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Vertebrates and Chordates

Vertebrates are a subphylum within the phylum Chordata
•Chordates are bilaterian animals that belong to the clade of animals known as Deuterostomia
•Two groups of invertebrate deuterostomes, the urochordates and cephalochordates, are more closely related to vertebrates than to other invertebrates


Derived Characters of Chordates

All chordates share a set of derived characters
•Some species have some of these traits only during embryonic development
•Four key characters of chordates:
–Dorsal, hollow nerve cord
–Pharyngeal slits or clefts
–Muscular, post-anal tail



The notochord is a longitudinal, flexible rod between the digestive tube and nerve cord
•It provides skeletal support throughout most of the length of a chordate
•In most vertebrates, a more complex, jointed skeleton develops, and the adult retains only remnants of the embryonic notochord


Dorsal, Hollow Nerve Cord

The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube dorsal to the notochord

•The nerve cord develops into the central nervous system: the brain and the spinal cordBIO


•In most chordates, grooves in the pharynx called pharyngeal clefts develop into slits that open to the outside of the body
•Functions of pharyngeal slits:
–Suspension-feeding structures in many invertebrate chordates
–Gas exchange in vertebrates (except vertebrates with limbs, the tetrapods)
–Develop into parts of the ear, head, and neck in tetrapods


Muscular, Post-Anal Tail

•Chordates have a tail posterior to the anus
•In many species, the tail is greatly reduced during embryonic development
•The tail contains skeletal elements and muscles
•It provides propelling force in many aquatic species



(Branchiostoma lanceolatum (Amphioxus))

•Lancelets(Cephalochordata) are named for their bladelike shape
•They are marine suspension feeders that retain characteristics of the chordate body plan as adults



•Tunicates(Urochordata) are more closely related to other chordates than are lancelets
•They are marine suspension feeders commonly called sea squirts
•As an adult, a tunicate draws in water through an incurrent siphon, filtering food particles



Cordates with heads, allowing for active predation. First arose at 530 Ma during Cambrian explosion. 

Craniates share some characteristics: a skull, brain, eyes, and other sensory organs, as well as a heart with at least 2 chambers, red blood cells, and kidneys

They have a higher metabolism and are more muscular than lancelets and tunicates

In aquatic craniates, the pharyngeal clefts evolved into gill slits

Image: Haikouella, a skullless transition species from the Cambrian explosion (530 Ma)


Hagfishes (Myxini)

The least derived surviving craniate lineage

Possess a cartilaginous skull and axial rod of cartilage derived from the notochord, but no jaws or vertebrae

Purty, aren't they


Subphylum Vertebrata

Chordates with vertebrae (!)

Evolved during Cambrian period, were more efficient at hunting and avoid being hunted

Have the following characteristics:

Vertebrae enclosing a spinal cord

An elaborate skull

Fin rays (in aquatic species)


Lampreys (Petromyzontida)

Oldest living lineage of vertebrates

Jawless, inhabit various freshwater and marine environments

Have cartilaginous segments enclosing notochord and arching partially over the nerve cord


Infraphylum Gnathostomata

99% of all living vertebrates (which is to say all the rest)

Have jaws that may have evolved from skeletal supports of pharyngeal slits

Have an enlarged forebrain associated with smell and vision

aquatics possess the lateral line system, which is sensitive to vibrations


Class Chondrichthyes

Cartilaginous fish (sharks, rays, etc)

Possess a primarily cartilaginous skeleton which evolved from an ancestral mineralised skeleton



Superorder Selachimorpha

Sharks! Most of which:

Are swift swimmers with streamlined bodies

Are carnivores

have a short digestive tract, the surface area of which is increased by a ridge called the spiral valve

have acute senses!

Shark eggs are fertilised internally but can develop in various ways:

Oviparous: eggs hatch outside the mother's body

Ovoviviparous: embryo develops within uterus, is nourished by egg yolk

Viviparous: embryo develops within the uterus and is nourished through a yolk sac placenta from the mother’s blood


Superclass Osteichthyes

The largest group of vertebrates.

include bony fish and tetrapods

•Nearly all living osteichthyans have a bony endoskeleton
•Aquatic osteichthyans are the vertebrates we informally call fishes
•Most fishes breathe by drawing water over gills protected by an operculum
•Fishes control their buoyancy with an air sac known as a swim bladder


Class Actinopterygii

Ray-Finned Fishes

Include the vast majority of what we would recognise as "fish"

The fins, supported mainly by long, flexible rays, are modified for maneuvering, defense, and other functions


Class Sarcopterygii

Lobe Fins

have muscular pelvic and pectoral fins

Three lineages survive and include coelacanths, lungfishes, and tetrapods