Phylum Chordata A Tribute to the Diversity We Know
Introduction • Members of the Phylum Chordata include animals with which we are probably most likely familiar (including fish, birds, reptiles, amphibians and beasts like ourselves) • As unlikely as it seems, based on embryological evidence, the echinoderms appear to be the most-likely ancestors to the early chordates • Primitive stemmed echinoderms are thought to have shifted from arm-feeding to filter-feeding acquiring a body plan similar to urochordates • Unfortunately, the fossil record is poor and intermediates are lacking. The rest of the evolutionary picture is better documented.
Four structural characteristics set chordates apart from all other phyla: a notochord a pharyngeal gill slits postanal tail a hollow dorsal nerve cord These attributes are always found in the larval forms or early embryo (although they may be absent in the adult). Introduction
The notochord is mainly composed of fibrous connective tissue For those animals in which it persists into the adult form, the notochord provides support (it acts like our backbone) and increases swimming efficiency In animals like ourselves, bony structures called vertebrae develop near the notochord and eventually replace it during embryogenesis 1) The Notochord
Pharyngeal gill slits are cuts in the pharynx that connect to a cavity surrounding the pharynx For organisms in which they remain in the adult, they are often elaborated into respiratory structures (and are sometimes involved in filtering food during feeding). The morphological equivalent of gill silts are seen briefly during our own development (weeks 4-5), but they usually close or develop into other structures Occasionally, the slits do not close, resulting in the newborn having an opening in the neck area (a cervical fistula). 2) Pharyngeal Gill Slits
In all the phyla we have studied up to this point, the anus was terminal (at the tip of the tail) Chordates, on the other hand, follow the anus with a tail of variable length (again, an adaptation for locomotion). In us, the tail is short and fused (the coccyx at the base of your spine). 3) The Postanal Tail
4) Hollow Dorsal Nerve Cord • Our nerve cord, like that of other chordates is hollow (even in the adult). • Well, what else is there to say?!?!?
Let’s start at the very beginning . . . • Phylum Chordata is divided into three subphyla: • Subphylum Urochordata • Subphylum Cephalochordata • Subphylum Vertebrata
Subphylum Urochordata • At first glance, you might mistake this creature for a sponge. • Adult tunicates look like small sacs (about 3 cm tall) and are stationary, lacking a nerve cord, a notochord, and a post-anal tail. • Lacking three of the four distinguishing hallmarks of the chordates, it would seem impossible for these animals to be placed in phylum Chordata.
Subphylum Urochordata • However, tunicates begin life in a larval state, which have a post-anal tail, a nerve cord, and a notochord. • Therefore, these immobile animals with tadpole-like larvae are considered chordates.
Subphylum Urochordata • Urochordates have a notochord that extends from just behind the tail to the head (rather than from head to tail; Urochordata means "tail-cord") • Many urochordates are more commonly referred to as “sea squirts” • Contain cancer-fighting chemicals
Subphylum Urochordata • The body of an adult tunicate is quite simple, being essentially a sack with large gill structures that form two siphons through which water enters and exits. Water is filtered inside the sack-shaped body.
Subphylum Cephalochordata • Lancelets are common bottom-dwelling forms that possess all four chordate characteristics • They dig into the sand and lie with their anterior end protruding from the burrow. • Unlike the urochordates, the notochord extends along the entire length of their body. • This structure imparts rigidity to their body and permits more coordinated swimming movements.
