Phylum Chordata Characteristics and Examples

• Hamza Ali

Introduction:

The topic chosen for this biological study was the Kingdom Animalia. This study will specifically be analyzing the Phylum Chordata, by providing an introduction to Chordates, Vertebrates, The Classification method of Cladistics, Fish, Amphibian, and Reptiles.

What are vertebrates?

Vertebrates are known as animals that have an internal spine made of bone.Vertebrates include reptiles, amphibians, fish, birds, mammals, primates, rodents and marsupials (Kidport, 2012).

There are about 57,000 species of vertebrates alive today, which only speaks for about 3% of species alive today. The other 97% of species alive today, are invertebrates. Vertebrates have a vertebral column in which the notochord is replaced by multiple vertebrae that form a backbone. The vertebrae also protect and surround a nerve cord which gives the animal structural support.

• Structural details:

Vertebrates have a well-formed head, a distinct brain protected by a skull, aswell as sense organs. They also have a highly efficient respiratory system, a muscular pharynx with slits and gills (in terrestrial vertebrates the slits and gills are greatly modified), a burly gut, and a chambered heart. Another notable characteristic that vertebrates have is their endoskeleton! An endoskeleton is an internal assemblage of notochord, bone or cartilage that provides the animal with structural support. The endoskeleton grows as the animal grows and provides a sturdy framework to which the animal’s muscles are attached.

What are Chordates?

Chordates are known as all animals in the phylum Chordata. A phylum is second most general taxon. The Chordata is the animal phylum that everyone is probably familiar with, as it includes humans and othervertebrates. However, not all chordates are vertebrates! Chordates consist of three subphylums or categories, called vertebrates, tunicates, and lancelets. All chordates are similar as they are known to possess the following features at some point in their life (humans and many other vertebrates only possess some of these features in the embryo) (University of California Berkelely, n.d. ).

General structure:

Figure 1. Features of a Chordate

Pharyngeal slits- a series of openings that connect the inside of the throat to the outside of the “neck”. These are often, but not always, used as gills.

Dorsal nerve cord- a bundle of nerve fibers which run down the “back” and connects the brain to organs and lateral muscles.

The Notochord- is a cartilaginous rod ( a rod made out of cartilage which is a type of soft bone ) running underneath, and supporting, the nerve cord.

The Post-anal tail- an extension of the body past the anal opening ( like a tail )(University of California Berkelely, n.d. ).

Chordates come in lots of shapes and sizes due to the large variety of animals that are covered in this phylum. Chordates are known to be as big as whales and elephants, to as small as tiny sea squirts!

• Taxonomy of Chordates:

Chordates can be divided into three subphylums which help us determine the differences between other Chordates.

The first subphylum isSubphylum Vertebrata(vertebrates). “They may have come from anAmphioxus-like ancestor, however vertebrates have a definite head andAmphioxusdoesn’t. Another theory suggests that vertebrates may have come from a larval form similar to tunicate larvae that were capable of reproduction in the larval stage, that is, they are sexually mature prior to metamorphosis, which is referred to aspaedogenesis(paedo= child — same root as in pediatrician;genesis= origin, birth), defined as the precocious attainment of sexual maturity in a larva”(Carter, 2014).

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Another subphylum which are play a large role as Chordates are lancelets. Lancelets (cephalochordata) aresmall and long marine invertebrates that resemble fish but do not possess jaws and obvious sense organs. Lancelets do have a notochord and are among the most primitive chordates. As lancelets are invertebrates they do not possess a spine!In most vertebrates, the embryonic notochord is eventually replaced by bony vertebrae or cartilaginous tissue yet with cephalochordates, the notochord is retained into adulthood and is never replaced by vertebrae. There are around 20 species in two families, each with a single genus.Branchiostomawas formerly calledAmphioxus, a name that is kept as an informal term. The other genus isEpigonichthys, is also calledAsymmetron. The genusAsymmetronis sometimes retained for some species. The cephalochordate fossil record goes back to about 525 million years ago during the early Cambrian Period (Ghiselin, 2015).

And the third subphylum of the phylum Chordata is Urochordata also known as Tunicates. Tunicates are small marine animals that are found in great numbers. According to research most chordate features of tunicates disappear during metamorphosis. Some examples of tunicates are sea squirts (Ghiselin, 2015).

What is the importance of Chordates in an ecosystem?

