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Descent with modification-All chondrichthyes have cartilage skeleton and denticle skin. This is clearly genetically fit for their environment. Their common ancestors were able to survive with these traits and were able to reproduce offspring with these characteristics so today they are able to, too.
Type of Natural Selection-Chondrichthyes are an example of stabilizing selection, their population is stabilized on a particular non-extreme trait value. They have many similar traits in the different species. There has been points of directional selection when rays and skates were formed, but it is still stabilizing because sharks, rays, and skates are all still living and evolved/evolving.

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Reproductive Isolation: Rays and Skates have been reproductively isolated. Although they are similar, rays usually mate in the winter and skates usually mate in May and October. This means that rays and skates can not breed together which is why they are considered different species.
Immigration and Emigration: Migration can cause genetic shifts in species. If the species is bigger, the impact of migration is smaller. Migration can cause the gene pool to change and therefore the species will have different traits that are not necessarily fit for the environment.
Non-random and Random Mating: Chondrichthyes do not mate randomly, they do not mate with all organisms. They are non-random mating organisms...they have certain characteristics that they look for, which is mostly their same kind, and only mate with those organisms(sharks with sharks, rays with rays, skates with skates).

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The Chondrichthyes represent gradual equilibrium because of how the early chondrichthyes evolved and what the species of Chondrichthyes looks like now, it shows evolution through the class over time is from time to time. Evolution of the chondrichthyes created new species gradually, which represents gradual equilibrium and not punctuated equilibrium.

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Common Name | Mako Shark  | Manta Ray | Little Skate
Kingdom | Animalia | Animalia | Animalia
Phylum | Vertebrata | Vertebrata | Vertebrata
Class | Chondrichthyes | Chondrichthyes | Chondrichthyes
Order | Lamniformes | Stingrays | Rajiformes
Family | Lamnidae | Eagle ray | Skate
Genus | Isurus | Manta  | Leucoraja
Species | oxyrinchus | Birostris | Erinacea
Taxonomy allows us to see how organisms are similar by seeing which groups they are classified as. The more groups the organisms are all in together, the more related they are. Kingdom Animalia Characteristics-General characteristics of the Kingdom Animalia are: They are eukaryotic, multicellular and heterotrophic organisms, they have multiple cells with mitochondria, and they depend on other organisms for food.  You can use taxonomy to determine this because you can tell that they split a long time ago and are very diverse by seeing what level the group starts to differentiate. Sharks, Skates, and Rays are in the same group up until the order because all three do not have bones they only have really hard cartilage. They have skeletons made out of cartilage. Aside from the obvious differences in appearance between sharks and rays, they also use different methods to breathe, and different ways of moving through the water. Rays and Skates evolved from Sharks.

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Manta Ray’s and Mako Shark’s have 13 protein coding genes. Manta Ray's do not have a venomous tail so it shows more similarity between Manta Ray's and Sharks than Sting Rays and Sharks. Their 13 protein coding genes are very similar if Manta Ray's do not have a protein for the venom. Manta Ray's are the type of Ray that is most similar to Sharks. Although they both share the same protein that makes up their cartilage skeleton and denticle skin.

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Little Skates have 3.42 billion base pairs and Mako Sharks have 8 billion base pairs. Bands of DNA that are taken from the same segments of chromosomes from the Mako shark and little skate, all share many similar DNA parts that are the same size but the whole strand is not the same size.

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Humans have 23 pairs of chromosomes, 46 in all. The Manta Ray has 32 sets of chromosomes, 16 from each parent, but females have 66 chromosomes in total because of the XX while the males only have 65 in total because they only get the Y not XY. The little skates have 49 chromosomes, so about 25 from each parent and a genome size of 3.42 billion base pairs. Mako Sharks have 82 chromsomes so they get 41 from each parent. The differences in chromosomes between these organisms tell you that they are genetically different, which tells you that the species in this taxonomic group are not all the same.

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Chondrichthyes represent divergent evolution. All the species in the class share similar structures that are used in similar ways. These similar structures may be formed a little differently but they were still evolved from a common ancestor and is still used in similar ways. The bull shark is the only shark that can survive in freshwater AND saltwater. So divergent evolution happened which made the bull shark a shark but gave it a gene that no other shark has, it can survive in freshwater along as salt water. Sharks are considered fish and there are multiple freshwater fish but the bull shark is the only shark that can live in freshwater AND saltwater.

