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:
- 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.
- 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
- 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
- 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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