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Chordates: Origin & Characteristics [Academic Script] Subject: Zoology Course: B.Sc. 2nd Year Paper No. & Title: Z-201B Animal Diversity-II Topic No. & Title: Topic – 1 Chordates: Origin & Characteristic Lecture Title: Chordates : Origin & Characteristic Academic Script 1.Introduction: Chordates occupy a greater variety of habitats and show more complicated mechanisms of self-maintenance than any other group in the whole animal kingdom. Chordates derive their name from one of their synapomorphies, or derived features indicating their common ancestry. The chordate is the animal phylum, with which we all are familiar, as it includes us – the human beings, along with other vertebrates. It is also true that all chordates are not vertebrates. Origin: Several hypotheses are produced by zoologists while attempting to find out the evolutionary relationships of the chordates. We will try to understand different theories of origin of chordates and their characteristics in this programme. It is said that the chordates are evolved from animals like Amphioxus. We may look farther backwards to understand the origin of the whole Chordate phylum from still earlier ancestors. They may have existed in the earliest Paleozoic times. The great difficulty of such an inquiry in itself is a stimulus and a challenge. The Fish-like forms developed in early Ordovician strata, as Scales and bones have been found from the fossil collection of this period. So the origin of the Chordates must be sought far back in the Cambrian period, perhaps nearly 600 million years ago. It is believed that Chordates evolved during the time of Cambrian Explosion, when Invertebrates were also evolving. Now we must think about which of the main lines of Invertebrates shows the closest affinity with the Chordates. Almost every phylum in the Animal kingdom is suggested, including the Arachnids and Nemertines to have affinity with Chordates. Because of their body segmentation, it has been suggested that Annelids Chordates. and However Arthropods the have detailed closer study affinity suggests with many differences between Chordates and these groups. Invertebrates like Arthropods possess segmentation affecting most organ systems, while in Chordates, only dorsal myotomal region is segmented. Even the mesoderm is not divided in its ventral region in most Chordates. In Annulates, the organisms marked with rings, the nerve cord is present on ventral side and the ‘brain’ on the dorsal side. The Vertebrate nerve cord is dorsal to the gut & the anterior part is modified in the comparative ‘brain’. The circulatory system in Annelids is closed and open in Arthropods, while all Chordates possess closed circulatory system. Patten and Gaskell carried out such theories to extreme and tried to show relationship of Chordates with the Eurypterids as well as Arachnids of the Cambrian and Silurian periods. These animals show a certain resemblance to some early fossil fishes, for example, the Cephalaspids of the Devonian period. The modern Annulates differ from that of Chordates according to the embryonic development point of view of evolution, like. 1) In Annulates, the arrangement of blastomeres during cleavage is spiral hence known as Spiralia, while in Chordates, it is irregular, hence called Irregularia. 2) Gastrulation occurs by immigration in Annulates, while it occurs by invagination in Chordates 3) Regeneration of ATP is accomplished from phosphoarginine by its kinase in all Arthropods and Molluscs. This method is primitive as it occurs in many Bacteria, Protozoa, Coelenterates and Platyhelminths. Both the systems are found in Annelids, Echinoderm and Tunicates, whereas in Amphioxus and Chordates, phosphocreatine and its kinase provide the sole phosphagen system. 2.Evolution: In Chordates, Echinoderms, Tunicates and Hemichordates, the opening of gastrula, the blastopore, becomes the anus and the mouth is a new opening. So these animals are grouped together as Deuterostomia, distinct from the other Non-Vertebrates, where the blastopore forms the mouth, hence they are called Protostomia. The mechanism for providing energy liberation also shows systematic difference. According to Chamberlin, all modern Chordates possess the glomerular kidneys to remove excess water from the body. They might have evolved from some fresh water forms. But the early fossils of Chordates were recovered from marine sediments and even modern Protochordates are all marine forms. Myxinoids and Sharks possess the glomerular kidneys. It supports the theory of marine origin of Chordates. As they have similarities in embryonic development, type of coelom and larval stages, Chordates might have evolved from the Deuterostome ancestors like Echinoderm, Hemichordates and Pogonophorans. Simply comparison of two groups is not enough to understand the evolution. Extending the same method, we may divide the whole world of Metazoa into two groups: 1) The Spiralia including Arthropods, Annelids, Molluscs, and Platyhelminthes and 2) The Irregularia including Chordates, Echinoderms, Brachiopods, Polyzoans, Graptolites and Phoronis. At first sight we cannot believe that there may be any similarities in the animals belonging to these groups, but when we find similarities in the fundamental organization, we become more convinced that the ancestors of Fish-like animals are to be found here. By study of living relics of early Chordates, it is possible to trace the history of this strange change with some plausibility, although some zoologists like Brien still find that ‘The idea of bringing such unlike animals together is amazing’. Fossils of the earliest Vertebrates are known from the Silurian-Devonian period i.e. about 400 million years ago. 3.Theories of Origin: Over the years, different theories have evolved to explain the Origin of Chordates. We will understand them in details. 