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They all developed in similar ways but they do have some differences. Sharks- the sperm fertilizes the egg inside the female and then the embryo starts to form. Some species(bullhead, whale, zebra sharks) of sharks lay eggs with the developing embryo covered by a tough, protective case(oviparous reproduction). The embryos are supplied with nutritious yolk. After some time, the egg hatches and a young shark emerges. Most female sharks are ovoviviparous(dogfish, angel sharks, tiger) live-bearers; they retain their eggs inside the body until the young hatch. Provides young with protection. White and mako sharks, the embryos hatch inside the mother at age 3 months, but then stay in the mother for additional time obtaining nourishment by eating nutrient-rich, unfertilized eggs the mother produces for them. The sand tiger shark, the first embryo to hatch in the uterus eats the siblings, so only two offspring are born(one from each uterus). Hammerheads-Early, the mother and embryo make a connection. Embryo contains nutrients through the placenta for the remainder of its growth before being born(viviparity). They breed a lot but it takes a long time. Rays- They are internal fertilizers. Viviparous- retain embryos until they’re completely developed. Placental- the embryos are initially dependent on stored yolk but are later nourished directly by the mother through a placental connection(no batoid). Ovoviviparity- Employed by rays. Embryos rely on the substantial yolk within the ovulated egg only during the initial stages of development. After the nutrients stored in the egg have been consumed, the embryo ingests an organically rich “uterine milk” produced by the mother and secreted into uterus. Lining of the uterus forms tiny, finger-like projections that increase the surface area for histotroph secretion. Nutrient supply-large offspring. Oviparous- retain eggs for short period of time and then deposited on ocean floor in tough, keratinized egg cases to then hatch(fully developed). Skates-Extended Oviparity- embryonic development occurs outside of the mother and can last for a long period of time(15 months).

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The anatomical structures are very similar but they all have gotten bigger in size over time. Sharks now have sharp teeth, are more wide than they are long, and their fins are more in a triangular shape and tougher. Rays/Skates have not really changed over time that much, but they have gotten bigger; their fins and tails are a little longer. The fins of a ray/skate and the fin of a shark, even though look different do the same for each species, for all the fins allow them to swim and move around. Tails and stingers do the same thing for the animals, the tail and stingers also help the animals swim.

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It shows evidence of evolution because the sharks/rays/skates now have a little bit of different characteristics from the fossils which benefits them in their environment today. So they are relatively the same but little things are different which shows evolution and they changed for their benefit.

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Name of fossil: Holocephali-Shark

Age of fossil-422 millions years ago(Devonian)

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Name of fossil: Elasmobranchii fossil-Ray/Skate

Age of fossil-360 million years ago(Devonian)

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Name of Fossil: Cladodoides wildungensis-Shark

Age of fossil-300 million years old(Devonian)

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Name of fossil: Doliodus problematicus-Shark

Age of fossil-400 million years old(Devonian)

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Name of fossil: Pucapampela(Orthacanthus fossil)-Shark

Age of fossil-260 million years old(Permian)

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Location found & habitat specific information-The little skate is a benthic species that lives primarily on the continental shelf over sand or gravel bottom, often in shallow water (at depths less than 111m).
Physical characteristics:

External:

Ampullae of Lorenzini

Eyes

Nostrils

Spiracles

Teeth

Gill slits

Scales

Fins

Tail

Electric organs

Internal:

Skeleton

Liver

Digestive Tract

Pancreas

Spleen

Rectal Gland

Kidneys

Reproductive tract Behavioral characteristics- Unlike bony fish, little skates lack a mechanism to pump oxygenated water over their gills. Because skates spend most of their lives on the bottom, they breathe through specialized organs called spiracles, which are slit-like openings near their eyes. Water is taken in through the spiracles, passes over the gills, and then leaves the body through five pairs of gill slits underneath the body. Little skates feed on a diverse diet of shellfish, crabs, sea squirts, worms, amphipods, squid, and small fish. Little skates copulate many times in a year. The female lays two large eggs that develop inside a capsule, or egg case, which is often attached to seaweed. The empty black capsules wash ashore after the young have hatched. Resembling square coin purses with prongs at each corner, the capsules are commonly called "mermaid's purses."

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Location found & habitat specific information-Manta rays are marine animals that pass quietly in salt water. Manta rays are distributed in tropical, subtropical and temperate oceans worldwide. They are not species that tolerate low temperatures, and instead live in warm waters.
Physical characteristics:

External:

cephalic horn

eye

pectoral fin

tail

dorsal fin

mouth

unique spot pattern

pelvic fins

cephalic fin

gill slits

Internal:

esophogus

tooth band

gill arches

gill rakers

heart

liver

gall baldder

kidney

cloaca

uterus and ovary(female)

rectal gland

intestine

stomach      Behavioral characteristics- Giant manta rays are primarily plankton feeders. They use the unfurled cephalic (head) fins on the head (which look like horns when they are not feeding) to funnel plankton-rich water into the mouth where gill rakers filter out the plankton. Some small crustaceans and fish may complement the diet. Like other filter feeders, manta rays have reduced, nonfunctional teeth. They are no threat to larger ocean animals unless threatened; their only defense is their size and large powerful wings. Mantas do not have venomous spines on the tail. According to Yapese myth, mantas can wrap their wings around humans to squeeze them to death, but this is untrue. They can, however, pack a very powerful punch with their wings.

