The term “vertebrate,” designates one of a group of animals with a spinal column. The Vertebrates are characterized by
the occurrence of a longitudinal dorsal skeletal rachis transformed into vertebrae, sometimes cartilaginous, bearing certain phosphatic hard tissues, and having a cranium or equivalent. This subphylum is named for the vertebral column (a series of compact bony elements) that surrounds the nerve cord.
Vertebrates are very numerous and about 40,000 living species have been described. These animals are easy to find and can be seen with the naked eye or reside in places inhospitable to humans, like the sea floor. Some have bones and others cartilages. Some of them have jaws, lungs, legs, feathers etc.
Feeding and Digestion
Vertebrates have a complete digestive system with complex accessory glands. They feed on a variety of feeds and most of them have hinged jaws. The food of the earlier vertebrates was collected by ciliary action but this habit has long been abandoned and only in rare cases today (such as planktivorus fishes, some whales etc).
The animals have a closed circulatory system i.e. a circulatory system in which blood flows in blood vessels. The blood system shows two of the most vertebrate features namely:
· Presence of a heart that has at least three chambers and thus provides a rapid circulation
· Presence of haemoglobin within corpuscles serving to carry large amount of oxygen to tissues. Efficiency of this system must have been a major factor in producing dominance of these vertebrates.
The excretory system of vertebrates consists of mesodermal funnels leading primarily from the coelom to the exterior. It may be that this type of kidney arose in connection with the abandoning of marine water for fresh water. It is believed that all vertebrates passed fresh water stage and it is significant that all except the hagfish have fewer salts in their blood than there is in marine water. Elaborate organs for regulation of osmotic pressure have been developed and they play a large part this regulation.
Vertebrates reproduces sexually, asexual reproduction don’t occur in these animals. Most of them are dioecious i.e. have separate sex. Some are oviparous, others ovoviviparous and some viviparous.
Vertebrate evolutionary relationship
Vertebrates have a long history on this earth -- more than 500 million, from the late Cambrian up to today. The origin of these vertebrates is still under debate but the first vertebrate is believed to have developed from sessile form such as ascidians that had a free swimming larva. The vertebrate body organization may have arisen as an adaptation for more active life style. This probably occurred by a process known as neoteny in which the length of the larval period is gradually extended until eventually sexual maturity is reached without undergoing metamorphosis and then the animal retains its larval form and features as a sexually reproducing adult. These first vertebrates lacked jaws, like the living hagfish and lampreys. Jawed vertebrates appeared 100 million years later.
Walter Garstang noted from study of marine animals that both echinoderms and chordates are deuterostomes (in which the blastopore forms the anus), while most other possible ancestors of Chordates are protostomes (the blastopore forms the mouth). This inspired Garstang to suggest an alternate route of evolution: from echinoderms to chordates.
There are many important differences between chordates and echinoderms. Most importantly, adult echinoderms show little likeness to chordates: echinoderms are radially symmetric, possess calcium carbonate plates in their skin and have tube feet. Garstang made the radical suggestion that perhaps it was echinoderm larvae, not adults, which had given rise to chordates.
Echinoderm larvae, like chordates, are bilaterally symmetric. Especially notable are their similarities to larvae of hemichordates, which are a step closer to chordates as they share two of the five most commonly noted chordate characteristics, namely a hollow neural tube and pharyngeal slits.
Garstang's idea has been expanded and is supported by many lines of evidence. Perhaps most interesting and compelling is the fact that some amphibians can stay in larval form and still reach sexual maturity—this shows that echinoderm larvae could, theoretically, have become sexually mature and simply stopped morphing into adults, instead evolving into chordate ancestors. Species that show this refusal to leave the larval stage include mud puppies and other salamanders, which either partially or completely show neoteny (also called pedomorphism): retention of juvenile traits or phenotypes after sexual maturity. These first vertebrates lacked jaws, like the living hagfish and lampreys. Jawed vertebrates appeared 100 million years later.
Garstang's hypothesis was revolutionary for both its time and idea: it suggests that not only may single species evolve, but that single life stages of species may evolve into separate organisms. The hypothesis, which Garstang proposed in the early 20th century, seemed far-fetched at the time of its conception and did not receive support until after Garstang's death.