1. Echinoderm Origin by Johannes Muller (1860) The theory is based on the comparative studies of larval stages of Echinoderms and Hemichordates. Tornaria larva of Hemichordates resembles Bipinnaria, Auricularia, Echinoderm larvae such as Dipleurula and Doliolaria, which all possess ciliary bands and apical tuft of cilia. Johannes Muller, W. Garstang and DeBeers proposed that Echinoderm larvae gave rise to Chordates by neoteny. Further, like Chordates, Echinoderms are also Deuterostomia and possess mesodermal skeletal elements. The discovery of fossil period, Echinoderms confirms called Calcichordata from Echinoderm ancestry of Ordovician Chordates. Calcichordates were asymmetrical animals which demonstrate affinities with both Echinoderms and Chordates but their skeleton is made of CaCO3, where as in Vertebrates the bones are made of hydrated Calcium and Phosphate. The Calcichordates had large pharynx with a series of gill slits, each covered with flaps for filter feeding, a small segmented body and a post anal tail. A perforated pharynx for filter feeding appears to have evolved in diverse groups of animals during Cambrian-Ordovician periods when Planktons were abundant in water. 2. Hemichordate Origin ( Stomochordata ) by Romer (1959) This theory suggests that ancestral Deuterostomes were sedentary tentacle feeders whose mucous-laden ciliated tentacles served to trap Plankton as they were waved in water as do the modern Lophophorates Hemichordates, Cephalodiscus & Pterobranch & Rhabdopleura. Due to mutation pharyngeal gill slits evolved in these ancestors, the pharynx became sieve-like to trap Plankton as the water current passed through it. Extant Pterobranch possess both ciliated arms and pharyngeal gill slits. Tornaria larva of Hemichordates show phylogenetic relationship with Echinoderm larvae and it also shows affinities with Chordates. 3. Urochordate Origin by W. Garstang (1928) and N.J. Berrill (1955) They gave Urochordates importance which to carries the tadpole-like typical Chordate larva of characters, namely, a notochord in tail along with segmented myotomes, Dorsal hollow nerve cord, Sense organs and Pharyngeal gill slits. Garstang suggested that Chordates evolved from some sessile filter feeding Urochordate. The larval stage evolved into adult by neoteny and lost the sedentary adult stage. 4. Cephalochordate Origin by Chamberlain (1900) He studied the Cephalochordates primitive and and advanced proposed that, characters while of extant Cephalochordates possess all the Chordate characters in typical state, they also show some primitive features of nonChordates, such as, absence of heart, head, sense organs, respiratory pigment, filter-feeding mode of food capture and excretion by solenocytes. The current consensus is that Chordates are monophyletic, meaning that the chordata contains the only descendants of a single common ancestor which is itself a Chordate, and the nearest relatives of Craniates are Cephalochordates. Fossils of 60 specimens from mid-Cambrian of the earliest Chordate, Pikaia gracilens have been discovered from Burgess Shale in British Columbia, Canada. The Amphioxus-like fossils show streamlined, ribbon-shaped, 5 cm long body having notochord in the posterior two-third of body and myomeres. It has a small head with two tentacles and gill slits in the neck region. Other Chordate-like fossils are Cathaymyrus from early Cambrian sediments in China and Palaeobranchiostomata from early Permian from South Africa that appears to be more similar to Amphioxus. 5. Combined theory by E.J.W. Barrington (1965) He combined all the theories and proposed that the common ancestor of Echinoderms and Chordates was, a sessile ciliary arm feeder that lived in the Plankton-rich environment of the Cambrian period. According to this theory, the Modern Crinoidea also called Echinodermata, Pogonophora and Pterobranch Hemichordates evolved from a similar ancestor by retaining the original mode of feeding. Perhaps because they have continued to inhabit the same environment as occurred in the ancestral days. 4.Further Transition: However, Pharyngotremy must have evolved in a large number of groups at the same time, which must have been much more superior method of food gathering by filtering water through pharynx, as compared to ciliatedarm feeding. Hence, the sedentary Protoascidians of that time, lost ciliated arm feeding and adopted pharyngeal filter feeding as the only method of food gathering. Sometime later, when the Plankton population in water declined, free-swimming tailed larvae of Urochordates did not metamorphose. They became neotenic adult, as free- swimming mode was superior in food searching at the time of food scarcity. By perfection and expansion of Chordate characters, which were already present in the Ascidians tadpole larva, Cephalochordate-like ancestors were evolved. We have fossils of such primitive Chordates, for example, Pikaia gracilens from mid-Cambrian. All of the earliest Chordate fossils have been found in the Early Cambrian Chengjiang fauna and include two species that are regarded as Fish, which implies that they are Vertebrates. Because the fossil record of Chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has also proved difficult to produce a detailed classification