Subphylum Vertebrata • Members of the Subphylum Vertebrata differ from the urochordates and cephalochordates in having the notochord replaced by a vertebral column composed of bone and/or cartilage. The vertebral column, along with the cranium, limb girdles, and limbs, make up the endoskeleton. This internal skeleton is an adaptation for efficient locomotion, as was the notochord
Subphylum Vertebrata • Subphylum Vertebrata has several divisions you need to be familiar with: • Superclass Agnatha – Jawless Fish; Lampry Eel; Ostracoderm (fossil) • Superclass Gnathostomata • Class Placodermi – First Jawed Fish (Fossils) • Class Chondricthyes – Cartilagenous Fish; Sharks; Rays • Class Osteicthyes • Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea Horse • Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth
Subphylum Vertebrata • Subphylum Vertebrata has several divisions you need to be familiar with: • Superclass Tetrapoda • Class Amphibia • Order Caudata – Tailed Amphibians; Salamanders; Newts • Order Anura – Tailless Amphibians; Frogs; Toads • Order Gymnophiona – Caecillians • Class Reptilia • Subclass Anapsida – Turtles; Tortoises • Subclass Diapsida – Crocodiles; Snakes; Lizards • Class Aves • Subclass Archaeornithes – Archaeopteryx (Extinct) • Subclass Neornithes – All Other Birds • Class Mammailia • Subclass Prototheria – Monotremes – Platypus; Echinda • Subclass Metatheria – Marsupials – Kangaroo; Koala • Subclass Eutheria – Placentals
Superclass Agnatha • Agnatha (Jawless fish) are primitive fishes with a fibrous skeleton and an eel-like body • They lack a jaw as well as the scales and paired fins we usually associate with fish • Agnathans are mostly represented as heavily armored fossil forms, but today’s living species are smooth-skinned, completely unprotected by armor
Superclass Agnatha • Many species are parasitic (they attach to the outer surface of a fish with their sucker-like mouth) • Rasping teeth (arranged in a whorl) cut into the host. The lamprey then sucks blood from the wound (a fish hickey?). • When it is finished its bloodmeal, the fish is released. The injured fish usually dies from blood loss or infection.
Superclass Agnatha • The agnaths are considered to be an evolutionary dead end, a side branch in the phyletic tree that did not lead to the more advanced fish • The first agnaths were ostracoderms • Though extinct, they give us clues into how vertebrates evolved from the cephalochordates
Superclass Agnatha • Notice the similarities of the agnath larve to the lancelet • There are two main classes under Agnatha: • Class Petromyzontes (lampreys) • Class Myxini (hagfish)
Superclass Gnathostomata • Gnathastomata (Jawed fish) contain a verticle biting appendage • Jaws are what makes it possible for some gnathostomes to crack nuts, rip of sections of meat, and even crop grass. • Teeth have modified into a wide range of forms, from teeth designed to bite off and chew, to teeth designed to strip bark, to teeth designed to inject poison, to teeth grown out into defensive tusks.
Superclass Gnathostomata Superclass Gnathostomata can be broken up into the following classes: • Class Placodermi – First Jawed Fish (Fossils) • Class Chondricthyes – Cartilagenous Fish; Sharks; Rays • Class Osteicthyes • Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea Horse • Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth
Class Placodermi • The Placodermi are characterized by a dermal armor consisting of a head armor and a thoracic armor. • In the thoracic armor, the foremost dermal plates form a complete "ring" around the body and always include at least one median dorsal plate • Jaws probably developed from the anterior visceral arch that first served to support the gills. The presence of jaws and development of paired appendages resulted in efficient eating and locomotion
Class Chondricthyes • Sharks, skates, rays, and chimaeras are all members of the ClassChondrichthyes. • Their endoskeleton is entirely cartilaginous (Chondros = "cartilage"; "icthys" = "fish") and all are carnivorous (as exemplified by the great white shark. • The notocord persists into the adult, and they have both median and paired fins
Class Chondricthyes • They have two-chambered hearts (one auricle, one ventricle). Only deoxygenated venous blood flows through the heart (which is then pumped through gills before going to the rest of the body). • Red blood cells are present, but they're nucleated and oval. • Respiration is by five to seven pairs of gills, each located in a cleft and supported by cartilaginous visceral arches. • Sexes are separate (dioecious), and the gonads are usually paired.
Class Chondricthyes • The caudal fin of sharks differs from that of bony fishes because it is asymmetrical. This heterocercal fin provides both lift and forward thrust for the animal when swimming (an important consideration since sharks are heavier than water and have no swim bladder to keep them buoyant). • Although there are many rows of teeth, only the outer row or two is functional. The inner rows are replacement teeth, which move forward when outer teeth break off. Unlike ours, the upper jaw is not rigidly connected to the braincase and can be moved to open the jaws. • Although most sharks are predatory or scavengers, the largest sharks (whale sharks and basking sharks) feed on planktonic organisms they filter from the sea. • The outer skin layer is the epidermis, beneath which a thick, fibrous dermis can be seen. The outer, hard portion of the scale is enameloid, one of the toughest materials produced in the animal kingdom.