Adaptation of vertebrates
· Vertebrates have paired appendages that facilitate locomotion
o In fish the fins facilitate balance, propulsion and directional movement.
o In other vertebrates fins have developed into jointed limbs for movement and support on land. Jointed limbs are well adapted for moving over surface that are not smooth or level.
· Vertebrates have ciliated pharyngeal gill structures which are modified for gaseous exchange.
o The gills are very proficient in gas exchange and the respiratory efficiency exceeds that of invertebrates.
o Oxygen is distributed around the body by a well developed circulatory system.
· Vertebrates have an endoskeleton which has several advantageous features. It is composed of living tissue and thus grows with the animal rendering moulting unnecessary.
o It permits greater body size by providing a lighter support structure.
o It provides an increased surface area for muscle attachment permitting a wider range of movement.
· Vertebrates have a highly developed and complex nervous system.
Vertebrates are classified as below
├─Class Placodermi — extinct (armored gnathostomes)
├─Class Chondrichthyes (cartilaginous fish)
├─Subclass Elasmobranchii — (plate gills)
├─Subclass Bradyodonti (slow tooth)
├─Class Osteichthyes (bony fish)
├─Subclass Acathodii (spiny)
├─Subclass Actinopterygii (ray-finned fish)
└─Subclass Sarcopterygii (lobe-finned fish)
├─Class Amphibia (amphibians)
├─Class Aves (birds)
├─Class Mammalia (mammals)
1. Superclass Agnatha
Super class Agnatha consists of an ancient group of animals similar to fish but with some very noticeable differences. The term Agnatha means “no jaws” in Greek. The Agnathans lack jaws and paired fins. As with sharks (another ancient group), the internal skeleton consists primarily of cartilage.
The only living Agnathous animals are the Cyclostomata (ringed-mouthed) lampreys and the hagfishes but the first vertebrates to appear in most fossil series mostly heavily armored and hence known as ostracoderms (bonny skinned) also show Agnathous condition and have some other features common with the cyclostomata.
· Agnathans have no jaws
· Most are bottom dwellers and wriggle along the sea floor or streambeds.
· They don’t have any appendages, external body parts.
· Digestion: Agnatha don’t have an identifiable stomach. Their metabolism is slow and due to this they don’t have to eat as much. They are Fluid feeders
· Osmoregulation: Some Agnathans are Osmoconformers e.g. hagfish and some are osmoregulators. Osmoconformers normally change the osmotic ionic conc. of their body to conform to that of the surrounding environment thus remain isotonic.
· Circulatory system: They have a simple chambered hearts for blood circulation. It is a closed circulatory system with 3 types of blood vessels. Agnathans have a single circulatory system.
· Reproduction: Fertilization and development is external. There is no parental care. They are Oviparous
The first jawless fishes were Ostracoderms (shell skin) which is an artificial designation and not a monophyletic group.
Characteristics of Ostracoderms
a) First fossils have well developed external bone, no internal bone: Cambrian (500 mya)
b) Dominant for 100 my (gone by 380 mya)
c) First fossils: marine; later marine and freshwater
Features of Ostracoderms
a) No jaw, muscular feeding pump (filter feeders)
b) Body armor - true bone
c) Some had paired fin-like appendages, but not true fins with bony support
d) Heterocercal tail
Two main Classes of Ostracoderms are Cephalaspidomorphi ( ancestor to lamprey) and Class Pteraspidomorphi (ancestor to hagfish)
i) Class Pteraspidomorphi
Pteraspidomorphi is an extinct class of early jawless fish which lived from the Early Ordovician to the Late Devonian (i.e. from 470 to 370 million years ago). The fossils show extensive shielding of the head. Some species may have lived in fresh water. The taxon contains the subgroups Heterostraci and Thelodonti.
Pteraspidormorphs have long been regarded as closely related -or even ancestral- to jawed vertebrates, but the few characteristics they share with the latter are now considered as primitive for all vertebrates.
· They were jawless and possessed a massive dermal skeleton with the body divided into a scaly trunk and a tail.
· They are characterized by possession of an extensive dermal armor covering the head having large, median, ventral and dorsal plates or "shields”.
· Their dermal plates and scales are made up of acellular bone, or aspidine, but this is thought to be a primitive condition for the dermal bone of all bony vertebrates.
· Pteraspidomorphs have no calcified endoskeleton however some show traces of calcified cartilage.
· They have no other fin than the caudal fin.
· The internal surface of the dermal plates -at any rate in heterostracan- shows impressions of the gills, brain, two vertical semicircular canal and distinctly paired nasal capsules.