within the living Chordates. Attempts to produce evolutionary "family trees" give results that differ from traditional classes, because several of those classes are not monophyletic. As a result vertebrate classification is in a state of flux. 5.Characteristics: Now let us understand the characteristics of chordates. Characteristics of Chordates: Phylum Chordata is a group of animals which are either Vertebrates or one of the several closely related Invertebrates. They are united for at least some period of their life cycle, by having a notochord, a hollow dorsal nerve cord, gill slits and an endostyle and a post-anal tail. We will understand them in detail. A. Notochord: The notochord is an elongated, rod-like structure, which is developed from the mesoderm. It is made up of skeletal tissue and located dorsal to the gut tube and ventral to the nerve cord. It is a flexible structure that forms the main support of the body in the lowest Chordates, such as the Lancelet, a primitive backbone. From a similar structure in embryos of higher Vertebrates, the vertebral column is developed.A semiflexible rod is running along the length of the animal. Chordates, which lack bone, muscles work against the notochord to move the animal. Mostly, all Chordates have a notochord at some stage in their lives, but in some, such as Tunicates the notochord is lost in the adult, whereas in others, such as the Vertebrates the notochord is present in the embryo, but in later stages is largely replaced and surrounded by the vertebrae, or backbones. The notochord should not be confused with the backbone or vertebral column of most adult Vertebrates. The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult Chordates the notochord disappears or becomes highly modified. In some Non-Vertebrate Chordates and Fishes the notochord persists as a laterally flexible, but incompressible skeletal rod, that prevents telescopic collapse of the body during swimming. B. The Nerve cord: The nerve cord of chordates develops dorsally in the body as a hollow tube above the notochord. This is in contrast to organisms such as Annelids and Arthropods, in which the main nerve cord is solid and located ventrally. In most species, it differentiates during embryogeny anteriorly into the brain and a spinal cord that runs through the trunk and tail. The Chordate nerve cord is hollow, with pairs of nerves branching from it at intervals and running to the muscles. Together, the brain and the spinal cord, form the central nervous system, to which peripheral sensory and motor nerves are connected. C. The Visceral Clefts and Arches: Gill slits are, at some stage of life, found in all chordates. They have been modified extensively in the course of evolution. The visceral, also called pharyngeal or gill, clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the oesophagus, meaning they are the openings between the pharynx and the outside. The visceral clefts: They appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slitlike passages connecting the pharynx to the exterior. Pharyngeal slits are filter-feeding organs found in nonvertebrate chordates like lancelets and tunicates, and hemichordates, living in aquatic environments. These repeated segments are controlled by similar developmental mechanisms. Pharyngeal slits, resembling the gill slits, are transiently present during the embryonic stages of Tetrapoda. In Fishes and some Amphibians, the slits bear gills and are used for gas exchange. In most land- living Chordates, the "gill slits" are present only in embryonic stages. The Visceral Arches: The soft and skeletal tissues between adjacent clefts are the visceral arches. The slits are supported by gill arches, which have also been highly modified in various groups of Vertebrates. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many NonVertebrate Chordates, such as Tunicates and Cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from the water. In typical Fish-like Vertebrates and juvenile Amphibians, the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. In adult Amphibians and the amniotic Tetrapods like Reptiles, Birds and Mammals, the anterior most cleft transforms into the auditory tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region. The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the Vertebrates. For example, in Humans and other Mammals, visceral arch derivatives include, the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the braincase, the three middle ear ossicles, the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx and the cartilaginous tracheal rings. D. The Post-anal tail: All Chordates have a post-anal tail, or extension of the notochord and nerve cord past the anus. This feature is also lost in the adult stages of many Chordates, such as Frogs and Primates including Human beings. 6.Sub-phyla: Now let us understand three Subphyla of Phylum Chordata namely Tunicata, Cephalochordata and Craniata. 1. Tunicata is represented by Tunicates. Tunicate larvae have both a notochord and a nerve cord which are lost in adulthood. The strictly marine Urochordata or Tunicata are commonly known Tunicates, Sea Squirts, and Salps. There are roughly 1,600 species of Urochordates. Most are small solitary animals but some are colonial organisms. Nearly all are, sessile as adults, but they have free-swimming active larval forms. Urochordates are unknown as fossils. 2. Cephalochordata, also known as Amphioxus, is represented by Lancelets. They have a notochord and a nerve cord, but no brain or specialized sensory organs, and a very simple circulatory system. The group contains only about 20 species of sand-burrowing marine creatures. 