Class Osteicthyes • The bony fish (Osteon = "bone"; "icthys" = "fish") are the most diverse and numerous of all vertebrates • They differ from most of the cartilaginous fishes in having a terminal mouth and a flap (operculum) covering the gills. • In addition, most have a swim bladder, which is ordinarily used to adjust their buoyancy, although among the air-breathing fishes it is attached to the pharynx and serves as a simple lung.
Class Osteicthyes • The skin has many mucus glands and is usually adorned with dermal scales. • Their jaws are well developed, articulated with the skull, and armed with teeth. • They have a two-chambered heart built on the same plan as the Chondrichthyes • The sexes are separate, most are oviparous, and fertilization is usually external.
Class Osteicthyes • There are two subclasses: Subclass Actinopterygii Subclass Sarcopterygii (ray-finned fishes) (lobe-finned fishes)
Subclass Actinopterygii • The Actinopterygii is the larger of the two subclasses • These animals have slender fin rays suporting their fins and lack the odd appendages of the lobe-finned fish. • Most have a symmetrical caudal fin (homocercal tail) and a swim bladder. • This group includes most of the fishes with which you are familiar (bass, goldfish, guppies, sea horses, sturgeons, and tuna).
Subclass Sarcopterygii • Sarcopterygians have a fleshy lobe at the base of their fins that is leg-like in appearance. • They include fossil forms that are ancestors of the amphibians, the true lungfishes, and the coelacanth. • Today's coelacanths are "living fossils," represented by a single species (Latimeria) found near the coast of Madagascar. • Until 1938, when a coelacanth was first captured by chance, they were known only from the fossil record. Since then several dozen have been captured and some of their behavior has been filmed using robotic cameras. • Latimeria is important because it provides an opportunity to compare observations from the fossil record with a living animal.
Superclass Tetrapoda • Tetrapoda means "four feet", and the group was so-named as its members primitively had four limbs, as opposed to fins. • This taxon includes about 3000 extant species of amphibians (frogs, salamanders, and caecilians) and approximately 18100 extant species of amniotes (mammals, reptiles, and birds).
Superclass Tetrapoda • Superclass Tetrapoda has four major class divisions: • Class Amphibia • Class Reptilia • Class Aves • Class Mammailia
Class Amphibia • The ancestors of today's amphibians were the first chordates to venture onto land • Although many changes in the anatomy, physiology, and behavior were required for terrestrial colonization, several pre-adaptations among the early amphibians eased their colonization of terrestrial habitats. • Two barriers, however, keep most amphibians from a totally terrestrial lifestyle: respiration and reproduction
Class Amphibia • Today's amphibians have a moist, glandular skin with no scales (with a few exceptions). • Most have two pair of limbs adapted for walking and/or swimming. Their hearts have three chambers (two auricles and one ventricle) • Respiration is by the skin, lining of the mouth, gills, and/or lungs, depending on the species or stage in their life history. Most cannot survive away from water for very long because they lose too much moisture through their thin respiratory surfaces. • In addition, most amphibians require water for reproduction and have an aquatic larval stage. Fertilization may be internal or external and most are oviparous.
Class Amphibia • There are three surviving orders: • Order Caudata (newts and salamanders) • Order Anura (frogs and toads) • Order Gymnophiona (caecilians) (Extinct Euryops)
Order Caudata • Caudatans (salamanders and newts) have poorly developed limbs and retain a tail as adults (the name Caudata refers to the presence of a tail). • They prey on worms, insects, and small mollusks. Some species have no lungs and depend entirely on cutaneous respiration. Others, such as the mudpuppy (Necturus) and the axolotl (Ambystoma), retain the larval gills as adults.
Order Anura • Frogs and toads belong to the order Anura ("Anura" refers to the lack of a tail in adults). • They differ from the caudates by having a more complex skeletal system with stronger limbs and developmental metamorphosis (from a tailed and limbless polliwog to a tailless limbed adult). • Of all the amphibians, anurans have been most successful in their occupation of terrestrial habitats (including trees).
