Before the sharks, the chordates and craniates… Features

Early on, and by early on I mean about 530 million years ago, many animals lived on earth. They were strange and had various adaptations to help them survive, but they lacked one feature that grants us and all other vertebrates the upper hand – they lacked a spine. Yeah, they were worms… literally. Although, that is not to say that they were ill adapted for the earth… no, they did well, very well, and obviously they still do well. I mean, there are about 1 million species of insects on earth, just tipping the iceberg for invertebrates, compare this to the some 52,000 species of vertebrates. Of course though, invertebrates are not as nearly as advanced as vertebrates, which leaves them at a disadvantage… I mean, just look at the food chain.

We must thank these creatures in the end however, seeming as they did give rise to the almighty vertebrae. In the midst of all these worms, and other puny creatures swimming around the seas, there was one slender little guy, a creature only 3 cm long, swimming abut the ocean, the myllokunminigia fengjioa. This animal was a chordate and a craniate, who appeared to have skeletal structures and a skull that was made out of cartilage. Although simplistic looking, small, and lacking any appendages, this animal left behind a remarkable legacy. This feature (the cartilage) marks a huge transition and step up from the previous animal forms.

Soon thereafter, with help from natural selection( a process in which organisms come to be, they “evolve” into modern organisms, because those with certain inherited characteristics are more likely to survive and reproduce than are organisms with other characteristics, and thus breed and proliferate, creating a modern organism with all of those beneficial characteristics) the vertebrates came about. The term vertebrate comes from the word vertebrae, which refers to the bones that make up the vertebral column, or backbone. Vertebrates belong to the phylum Chordata, also called the chordates. These Chordates have bilateral symmetry, and they all share a set of features that define them as chordates, even if the features are only present in the embryo. The features are:

  1. Notochord: this is a long and flexible rod that is situated between the digestive tube and the nerve cord. It is made of fluid filled cells that are large and are encased in fibrous tissue. This notochord will serve the important purpose of providing the support of the creature, and it spans almost through to the entire length of the creature. The notochord also provides a way for the muscles to work against, as the notochord is a very flexible structure. This notochord is present in all chordates for at least the embryonic period, but in most creatures a more complex skeleton will develop around the notochord and the adult will remain only fragments of the notochord.
  2. Nerve Cord: The nerve cord of the chordate embryo will develop from the plate of ectoderm ( of the three germ layers in the embryo period the ectoderm, the mesoderm, and the endoderm, the ectoderm develops first and will later form the nervous system, the epidermis, and the outer part of the organism) that is dorsal to the notochord. This hollow nerve cord is a unique feature of all chordates. Many other animals that are invertebrates have nerve cords that are solid (not hollow) and lie ventral (not dorsal). This nerve cord will later develop into the animals CNS ( central nervous system) which is composed of the animals vital control centers, the brain and spinal cord
  3. Pharyngeal slits or Clefts : The area just below the mouth is the pharynx. In all embryos of the chordates, these are pouches that form in series separated by grooves along the pharynx. In most chordates these grooves will eventually develop into slits that will open up outside of the body, and eventually become gill slits which allow water to enter the body and exit the body, without going through the digestive tract. This will allow the organism to obtain the needed oxygen. They also function as feeding devices in many invertebrates
  4. The muscular tail : the chordates all have a tail in the embryo stage that extends after the anus. This tail is oftentimes very muscular and contains skeletal elements which will help to propel the creature in the water.

Another important part of the evolution process was the development of a head. The chordates who have a head are known as the crainiates, and if you haven’t already guessed, the word comes from the word cranium, or skull. The formation of the head with the brain the eyes, and other sensory organs, allowed the organisms to have more advanced movements and behavior.

Once the early vertebrates came about, they were restricted to the oceans, and stayed for about 200 million years. What broke the trend was the eventual development of limbs, allowing the animals to colonize land—but here we’ll focus on the underwater creatures, where the sharks dwell.

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The development of a backbone.. Vertebrates come about!

Starting from the formation of the Crainiates, the very early crainiates were the Haikouella. These creatures had a large brain that was well-formed; they also had eyes and muscles throughout the body and had gills that ran along the pharynx. They didn’t have a skull or ear organs, these characteristics developed further along the line. The true first craniate was about the same size as the Haikouella but had ear capsules and a skull, it was the Myllokunmingia.

Now, the present era, the longest lived craniate lineage belongs to the hagfishes, they have a skull made of cartilage, but they lack a jaw and a vertebrae. They look like eels and they swim in a snakelike way by using their muscles to push against their notochord, which they will keep well into adulthood. They have small brains and eyes, ears, and a nasal opening. Their mouths do have formations that resemble teeth, they are made of keratin, a tough protein found in our skin and hair. These hagfishes live on the bottom of the ocean, and they are scavengers. An interesting feature of this organism, is their defense system, they can produce as slime, in fact several liters of this slime in less than a minute! This slime will then coat the gills of other creatures trying to attack the hagfish, which will cause the other organism to suffocate. This defense system is actually amazing, and as of now, biologists are trying to mimic the slime, in order to create a gel like substance that will stop bleeding during surgery.