Chordates are mainly essential for an ecosystem as these vertebrates or invertebrates are the carnivores or herbivores or omnivores which help sustain said ecosystem. The phylum Chordata consists of many animals which consume and hunt down other animals, hence maintaining the numbers of predator and prey. Omnivores also help an ecosystem sustain itself as the more they eat plants the more room there will be for other plants to grow. Chordates also help the environment as their excretion, bones, and remains provide nutrients and minerals to ecosystem. All in all, Chordates are detrimentally important to ecosystems, as their actions affect the ecosystems sustainability in a very large method.

What are cladistics?

According to research; “Cladistics is a particular method of hypothesizing relationships among organisms. Like other methods, it has its own set of assumptions, procedures, and limitations. Cladistics is now accepted as the best method available for phylogenetic analysis, for it provides an explicit and testable hypothesis of organismal relationships” (University of California Berkelely, n.d.) .

The basic concept of cladistics is that members of a group are derived from a common evolutionary history and therefore, are more closely related to members of that same group than to other organisms. These groups are recognized by sharing unique features which were not present in distant ancestors. Theseshared an derivedcharacteristics are calledSynapomorphies (University of California Berkelely, n.d.).

• What assumptions do Cladists make?

There are some common assumptions that are made in cladistics:

1. Any group of organisms arerelated by descentfrom a common ancestor.
2. There is abifurcating patternof cladogenesis.
3. Changein characteristics occurs in lineages over time(University of California Berkelely, n.d.) .

Cladistics General Structure:

Figure 2. An example of a basic Caladogram,

In this example of a cladistic, 5 animals are compared, and their similarities suggest that some animals are more closely related to others, such as the lizard and the chicken, versus the chipmunk and the frog.

• Types of features:

– Apomorphic, or derived features are features that originate within different members of the group, and therefore differentiate them. Apomorphic features are very useful in cladistic classification as they help us see differences between organisms.

– Plesiomorphic features are primitive features that are shared with all members of the group. Plesiomorphic features are not useful in Cladistics.

In short, Cladisticsare a method of classifying plants and animals according to the proportion of common characteristics they share. It is obvious that the higher the proportion of common characteristics, the more recently the organisms diverged from a common ancestor!

Fish:

Fish are the most numerous and widespread of all the vertebrates as there are so many fish species! In short, there are three categories of fish; Jawless fish, bony fish, and cartilaginous fish. All fish possess hills and live under water. Of the 22000 species of fish, it is known that 40% live in fresh water and 60% live in salt water.

What are Jawless fish?

Jawless fish are also known as class Agatha. Jawless fish are covered with slimy skin and don’t have paired fins. They have soft eel like bodies and possess a notochord. Jawless fish are made up of cartilage and not bone. Class agatha is made up of 70 species of lampreys and hag fish. Lampreys are small fish that have a large sucker with many tiny teeth. They are most known for the damage they cause in the Great Lakes. Hagfish are scavengers that eat dead or dying fish, and are more like scavengers who stay at the bottom of the ocean. They have a mouth surrounded by tentacles or “feelers”, but also have a row of ‘toothlike’ teeth which help them devour their prey.

What are Bony Fish?

Bony fish are also known as class Osteichthyes. Bony fish are the most numerous of all vertebrates; they include salmon, trout, cod, eel, halibut and many more. All living species of bony fish can be divided into three subclasses; Ray finned fish, primitive fish, and lung fish. Ray finned fish are your average fish, they have a flat body with covered with flexible scales. And their fertilization is usually external. Primitive fish such as sharks must keep swimming constantly as of they don’t they will sink. This is because sharks do not have a swim bladder.

What are Cartilaginous fish?

Lastly, cartilaginous fish ( class Chondrichthyes ) consists of sharks, skates and Ray fish that have a cartilaginous body. Cartilage is a soft type of bone. Cartilaginous fish have two major advantages over jawless fish, paired fins and biting jaws! The best known cartilaginous fish are the sharks! The largest sharks such as the whale shark, feed on plankton in the sea, but most sharks feed on other fish, bottom dwellers, molluscs and lobsters. Skates and rays are very similar to sharks but have a flat streamline body. This is because their bodies have adapted to living on the bottom of the ocean.