· The nasal capsules of heterostracans seem to have opened ventrally into a separate median duct comparable to the nasopharyngeal duct of hagfishes.
· Most Pteraspidomorphs were marine but lived very near to the shore, in lagoons and deltas. Some groups are thought to have been fresh water. They were certainly bottom-dwellers, as shown by traces of abrasion of the ventral surface of their head shield. Examples, Pteraspis, Psammosteus, etc
Limitations of Ostracoderms
· Habitat limitations
o Restricted to benthos
o Weak swimmers due to heavy armor that also result into inflexibility
· Food limitations
o No jaws - restricted to plankton, suspended organics - slow growth
2. Class Cephalaspidomorphi
· This is a taxon of jawless fishes named for the Cephalaspids, a group of Osteostracans. Most of the members of this group are extinct; however, it interests modern biologists because it includes the lampreys.
· Members of this taxa have a single nostril and are jawless fish
Order Osteostraci =Cephalaspida
The order contains fossil Agnathans that show more similarity to modern cyclostomes than the pteraspids.
· First fossils have well developed external bone, no internal bone (bone shell)
· First fossils were marine later they were found in marine and freshwater bodies.
· No jaws, they are filter feeders
· Ventral surface of head was flat and it was covered by small scales
· Some had paired fin-like appendages, but not true fins with bony support
· They have a heterocercal tail
· Had paired eyes and pineal eye
· On the dorsal surface there are sunken areas that are believed to contain electric organs.
Examples Hemicyclaspis, Kiaeraspis etc.
Members include lampreys and hagfish. Lampreys belong to suborder Petromyzontidae (stone suckers) and hagfish to suborder Myxinoidea (slime ones)
Characteristics of Cyclostomes
· Eel- like in structure with a size of 10-90cm in length
· Lack exoskeleton/scales
· Prey on fishes (fluid feeders).
· Notochord persists in adults
· Lack jaws but have rows of horny teeth that move in circular motion
· Axons of neurons are unmyelinated
· Generally marine (some anadromous)
· Skeleton cartilaginous or fibrous
· No paired fins
· No body armor
· Single median nostril
· Series of round gill openings, no true gill archesl
Two Important Sub-Orders of Cyclostomata
Lampreys are found in marine, brackish and freshwater environments. They are the most familiar cyclostomes. Some species are parasitic, attaching their sucker-like mouth to a fish and then using sharp teeth to rasp away at the animal's flesh. The lamprey has a larval stage. The ammocoete larva lives in fresh water, buried in mud. The ammocoete larva has to undergo metamorphosis to change into an adult.
Size and Habitat
Lampreys include marine and freshwater species example are the sea lamprey (Petromyzon marinus), pacific lamprey (Entosphenus tridentate) and freshwater lampreys (Entosphenus similes)
The adult lamprey is an eel like animal about 30 cm long and lives parasitically in the sea probably for about 2.5 years. They then migrate to fresh water where they spawn. Fertilized eggs develop into an ammocoete larvae which don’t have suckers and lives buried in mud feeding on micro organisms for 5 years. After metamorphosis the young adult migrates to the sea. This kind of migration is called anadromy and species with this kind of migration are said to be anadromous.
Adult Lamprey is dark on the back and white below. The body surface is smooth with no scales. The skin is many layered. The outer most cells have striated cuticular border. The skin secretes slime with antibacterial effects. The dermis contains pigmented cells called the chromatophores. The chromatophores are star shaped cells whose pigment is able to migrate hence making the animal pale or dark. This change is especially marked in larvae and it is produced by variation in the amount of pituitary secretion
Lamprey larva is sedentary filter feeders but the adults are ectoparasites primarily of other fishes. They attach to the host with the oral disk and use the keratin plates that cover the tongue and oral disk to rasp a hole through the skin and absorb the fish’s blood and tissue.
Lampreys have a simplified digestive system. Food travels from the mouth through the oesophagus to the intestines. There is no true stomach. They have a liver, gall bladder and bile duct but no separate pancreas.
Lampreys are dioecious. They have external fertilization but there are some modifications of the cloaca in both sex to ensure fertilization and proper placing of eggs in a nest. The lips of the cloaca of a ripe male are united to form a narrow penis like tube. The cloacal lips of a female are enlarged and often red.