So… the lineages of craniates finally gave rise to the vertebrates. These vertebrates were much more complex, as they had developed a skeletal system and a nervous system that was complex. These vertebrates were more successful at avoiding predators and they were better suited at obtaining food. The vertebrae they developed served to enclose the spinal cord, and take over the role of the notochord. The vertebrates in the water (which we will concentrate on) have fins running dorsally, and ventrally, to help them steer and propel themselves in the water. They also had more advanced gills, allowing for better gas exchange.

Now, the oldest still living vertebrates belong to the Lampreys. Most of these odd looking creatures are parasites and they use their large jaws to get a hold of other fish. They then use their tongue to ingest the fish’s blood. The skeleton of these lampreys is made of cartilage, but unlike the cartilage of other vertebrates, this cartilage has no collagen, but contains a protein matrix.

Then actual Jaws came about (not the movie star…)

Jawless craniates were common until the gnathostomes came about. These are a diverse group that contain the sharks, rays, amphibians, reptiles, birds, and mammals… diverse indeed! The word gnathostomes means jaw mouth. The jaw enables these creatures to grab onto food and slice them up. Thus the gill slits were no longer required for suspended feeding, as the jaws allowed for better feeding. Thus the gills only served the important purpose of gas exchange. The earliest gnathostomes were the placoderms “plate skinned” and the acanthodians.

Afterwards, the sharks finally came about. They belong to a group known as the Chondrichthyans. In this group are the sharks, the rays, and their relatives. This group contains what is obviously one of the biggest and most successful predators in the ocean. The term chondrichthyans means “ cartilage fish” and they have a skeleton that is mainly made of cartilage. There are about 750 species of this group.

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Prehistoric Sharks

Prehistoric Sharks

Denizens of the sea, monsters, beasts, demons, mutants, devils: sharks. Yes, the “big bad” shark, an animal who haunts the dreams of many each and every night. But is it so fair, to label this animal as such? Are all sharks truly monsters? Certainly not, although our fear of these creatures is so large, that it may seem that way. In fact, there are only a few sharks that exist that have been known to “attack” However true, people will continue to unjustly label sharks as “killers”. That is, until we are able to overcome our overpowering fear of them.
On the other hand, if in our perspective, the “Great white” and “Bull Shark” are mighty creatures capable of only destruction, what would we have to say about say the Megalodon? For many do not know that this creature, who measured over 40 feet in length, even existed. Yet this shark was a great white on steroids! It’s a good thing that the Megalodon is extinct, because if it wasn’t, it’s safe to say that the beach would no longer be a popular attraction. Yes, this shark was a prehistoric shark of epic proportions and there are many just like it. Specifically, this website takes a look at some of the “ancients” of the shark world.

Megalodon: Measured over 40 feet in length, with 6 inch teeth. Fossils of its teeth have been found throughout North America an in several other parts of the world. The fossils have been recorded to be about 5 million to 25 million years old.

Orthacanthus: yet another “Big and Bad” shark. This one was long and eel like and was about 10 feet in length (not quite as big as the Megalodon, but enough to spark fear in many) A shark with a very powerful jaw and fanged teeth. Fossils of this shark have been recorded to be about 400 million to 225 million years old.

Hybodus : 6 feet of prehistoric terror! This shark lived about 180 million years ago during the “dinosaur age”. Fossils of these sharks are very common and have been found throughout the world. The Hybodus, like many sharks displayed the typical “long and slender, but muscular” body shape.

Paleocarcharias: This bad boy was found throughout Germany about 150 million years ago. Its appearance resembled the Bull and Hammerhead sharks of today.

Of course, these are just to name a few. You see, unfortunately, fossils of sharks are rare. This is due to the fact that Sharks have skeletons made of cartilage. For the most part, because of this, the only fossils that have been found are of teeth, scales, and fins. These are the only “hard parts” of the shark that will fossilize. Although, under very rare conditions, a complete shark will preserve. which brings us to yet another problem : the earliest shark fossils have been found only from late Silurian to early Devonian period.
In fact, it should also be noted that even the fossil record of modern sharks is poor. Again, only fossils of teeth and scales are normally found. With that said, it should also be noted that although shark fossils are a rare find, it should be enough that sharks themselves are a form of “living fossils” That is because they still exhibit a cartilaginous body and share many of the same characteristics as their ancestors.

Sharks have had to evolve little throughout the last 150 million years. Which proves that sharks really do have the ideal body plan, while on the other hand, most animal species have changed a lot throughout the years. Despite their ancient appearance, sharks are actually very complex animals, and research has been shown to prove that sharks possess powerful problem solving skills, social complexity, and a natural curiosity!

Here are some fun facts about ancient and modern sharks :

Interesting Shark Facts
1.Sharks have been around for about 400 million years.
2. When a sharks tooth falls out, a new one is grown in the place of it. In fact, a shark may go through about a thousand teeth throughout its lifetime.
3. a Shark’s skin is so rough, that simply rubbing against it may cause bleeding.
4. Sharks live everywhere, literally! They inhabit areas from warm lagoons to polar seas, they even live in freshwater rivers and lakes.

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