Order Gymnophiona • Caecilians, as they are more commonly called, are wormlike creatures found in tropical forests. They make their livings by burrowing through the soft soil searching for worms and other tasty invertebrates. • Several adaptations are related to their subterranean habits: They have no legs, are almost totally blind (atrophied eyes are hidden under the skin), and have sensory tentacles. • Their skin is annulated, and some have minute dermal scales. This combination of characteristics makes them look very much like overgrown earthworms.
Class Reptilia • Reptiles have acquired several advances over amphibians that have allowed them to move successfully into terrestrial habitats. • Their skin, for example, is more heavily thickened and is protected with surface scales that are impervious to water. • A reptile's scales are very different in structure from that of fish. The outer layer of skin is a thick layer of dead, keratin-filled cornified cells. These cells are organized into horny scales covering the entire outer surface. • Since reptiles have internal fertilization, water isn't even needed for mating.
Class Reptilia • There are two major subclasses: • Subclass Anapsida – Turtles; Tortoises; some dinosaurs • Subclass Diapsida – Crocodiles; Snakes; Lizards; most dinosaurs • This is based on the presence or absence of certain temporal openings:
Subclass Anapsida • The term Anapsida ("no arch") refers to all those reptile groups that lack skull openings behind the eyes. • Subclass Anapsida contains most of the extremely primitive ancestral reptiles as well as turtles and tortoises
Subclass Diapsida • Most all other reptiles are diapsids (including lizards, crocodiles, snakes, and most dinosaurs) • The main diagnostic physical character for a diapsid is the presence of two openings on each side of the skull; the upper and lower temporal openings. • Birds even exhibit this temporal arrangement
Class Aves • Birds are endothermic (warm-blooded) vertebrates with feathers. Their anterior limbs are modified as wings for flight, while the posterior pair is adapted for walking, swimming, or perching. • Other adaptations related to flight include changes in the skeletal, respiratory, circulatory, reproductive, digestive, and excretory systems. • Today's birds still retain many reptilian characteristics such as similarities in behavior, skull structure, and scales on their beak, legs, and feet.
Class Aves • The long hollow and porous bones of birds are thin and slender to aid in flight. Many bones overtime have fused together to give support. • Teeth have been lost and replaced by a light beak. • Feathers are cornified epidermal appendages that are probably related to scales. They are used for thermoregulation, communication, and as a flight surface. • There are two subclasses of Aves: • Subclass Archaeornithes (Extinct Archaeopteryx) • Subclass Neornithes (All other birds)
Subclass Archaeornithes • The Archaeornithes, are represented by a single extinct species (Archaeopteryx). • The first Archaeopteryx specimen was discovered during the nineteenth century. It was about the size of a crow, had a long reptilian tail and a reptilian skull with no beak. It had three fingers on its wings, each bearing a claw. Because of its small sternum and flexible trunk, it's unlikely that Archaeopteryx was a strong flier. • Its characteristics are so reptilian that, were it not for the feathers fossilized with the specimen, it would not be recognizable as a potential ancestor of birds.
Subclass Neornithes • All birds other than Archaeopteryx belong to the subclass Neornithes. • While most neornithes fly, ratites can not. Examples of living ratites include emus, rheas, ostriches, and penguins. • Neornithes that fly are carinates (they have a large carina).
Class Mammalia • Members of the class Mammalia possess both hair and mammary glands. • Their integument is complex and has many glands used for a variety of purposes: thermoregulation and excretion (sweat glands), communication (scent glands), care of the hair and skin (sebaceous oil glands), and for feeding of the young (mammary glands).
Class Mammalia • They are thermic and have relatively high rates of metabolism. In keeping with their higher metabolic rates, adaptations for efficient feeding include heterodont teeth in most species and a secondary palate to separate the respiratory and food passages (so they can breathe and chew at the same time). • The circulatory systems are efficient, and they have a four-chambered heart with separate pulmonary and systemic circulations. • Their brains are highly developed, fertilization is internal, and most have placental attachment of the young.
Class Mammalia • There are three subclasses based on birthing systems: • Subclass Prototheria (egg-laying monotremes) • Subclass Metatheria (marsupials) • Subclass Eutheria (placental)