Amphibians versus Reptiles:

Amphibians are cold-blooded vertebrate animals of a class that comprises offrogs, toads, newts, and salamanders. They are different by having an aquatic gill-breathing stage followed (typically) by a terrestrial lung-breathing stage in adulthood (Zug, 2014) Amphibians live on land and water.

According to research more than 6,500 species of living amphibians are known. They first appeared about 340 million years ago during the Middle Mississippian Epoch, and they were one of the earliest groups to diverge from ancestral fish-tetrapod stock during the evolution of animals from strictly aquatic forms to terrestrial types. More recently, amphibians are represented bytoads, frogs, (order Anura), newts and salamanders (order Caudata), and caecilians (order Gymnophiona). As a group, the three orders create the subclass called Lissamphibia. Neither the lissamphibians nor any of the extinct groups of amphibians are the ancestors of the group of tetrapods that gave rise toreptiles. Though some aspects of the anatomy and biology of various amphibian groups might seem closely related to reptilian ancestors, amphibians are not the intermediate step in the evolution of reptiles from fishes (Zug, 2014).

Furthermore, Encyclopedia Brittanica states that; “modern amphibians are united by several unique traits. They typically have a moist skin and rely heavily on cutaneous (skin-surface)respiration. They possess a double-channeled hearing system, greenrodsin their retinas to discriminate hues, and pedicellate (two-part) teeth. Some of these traits may have also existed in extinct groups” (Zug, 2014. Para 1).

Reptiles:

Reptilesare cold-blooded vertebrates of a class that includes lizards, snakes, crocodiles, tortoises and turtles. They are distinguished by having a dry scaly skin, and typically lay soft shell eggs on land.

Reptiles appeared about 310 years ago and they are the first vertebrates to be fully terrestrial! There are approximately 7 thousand species of living reptiles in the world. The class reptilia can be classified into three subclasses;

1. Anapsida (turtles and tortoises)
2. Lepidosaura (lizards snakes and the tuatara)
3. Archosaura ( crocodiles, alligators and dinosaurs)

Furthermore, according to research: “The first reptile is believed to be the Hylonomus (“forest mouse”), which lived 315 million years ago in what is now eastern Canada. It was small animal, about 8 to 12 inches (20 to 30 cm) in length, and it ate insects. Around 230 million years ago, during the Triassic period, new, larger animals — the first dinosaurs — evolved, and these would go on to dominate terrestrial ecosystems for 160 million years, until they were wiped out during the Cretaceous-Tertiary extinction event 65 million years ago. Because of their dominance during this time, the Mesozoic Era is often called the Age of Reptiles” (What are Reptiles, 2015).

Why and how did reptiles evolve?

Figure 3. This is one of the first reptiles

Hylonomus-Reptiles first come fromamphibiansin the swamps of the lateCarboniferous. Increasing evolutionary pressure of the land forced the evolutionary changes in amphibians to gradually become more land based. Due to the abundance of available land and the lack of swamp area, small amphibians were forced to move into terrestrial environments. Naturalselection projected the development of traits which were necessary, such as a stronger skeletal structure, muscles, and more protective covering such as scales. This was the basics of the evolutionary movement for reptiles. And so reptiles like the first were created.

The evolution of legs and lungs were the main transitional steps towards reptiles, but the development ofthe soft shelled eggs from the aquatic amphibian eggs was the real key turn point for reptiles as this allowed the creatures to be fully distinguished, and leave the aquatic environments.Another major difference from amphibians to reptiles is the increased brain size! Reptiles have an enlargedcerebrumandcerebellum. Although their brain size is small when compared to birds and mammals, these enhancements prove vital in hunting strategies for reptiles. Moreover, reptiles evolved from amphibians and sauropsida because of the environmental pressures that forced them to adapt to their new terrestrial environment (Pawley, 2006).

Importance of Amphibians to the Ecosystem:

Amphibians play an important ecological role as their existence affects the ecosystem greatly. Because amphibians lay their eggs in water, they are restricted to stay in wet areas. Many amphibians, frogs in particular, help the ecosystem by simply breathing. This is because frogs breath through their skin, and can absorb many gases, this helps the environment a great deal. They also help sustain the environment by controlling plant and animal levels in the ecosystem, as frogs and many amphibians consume both animal and plant material. Finally, over the years, many scientist have concluded that the degradation of a semi aquatic ecosystem has something to do with the degradation of amphibian populations in those ecosystems. Hence proving that amphibians do play an important ecological role in the sustenance of an ecosystem.