Vertebrates:Chapter 34  

 

Chordates

 

The chordates are the basic origin of the body structure of vertebrates. They are bilaterian animals, and they also go with the clade Deuterostomia, which goes with invertebrates (Nanci's page). The chordates are the animals that started with the notochord, dorsal hollow nerve cord, the phrayngeal slits, and the muscular post-anal tail.

Another group, called the tunicates, are closely related to the deepest branching lineage of the vertebrates. The larav probably used the post-anal tail to help it swim and get through places. Once it settles, it goes through a sudden metamorphosis, and all the organs rotate. Many of the chordate characters dissapear, they get their foood through pumping in a siphon, and they are known informally as "sea squirts".

Basically, it takes in water through the incurrent siphon, and passes through the pharyngeal slits into a chamber called the atrium, and then comes out the excurrent siphon. Food particels are filtered by a mucous net (nasty), and the cilia also clings onto it.

 

 A third group, called the lancelets, are also put with a close relation to the vertebrates, being more related to them than to any other invertebrate. These get the name for their balde-like shape. As a larvae, they develop the PNDM, and also live in the water, feeding on plankton. Following metamorphosis, they undergo a sudden change once settling. They leave their fronts out, sitcking out from the ground, and they feed by drawing water in through ciliary pumping. Also have gas exchange, and they don't really use their pharyngeal slits. In a simple form, actually, they demonstrate and show the basic swimming mechanisms of fish. Contradiction of the muscles along the notochord flexes it, making the side-to-side movement, making them swim. Very rare to find them.

The rise of evolutionary biology focuses much on these organisms. They don't posses all the characteristics of vertebrates, since there is alot of diversity in the world, but the chordates, lancelets, and tunicates can give a basic idea of how the vertebrates came from and what it means. Tunicates have some  chordate characteristics as a larvae, while a tunicate shows them during adult form. Scientists propose an idea that the tunicates had obtained more sexual maturity. Having some traits form the past, such as in the larvae stage, and still showing them in the adult stage, that is an example of paedomorphosis. Still, that was a while ago, and now the idea isn't so strong anymore. Hox genes- DNa sequence in genes that involve in body segmentationduring embryonic devevelopment. Studying these show that tunicate larvae has a heart and that there is something wrong. The brain is also a freaky topic. The brain of ours was not common at all in the past times, and the lancelets just had a tip as a head. Still, the same Hox genes are in the same order displaying these traits. 

 

 

 

During the Cambrina period, there was a lot of diversity, and more vertbebrates came out and developed. Vertbrates underwent another gene development, and they soon started getting more ways to get food. The skull also became more extensive, and the spinal cord was adjusted a little to make the animals a little more upright. Aquatic vertebrates had aslo changed and had better gas exchange ways, thanks to the gills. Lampreys are the oldes living lineage of vertebrates today, and they act mostly as suckers, with thier jawless round mouth, peeling skin and taking stuff such as sucking fish's blood. The skeleton of lampreys is mostly made of cartilage-a firm, elastic, flexible type of connective tissue of a translucent whitish or yellowish color; gristle. Still, they don't contain any collagen. Conodonts were early vertebrates, and also had no jaw, but instead a type of hook with little dental tissues. They had good predatoyry skills, mostly because of their large eyes. Later on, there were more vertebrates developing, some with armor, and more characteristics. They all became extinct by the Devonian period.

 

The mineralization of the skeleton was actually after the development of lampreys. Philip Donoghue, hypothesized that mineralization was mostly associated with the feeding of organisms. Early chordates were suspensio feeders, and then eventually became into more active predators. The earliest known mineralization were the dental elements of conodonts, the teeth, eventually became the source of teeth of becoming scavengers and predators. Armor was from the dental elements, so that basically hypothesized that everything came from the dental mineralization of conodonts, Donoghue said. Then again, only other vertebrates had characteristics that helped show that all characteristics came from the skull.

 

 

 Chondrichthyans are vertberates that include the most succesful and ferocious predators of the ocean (that is right, sharks). They have over 750 species of chondrichthyans. It belongs to the class of Chondrichthyes, which means"cartilage fish". As the name is, they have a skeleton that is mostly made out of cartilage. Scientists thought that thses organisms were representing a primitve stage in the evolution of the skeleton. This was supported by the fact that most of the time in the embryo, the skeleton was made mostly of cartilage. Later, much of the cartialge is made with bone, a hard matrix. Then again, the idea was also rejected for a while, because the mineralization of the vertebrate skeleton happened before the chondrichthyan lineage. Most sharks ahve a swift swimming technique and their powerful movements help them move forward a lot. By a huge amount of oil in their liver, it help them gain bouyancy in the atwer, and if they were to stop swimmming in the water, then they would sink. Continual swimming also helps with having the water in their gill slits. Sharks have good sensory organs, and they can feel vibrations that help with getting the other organisms and eating them. Some sharks are vivparous(eggs in Mom), some are ovoviviparous( having eggs that get nourishment a different way).

Ray-finned fishes and lobe-fins are also in the same category, and are known for being mostly bony. Unlike chondrichthyans, osteichthyans have an endoskeleton with a hard matrix. It is not really clear that when the evolution of the skeleton happened. It is possible that the chondrichthyans had mostly lost the bones before. Aquatifc osteichthyans are known as fishes. Water is drawn into the mouth, through the pharynx, and out between the gills by movement of the operculum. Most aquatics can control their buoyancy by the air sac called the swim bladder. Movement of the blood shows if they get any higher or lower. Some lineages show that could have had lungs at an earlier time. The skin is often covered in scales that help with gas exchange.

 

 

360 million years ago, some lobe-finned fishes started developing legs and limbs. Once that had happened, a few years later would turn into the developing of a new lineage that would give rise to even mor evertebrates, and omst being non-aquatic. The event gave rise to the tetrapods. The name is Greek for "four-legged", and it means of these organisms that have limbs that help to support the wight and move through terrain. The bones of the pelvic girdle are connected to the backbone, and the gill slits are lost during embryonic development. Instead, it gave rise to different organs such as the ear, glands ,and other sensory organs. The tetrappod body plan was actually a modification of a preexisting body plan. Some bones just started becoming limbs, and more tetrapods were able to walk on land. Some were aqautic as well, which goes to the topic of reptile and amphibians.