3. Craniata. Craniates are the only sub-phylum whose members have skulls. The Vertebrates and Hagfishes together comprise the taxon Craniata. In all Craniates, except for Hagfish, the dorsal hollow nerve cord is surrounded with cartilaginous or bony vertebrae and the notochord is generally reduced. Hence, Hagfish is not regarded as Vertebrates. The remaining Chordates are possibly, some odd extinct groups. The Chordates and three sister phyla, the Hemichordata, the Echinodermata and the Xenoturbellida, make up the Deuterostomes, one of the two superphyla that encompass all fairly complex animals. 7.Summing Up: The Phylum Chordata includes the well-known Vertebrates, namely fishes, amphibians, reptiles, birds and mammals. General characteristics of Vertebrates We need to be familiar with some of the traits to identify how they are expressed in the Vertebrates, either in the laboratory or in nature. Vertebrates may be characterized by 12 general derived characteristics. They are, 1. Bilateral symmetry 2. Two pairs of jointed locomotor appendages 3. Skin - modified as scales, hair or feathers 4. Because of Metamerism, the ribs, vertebrae, muscles and ganglia or peripheral nerves are formed 5. Coelom or body cavity is well-developed 6. Well-developed axial skeleton and appendicular skeleton 7. Highly developed brain enclosed by skull, and nerve cord enclosed by vertebrae. 8. Well-developed sense organs i.e. eyes, ears, nostrils 9. Gills or lungs are located closely to the pharynx or throat. 10. Closed circulatory system with ventral heart and median dorsal artery. 11. Genital and excretory systems closely associated to form the urinogenital system 12. Digestive tract with two major digestive glands - liver and pancreas With few exceptions, Chordates possess Bilateral symmetry. Body is divided into three parts namely Head, trunk and tail. Chordates are well represented in marine, freshwater and terrestrial habitats from the Equator to the high northern and southern latitudes. Chordates other than Craniates, include entirely aquatic forms. The smallest Chordates, for example, are some of the Tunicates and Gobioid fishes. They are mature at a length of about 1 cm. The largest Chordates existed were Sauropod Dinosaurs, which reached more than 20 meter height and living blue whales, grow to about 30 meter length. So in this programme, we learnt about the origin and general characteristics of Chordates. Downloads: The Fossil record to Remember: The oldest fossil Chordates are of Cambrian age. The earliest is Yunnanozoon lividum of China from the Early Cambrian, 525 ma (= million years ago). This fossil was recently described and placed with the Cephalochordates (Chen et al., 1995). Another possible Cephalochordate is Pikaia (Nelson, 1994) from the Middle Cambrian. These fossils are highly significant because they imply the contemporary existence of the Tunicates and Craniates in the Early Cambrian period during the so-called Cambrian Explosion of animal life. Two other extinct Cambrian taxa, the Calcichordates and Conodonts, are uncertainly related to other Chordata (Nelson, 1994). In the Tree of Life project, Conodonts are placed as a subgroup of Vertebrates. During the Ordovician Period (510 - 439 mya), jawless or Agnathan fishes appeared and diversified. These are the earliest known members of Vertebrata, the Chordate subgroup that is most familiar to us. Fossils representing major lineages of Fish-like Vertebrates and the earliest Tetrapods (Amphibian) were in existence before the end of the Devonian Period (363 mya). Reptile-like Tetrapods originated during the Carboniferous Period (363 - 290 mya). Mammals differentiated before the end of the Triassic Period (208 mya) and Birds before the end of the Jurassic Period (146 mya). Remember General characteristics of Vertebrates with clear understanding: One should become very familiar with these traits, and identify how they are expressed in the Vertebrates, in the laboratory or in nature. Vertebrates may characteristics. be characterized by 12 general derived 1. Bilateral symmetry 2. Two pairs of jointed locomotor appendages, which can include fins (pectoral and anal/dorsal fins, as well as the forelimbs and hind limbs). 3. Outer covering of protective cellular skin, which can be modified into special structures such as scales, hair and feathers 4. Metamerism found in skeletal, muscular and nervous systems. The structures can include ribs, vertebrae, muscles and ganglia / peripheral nerves. 5. Well-developed coelom, or body cavity completely lined with epithelium (cellular tissue), that may be divided into 2 to 4 compartments. 6. Well-developed internal skeleton of cartilage and bone, separated into sternum) and axial skeleton appendicular (skull, skeleton vertebrae, ribs, (girdles and appendages). 7. Highly developed brain enclosed by skull, and nerve cord enclosed by vertebrae. This provides advanced neural structures that are highly protected from damage. 8. Well-developed sense organs (eyes, ears, nostrils) located on the head (cephalisation). 9. Respiratory system, including either gills or lungs, and located closely to the pharynx or throat. 10. Closed circulatory system with ventral heart and median dorsal artery. 11. Genital and excretory systems closely associated, utilizing common ducts and pathways. Hence also known as the urinogenital system 12. Digestive tract with two major digestive glands (liver and pancreas) that secrete the bile and pancreatic juice into it.