FOSSILS

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).

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

Tertiary.

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). 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).

VARIATION

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).

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.

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.

The relative proportions of fin sizes can vary in groups of sharks.

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).

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).

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)

Fossil sharks of the Paleozoic had a variety of kinds of 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).

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).

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).

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).

 

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. 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.

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 sharks can inflate their stomachs to function in buoyancy or to

appear larger (Steel, p. 60).

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).

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)

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).

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.

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.

Some possessed a tooth whorl and others possessed teeth on

scissor-like jaws.

Some possessed a bony dorsal brush and others modified dorsal

spines that may have functioned in courtship (Perrinne, 1999).