flat and ray-like (as in angel sharks). Saw sharks and the unrelated sawfish (which are batoids, related to rays) possess a long snout bearing tooth-like denticles. Some possess sensitive sensory barbels hear the mouth while others possess worm-like tissue which attracts potential prey. Some are long and thin with very low fins (like the false cat shark). The goblin shark possesses a long snout. The basking shark strain water for plankton using elaborate gill rakers in gill slits so long that they almost encircle the body. Mako sharks can reach speeds of 20 mph. Some hammerheads possess a head whose width is half the length of the body (Allen, 1999).


An enormous degree of variation occurs within sharks, even within groups of closely related sharks. More than half of all shark species measure less than a meter although body length can vary from under 8 inches (in dwarf lanternsharks) to whale sharks which reach 39 feet (or perhaps as much as 59 ft), basking sharks which reach 32 feet (or perhaps as much as 49 feet), the extinct Carcharodon megalodon which is estimated to have reached 42 feet (and became extinct by 11,000 years ago) and great white sharks which can reach 23 feet (Farino, 1990).
shark
The size of gray sharks (genus Carcharhinus) varies from 3 to 12 feet (Gilbert, p. 85).
Different species of shark have adapted to coastal waters, open ocean, shallow water, depths of more than 11,000 feet, fresh water, salt water, tropical waters and polar regions. Some deep water sharks are bioluminescent (Farino, 1990).
Angel sharks resemble rays in the flattening of their bodies.
shark

An angel shark and a ray are depicted below, two independent modifications of shark design for a flat shape.


Closely related organisms often vary considerably in overall body shape. In the shark family Squalidae, there are significant variations in the shape and relative size of the head, the position of the mouth, the position and size of the dorsal and pectoral fins, and the overall body size and shape.

sharkssharks
The relative proportions of fin sizes can vary in groups of sharks.
shark
In the hammerhead sharks (family Sphyrnidae), the width of the head ranges from almost ½ the length of the body to well under 1/3 of the body length in most (Gilbert, p. 73).
hammerheads
Vertebrae numbers vary among sharks, even among members of the same genus. Carcharhinus galapensis individuals possess 103-109 precaudal vertebrae and 94-107 caudal vertebrae while C. obscurus possess 86-94 precaudal vertebrae and 87-101 caudal vertebrae (Gilbert, p. 86). The frilled shark is the most primitive living today and its notochord is only partially restricted by its vertebrae (Steel, p.74).
frill shark
Some sharks lack dorsal fins above the vertebral column. Sharks can vary as to whether or not there is a spine in dorsal fins, even within a family (as evident in the following illustration; Castro). Some sharks have ribs but the earliest sharks lack them (Carroll, p. 66)
fins

Fossil sharks of the Paleozoic had a variety of kinds of fins.
fins
There are clasper gaffs in two species of Squalus which closely related species lack (Gilbert, p. 54). In some sharks the coracoid bones are united in the midline, in others they remain separate (Gilbert, p. 56).

Shark teeth vary. Although the earliest shark-like scales are known from the Early Siluran, shark teeth are not known until later suggesting that the fish from which these scales originated may have lacked teeth or even jaws (Long, 1995). Fossils of denticles which are slightly larger than normal denticles are known-these may be remains of the early forms of teeth. A shark may produce tens of thousands of teeth in only a few years . The teeth of early sharks are often blunted, suggesting that they were not replaced as often as those of modern sharks (Stephens, 1989; Perrinne, 1999). The first teeth of cartilaginous fish are known from the Early Devonian and measure less than 4 mm(Long, 1995).
saw shark
Sawsharks and sawfish (a type of ray) both possess a saw-like protrusion from their rostrum (Farino, 1990).
Shark teeth can vary from slender forms adapted for slippery prey to crushing platforms for marine invertebrates. Species which feed on plankton may possess small, vestigial teeth. Some species, such as the sawsharks, possess elongated rostrums to stir sediments and dislodge prey. The elongated tail of thresher sharks is used to stun prey (Farino, 1990).
There are a number of variations in teeth in modern groups of organisms. Whale sharks may possess 15,000 small teeth (Allen, 1999). Frilled sharks may possess about 300 teeth, each of which forms from the fusion of 3 embryonic denticles. In pristiophorid sharks, denticles on the side of the face have been modified into teeth (Gilbert, p. 50). In the family Squalidae, the upper teeth may possess 1-7 cusps. Heterodontus has front teeth which seize prey and back teeth which crush crustaceans. The majority of the Paleozoic petalodontid sharks had a hypermineralized homolog of the component of modern shark teeth (Zangerl, 1993).
There was considerable variation in the teeth of fossil sharks.

teethteeth

By the Late Devonian, sharks had evolved a diversity of tooth types including some with 8 cusps (compared to the two cusps of the earliest known shark teeth) (Long, 1995). Most sharks possess a homodont set of teeth (all the teeth have the same shape), but a few sharks possess a heterodont dentition with sharp tearing teeth in front and crushing teeth located posteriorly. (Webster, 1974). Some denticles are much larger than the common ones which compose the integumentary surface. These can give the shark a rough appearance as in the bramble shark, spiny dogfish, and some rays. They can also form fin spines in various species, the stingers of stingrays, and the teeth of sawfish. In early shark embryos, developing teeth and denticles are essentially the same (Allen, 1999).

Cowsharks, catsharks, and horn sharks were among the first modern families to evolve. Horn sharks possess more molar-like teeth in the backs of their mouths (Perrinne, 1999). Mackerel sharks can maintain their body temperatures above that of the surrounding ocean. The upper tail lobe of the thresher shark can be half its body length (Stephens, 1989). The batoids (skates, rays, and their relatives) have diversified into a large number of species—greater than the number of shark species. Rays also vary. Manta rays can reach 22 feet in width and use their prominent cephalic fins to direct water towards their mouths. Two families of rays (the electric rays) can generate electric pulses (Allen, 1999). In all rays, the mouth is positioned ventrally except in the manta ray. In manta rays, the teeth are reduced or absent and gill rakers have been modified for filter feeding. Many rays possess flat teeth. While most skates and rays are extremely flattened, some are less so (such as guitarfish) and some are sharklike in their appearance (such as sawfish) (Moss, 1984).

While many sharks possess a brain weight:body weight ration which is consistent with those of bony fish, some possess ratios which fall into the bird and even mammal range (Moss, 1984). Great white sharks can maintain a body temperature higher than their surroundings (Long, 1995).

In chimeras, the upper jaw is fused to the braincase. One chimera possesses a long snout (Allen, 1999).