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.


Significant differences in modes of reproduction occur within shark lineages. Some sharks lay eggs, some retain the eggs inside the body of the female, some develop placentas and give birth to live young. In some, the young are born alive without the aid of a placenta, they develop by eating yolk and other eggs for food. Variation can even exist within a genus: some species of the genus Galerus lay eggs while others give birth to their young live (Steel, p. 58-9). In some cartilaginous fish the yolk sac is an appendage to the embryonic body which does not compose any part of the adult (Mossman, p. 16).

embryos
Sharks are second only to mammals in the frequency of live birth (Lopez, 2006).
In the shark order, Carcharhiniformes, the more primitive families (such as Scyliorhinidae and Proscyllidae) reproduce by laying eggs or through live birth in which the fetus depend on the yolk sac for nutrition. The family Triakidae is thought to represent an intermediate stage in which species reproduce through live birth and some utilize placentas. The majority of higher carcharhinid groups utilize placentas and live birth. Within lineages which evolved placental nutrition, a few groups subsequently lost it, and, in at least one genus (Hypogaleus), it was secondarily regained (Lopez, 2006).

Some sharks can inflate their stomachs to function in buoyancy or to appear larger (Steel, p. 60).

shark

The number of gills can vary as well. Notoryhchus possesses 7 gills, a few sharks possess 6 gills (such as the frill shark), and most sharks possess 5 gills (Steel, p.46).

The anatomy of the face can vary considerably in other groups as well. Some sharks such as Heterodontus and Heptranchias have a supplemental jaw articulation between the palatoquadrate and the mandible (Gilbert, p. 9).

shark

Some sharks even developed bone tissue. The fossil shark Stethacanthus is interesting because it possessed a 'spine-brush complex' composed of acellular bone. It also possessed a type of calcified cartilage (globular) like that of placoderms and advanced jawless fish (Coates, from Ahlberg, 2001). Others modified dorsal spines that may have functioned in courtship (Perrinne, 1999).

brushes


Sharks and rays vary in the sizes and structures of their brains. Some, such as the squaloid sharks, have small brains while those of others, such as makos, grey sharks, hammerhead sharks, and great white sharks, have much larger brains. While many sharks have brain size: body size ratios which are similar to bony fish, other possess ratios observed in birds and even mammals (Steel, p. 62). Among sharks and rays, this ratio (the encephalization quotient) can range from .25 to 2.77 (Hodgson, p. 125). If the ratio of the telencephalon size to body size is determined, some rays have a value 9x that of Squalus and hammerhead sharks have a value 5x that of Squalus. Other brain areas also have been observed to vary in size including a 9 fold variation in the cerebellum, an 8 fold difference in the size of the midbrain, a 5 fold difference in the size of the diencephalon, a 7 fold difference in the olfactory bulbs, and a 4 fold difference in the medulla (Hodgson, p. 146). The variability observed among the rays is about twice that observed among the sharks (Hodgson, p. 146). Some elasmobranch brains possess nuclei that are not present in others. Many of them develop a prominent central nucleus in the telencephalon. (Hodgson, p. 153, 185).

brains

In general, the squalimorph sharks have a smooth cerebellum, an exposed optic tectum, few migrated nuclei in the diencephalon, and a poorly developed telencephalon, while galeomorph sharks in general, have the opposite condition. In rays, the telencephalon is modified to the point that the lateral ventricles are reduced to vestiges. Some rays have smooth brains, some have slightly convoluted brains, in others the large, convoluted brains have considerable asymmetry.

brains
In some sharks, embryonic neuromasts, which usually form the lateral line system, remain free and form pit organs under modified scales (Gilbert, p. 151). The eyelids are mobile in some species and immobile in others; some sharks have a 3rd eyelid similar to the nictitating membrane (Hodgson, p. 12-3). Some species of shark have a superchoroidea over the choroid composed of connective tissue, blood vessels, and lymphatic vessels (Hodgson, p. 22). The iris is immobile in some species and can move rapidly in others. The tapetum is fixed in some and can be darkened in others (Hodgson, p. 89).

Fossil sharks have varied significantly. Following the extinction of the placoderms, sharks diversified in the Carboniferous. Some reached estimated sizes of 6-7 meters with meter-wide mouths. One group (which includes Stenacanthus) evolved a bony spine on their backs. Others evolved odd whorls of teeth (such as Heliocoprion) which may have helped them catch fish while swimming through schools of fish (Long, 1995).
The diversity of Paleozoic sharks included some 4 to 6 inch forms whose long pectoral fins located near the top of the head might have permitted the same type of aerial gliding.

odd fish
Some possessed a tooth whorl and others possessed teeth on scissor-like jaws.

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