The generalized life cycle of lampreys is as summarized below
Parasitic forms are anadromous- they spawn in clear flowing streams in gravel and later juveniles migrate back to the sea. Like salmon – they are semelparous i.e. they invest heavily in reproduction and die after spawning
Lampreys produce small eggs (1 mm) also one female can produce 250,000 eggs
Their larvae and juveniles are called ammocoetes – they live in gravel and filter feed, parasitic forms undergoes metamorphosis as they migrate downstream and adopt a parasitic lifestyle
Nonparasitic lampreys are called brook lampreys - retain the juvenile form, filter feed, and mature earlier and spawn fewer eggs. Brook lamprey are derived from the parasitic form by loss of the parasitic phase. Brook lampreys are not anadromous, they inhabit fresh water bodies.
Circulation and gaseous exchange
The blood vascular system is arranged on the same general plan as in amphioxus but there is a well developed heart. The heart lies behind the gills and it is three chambered. Blood from the heart passes the ventral aorta to the gill pouches (external gill pouches). Oxygenated blood from gill pouches is passed through arteries to organs such as kidneys, gonads, etc. deoxygenated blood from these organs is transported back to the heart through veins so that it may be pumped back to the gills. Blood of lamprey contain haemoglobin as that of other vertebrates. These animals have about 7 pairs of gills.
Excretion and Osmoregulation
The blood of lamprey contains a much higher concentration of salts f than the surrounding water when it is in fresh water and vice versa when in marine water. When they are in fresh water they must deal with the tendency of water to flow in and minerals out.
In fresh water lampreys the excess water that moves in osmotically is removed by passing the filtrate in the kidney and the salts are re absorbed back in the kidney tubules. Fresh water lamprey secretes a lot of urine that is hypotonic to blood. Their bodies are also impermeable to water however not completely impermeable; this reduces the amount of fresh water that may enter their body. Lampreys can also take up Na+ and Cl- from the environment by special ion uptake cells in the gills (this transport of mineral ions is by active transport). Lampreys are therefore the osmoregulators.
Nervous system and Sense organs
Lampreys have a well developed sensory structure including eyes and a lateral line system. In addition to possession of eyes the lampreys have a pineal eye which is normally associated with reproduction, metamorphosis and change in colouration in larvae (in day larvae becomes dark and at night pale). The lateral line provides a distant touch sense. Mechanoreceptors detect vibration and water movement.
The modern hagfish are all marine, living on the bottom and usually in burrows. Rather worm-like in appearance, the hagfish lack paired fins, only having a slight tail fin. They have no eyes and no scales. They also have rows of horny teeth on the tongue which rasp at the food. Surrounding the mouth is a ring of tentacles. Hagfish produce a great deal of slime.
Habitat and Nutrition
Hagfish lives buried in mud or sand and eat polychaetes and other invertebrates as well as scavenging dead and decaying fish. Have keratin plates on teeth. They use their tongue to rasp and burrow into the body of prey usually through the gills or anus and use their keratinized teeth to tear of pieces of the host tissue. They don’t have a stomach
Nervous system and sense organs
Hag fish have poor vision and the lateral line system is greatly reduced. The eyes are functionless rudiments though the animals are sensitive to changes of illumination through skin receptors. They also have sensory tentacles around the mouth (barbels).
Hag fish are monoecious i.e. they have both male and female sex organs but the gonads of only one sex are functional. Fertilization is external and development is direct, there is no larval stage. They produce large eggs with hooks.
Hagfish have up to 16 pairs of gills. Each gill pouch connects through one or more ducts to the pharynx. The median nostril connects to the pharynx; it allows water to pass through the pharynx and gills while food is in the mouth. The internal gill openings combine into a single tube connecting with the pharynx. With this structure feeding doesn’t impede the passage of water through the pharynx and over gills.
Excretion and Osmoregulation
Hagfish are osmoconformers; they are the only vertebrates with body fluids isotonic to seawater. They are also the only vertebrates to have both pronephric (a 1st functional kidney of embryo) and mesonephric kidneys in adult. In most vertebrates the pronephric kidney degenerates in early embryonic development and is replaced by mesonephric kidney. Mesonephros is a full functional kidney in other fish and amphibians but it is an embryonic kidney in birds, reptiles and mammals which is replaced by the metanephros.