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Location found & habitat specific information-The Mako Shark have the latin name Isurus Oxyrinchus, lives in warm and temperate waters between 50 degrees north and 50 degrees south. Mako Sharks can be found near Australia and New Zealand.
Physical characteristics:

External:

snout

nostrile

eye

mouth

spriacle

labial furrows

gill openings

petoral fin

dorsal fin spine

2 dorsal fins

pelvic fin

clasper(males)

anal fin

precaudal pit

caudal keel

caudal fin

Internal:

skull

inner spiracle openings

stomach

kindey

spinal cord

cord

cloaca

encephalon

pharynx

pariental muscles

ventral oarta

heart

vertebra

anterior intestine

liver

posterior intestine Behavioral characteristics-The Mako’s of today feed on some of the world's fastest and highly developed tunas and bill-fish and are therefore considered apex predators. Makos are also known to prey on squid, dolphins, porpoises, blue sharks, bonito, sea turtles, and mackerels.

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Majority live in salt water but some live in freshwater.

Tropical to cold waters.

Their habitat helps protect them from predators but also experiences rapid increases in water temperature.

Skates and rays are native to the Atlantic, Pacific, and Indian Oceans.

Living between 8m and more than 3000 m below the surface.

Skates and rays can also live in estuaries.

Small sharks burrow itself into the sand, hide in coral reefs, and hide underneath rocks to hide and seek protection.

They change their habitat to avoid predators.

The saltwater sharks live between the surface of the ocean and about 2,000 meters down.

Some sharks prefer the open ocean; these sharks are called pelagic sharks.

Some sharks prefer warmer waters, while other sharks prefer colder waters.

Rays normally habituate in the coastal tropical areas and hide in the sand on the ocean floor. They can be found inland or offshore in warmer waters.

Many think that Rays can only be found on the ocean floor, but the majority of their time is spent feeding near the ocean’s surface.

Rays often mass in groups of up to thousands but some species of rays prefer to live alone.

Skates like to live in soft sandy bottoms and only live from the shallows to 300 ft. deep.

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Skates, similar to rays, are cartilaginous fish. There are more than 200 different types of skates but are grouped into 2 subfamilies; Rajinae (hardnose skates) and Arhynchobatinae (softnose skates). Skates, like rays, are related to sharks but unlike rays, skates are harmless. Skates lay flat, rectangular eggs that are enveloped by a leathery shell. Empty egg cases have been found on beaches and have come to be called mermaid's purses. Skates obtain their food by laying their body over their prey and then directing it to it's mouth where two bony toothless jaws crushes it's prey.

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Rays display internal fertilization. Most species will give birth to live young. Rays, like sharks, feed on a variety of seafood. Rays eat mostly fishes and invertebrates with their blunt teeth. Some even consume plankton and small organisms. Between the shark and the ray, the two animals prey on nearly everything that swims.

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Sharks are sexually dimorphic(differences between male & female bodies). A male shark will select a female and follow her very closely. He may even encircle the female. At a point he will quickly bite the female's pectoral fin to stimulate her. After a while, the female will give in to the male who then positions his ventral surface along the female's ventral surface. The male then inserts his claspers into the female's cloaca. Fertilization occurs internally. Sharks reproduce in three ways. Oviparity: The female lays eggs that develop over a period of a few months. Young hatch at sea and are on their own.Oviviparity: The eggs are hatched in the oviduct of the female.The embryos develop in the uterus until fully grown. Then they are released into the sea.Viviparity: The embryo is nourished inside of the female by a placenta. The gestation period lasts almost a year before the newborn is released.
The shark is truly the ultimate predator. It is perfectly adapted in every way to locate, hunt and kill prey. The carnivorous diet of the shark consists of fish, other sharks, young seals and whatever else it feels like eating.

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Cartilaginous Skeleton

Pelvic Claspers

Do not have a swim bladder

Internal Gills

Coloration Patterns

Placoid Scales

Pectoral Fins

Ampullae of Lorenzini

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Sharks

Skates

Stingrays

Sawfiah

Batoids

Chimera

Helicoprion

Edestus

Electric Ray

Manta Rays