Ambulacraria Group Includes Echinodermata and Hemichordates - Science Assignment Help

Assignment Task

1. Introduction
According to Cameron et al.  (2005), Deuterostome organisms developed a distinct chordate body plan earlier around the Cambrian period. To elucidate, chordates are made up of five distinct morphological traits that differentiate them from their ancestral species that include, a notochord, a dorsal nerve cord, a pharyngeal slits, a post-anal tail, and an endostyle, all of which are present during their developmental period (Sotoh, 2016). Furthermore, chordates are bilaterally symmetric, their body can be divided into two indistinguishable halves, they have a coelom, circulatory system, and metameric segmentation. Chordates are the species that belong to the phylum Chordata, this includes mammals, birds, amphibians, amphioxus, reptiles, and tunicates. 
According to Cannon et al. (2014), the ambulacraria group includes Echinodermata and Hemichordates, while Deuterostomia is a superphylum that includes ambulacraria and Chordata. To clarify, there are three main clades of deuterostomes namely, Chordata, Echinodermata, and hemichordate. The phylum Chordata is classified into three subphyla which include, a Vertebrata, the most developed and diverse subphylum of chordates, which comprises all amphibians, birds, mammals, fish, and reptiles, Urochordata identified as tunicates, and Chephalochordata, also known as lancelets (Hickman et al., 2017).
Chordate fossils were discovered about 500 million years ago, during the Cambrian period, when invertebrates initially emerged (Ghiselin, 2019). Chordates originated from sessile or semi-sessile organisms, according to Williams (1996). The origin of chordates, on the other hand, remains a mystery, as all theories lack adequate evidence to support the main argument. This paper’s main objective is to discuss the origin and evolution of the phylum Chordata. Anatomy, phylogeny, origin, and their classification within the animal kingdom will be discussed as a purpose of archiving the main objective of this study.

1.1 Hypothesis
Although there are numerous assumptions regarding the evolution and origin of chordates, some assumptions tend to be validated by facts. To expound, Williams (1996) claims that chordates originated from sessile or semi-sessile marine organisms. Ambulacraria and chordates evolved from sessile or semi-sessile relatives with a bilaterally symmetrical and tripartite structure.
 

1.2 Problem statement 
According to Ghiselin (2019), the earliest chordates had soft bodies, which implies that there was no possibility of fossils being preserved. In addition, the fossil record is limited considering the observation of Cambrian chordates. As a result, these studies are primarily based on living creatures, specifically early developmental phases, which are far more evolutionary preserved than the distinctive adult forms they gradually become. This is the main reason why the origin of chordates appears to be controversial since there is inadequate evidence to support their existence.

2. Discussion 
2.1 The origin of chordates 
As stated by Ghiselin (2019) chordate fossils were discovered about 500 million years ago, during the Cambrian eruption, when invertebrates first evolved, the earliest chordate fossil namely, Pikaia grecilens, the ancient cephalochordate that lived about 505 million years ago. Williams (1996) claims that chordates originated from sessile or semi-sessile marine organisms. To elucidate, Glomerular kidneys are present in all modern chordates and are responsible for removing excess water from the body. Aquatic sediments, on the other hand, also yielded early chordate fossils, and all recent protochordate are aquatic creatures. This demonstrates that the first chordates evolved from sessile or semi-sessile marine organisms. Figure 1.2 shows the evolution of chordates from the sessile marine organism. However, as stated by Hendricks (2020) since the earliest chordates were mostly small soft-bodied species without any mineralized tissues to serve as a protective shield, their fossil record is sparse.

The early evolution of chordates from the sessile marine organism (Sabbotin, 2017)

2.1 Phylogeny
There is much ongoing comparative research that tries to differentiate between the simplest forms of Chordate. Chordates are difficult to group because, over time, some bloodlines of the majority of animals without a spinal cord could have lost these structures. Hickman et al. (2017) state that If chordate-like structures provide no physical proof, these chordate lineages could only be found by experiments on DNA. A series of ideas has resulted in initiatives to work out the genetic relationships of the chordates. To clarify, the Chordata comprise all and only the members of a single shared ancestor, which means that chordates and craniates are tunicates' closest relatives. Figure 2.1 illustrates a phylogenetic tree reflecting chordate evolution. Lines represent possible evolutionary relationships, including extinct taxa.

 

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