An interesting feature of hagfish
Hagfish secrete a lot of mucus, a 50 cm hagfish can fill 8 litres bucket with mucus in minutes. Hagfish mucous is composed of proteins and carbohydrates that bind to water. Mucous glands are along the ventral surface
Functions of slime
· Suffocate prey fishes
· Protection from digestive enzymes
· Discourage other scavengers
· Secure burrow walls
· Defence from predators
Summary of the characteristics of Lampreys & hagfish
1 or 2
Lateral line system
on top of head
External gill openings
hyper- or hypoosmotic
small, no hooks
large, with hooks
Summary of the Key features of Agnathans
· Jaws are absent.
· Paired fins are generally absent.
· Early species had heavy bony scales and plates in their skin, but these are not present in living species.
· In most cases the skeleton is cartilaginous. The embryonic notochord persists in the adult.
· Seven or more paired gill pouches are present.
· The digestive system lacks a stomach
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth".
The Gnathostomata, or gnathostomes, are the majority of the Middle Devonian (-380 million years ago) to Recent vertebrates. They differ from all other craniates or vertebrates in having a vertically biting device, the jaws, which consist of an endoskeletal mandibular arch and a variety of exoskeletal grasping, crushing, or shearing organs, i.e. the teeth, and jaw bones.
It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively — the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would with time become stronger and tougher, being transformed into real jaws.
Gnathostomes are characterized by:
· A vertically biting device called jaws, and which is primitively made up by two endoskeletal elements and a number of dermal elements called teeth, sometimes attached to large dermal bones.
· The skull of a gnathostome (here a shark), are characterized by vertically biting jaws (Y). The gill arches (R) are situated internally to the gill filaments, and the nasal capsules (C) open to the exterior by means of paired nostrils.
· Possess jointed, paired lateral appendages e.g. paired pectoral fins, limbs etc
· Gill arches lie internally to the gills and branchial blood vessels, contrary to the gill arches of all jawless craniates, which are external to the gills and blood vessels.
· Possess three (3) semicircular canals in the inner ear.
· Paired nasal sacs which are independent from the hypophysial tube. Nasal sacs open separately to the exterior by external nostrils. In all extant and fossil jawless craniates, the nasal sacs, which contain the olfactory organs, open into a median duct, the nasohypophysial duct, which takes part to the formation of the pituitary gland and either leads postriorly to the pharynx (e.g. in hagfish) or ends as a blind pouch (e.g. in lampreys and osteostracans). In the gnathostomes, this pouch remains as a thin canal in the palate, the buccohypophysial canal,
· There are numerous other characteristics related to anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to gnathostomes among extant craniates, but cannot be served in fossils.
Discussion of Phylogenetic Relationships
Gnathostomes have long been placed as the sister-group of the Agnatha as Clade. Current phylogenies, however, suggest that the Agnatha are not a clade, and that, among recent craniates, the gnathostomes are the sister-group of the lampreys only. Among fossil craniates, the Osteostraci share the largest number of uniquely shared derived characteristics with the gnathostomes.
Placoderms were armored jawed fishes that first appeared about 420 million years ago (MYA) during the Silurian Period. They had diversified dramatically by the beginning of the Devonian and came to dominate most marine and freshwater ecosystems. Placoderms became extinct during the early carboniferous era (355 MYA).
About 200 genera of placoderms have been discovered, with most of these occurring during the Devonian radiations. The rapid evolution and diversity of placoderms have made them useful in dating Devonian rocks.
Key features of Placoderms
· Placoderms (= plated skin) were named for their heavy armor of dermal bone, which formed large shields on the head and thorax. The rest of their bodies was covered with small bony scales or was without dental armor.
· They had jaws but no ability to extrude them
· Placoderms lacked teeth, but they had sharp bony plates that they used as teeth.
· They had true paired fins
· Most placoderms were less than 30 cm (2 feet) in length, but some members reached or exceeded 6 m (20 ft), making them the first giants of the vertebrate lineage.
· Possess a bony internal skeleton
· Most had a heterocercal tail
Traits shared with Ostracoderms
· Negatively buoyant (due to heavy plates)
· Occupied benthic and near-benthic habitats (epi-benthic)
· Dorsoventrally depressed (common among benthic fishes)
Differences from Ostracoderms
· Placoderms reached much greater sizes
o up to 10 m (33 feet) in length
o Why? - food source, mobility
· Placoderms had slightly lighter and more flexible (articulated) armor