ORIGEN AND

CLASSIFICATION OF VERTEBRATES

Essam Eldin Abdelhady Salama

Origen of vertebrates • The efforts of vertebrate paleontologists to discover

countless fossils.• These fossils tell us where the living vertebrates had come

from • They show us views of the different areas of the worlds

Con.• Vertebrates are a

minor twig on the ‘Universal Tree of Life’.

Con.•Molecular studies through the 1990s showed that previous

conceptions of the tree were wrong, and that the fundamental splits in the tree of life were all among Bacteria, separating the two major groups Bacteria and Archaea. •Modern studies confirm that a major clade within Metazoa,

the animals, is supported by both morphological and molecular evidence.

Con. • The familiar plants, animals and fungi are part of Eukaryotes,

the major group characterized by complex cells with a nucleus,• Examination of living animals; • The vertebrates are members of a larger group, termed the

Phylum Chordata, and their closest living relatives• Vertebrates are all the animals with backbones, the fishes,

amphibians, reptiles, birds, and mammals.

Classification

PHYLUM CHORDATA• PHYLUM CHORDATA• Subphylum I. Urochordata or Tunicata, the tunicates or sea squirts.

Subphylum II. Cephalochordata or Acranla, Amphioxus and its allies: Subphylum III. Vertebrata or Craniata, all chordates with an endoskeleton of cartilage or bone or a combination of these.

Vertebrate Classification • All animals are consumers; that means that they do not

make their own food. They need to eat other living things in order to survive. • Animals that eat meat are carnivores (kahr-nuh-vohrs). • Some animals only eat plants. They are herbivores (hur-buh-

vohr). • Some animals eat both plants and animals. They are

omnivores (om-nuh-vawrs).

CHORDATES• Four features characterize the chordates and have played an

important role in the evolution of the phylum• 1. A single, hollow nerve cord runs just beneath the dorsal

surface of the animal. In vertebrates, the dorsal nerve cord differentiates into the brain and spinal cord.

• 2. A flexible rod, the notochord, forms on the dorsal side of the primitive gut in the early embryo and is present at some developmental stage in all chordates.• The notochord is located just below the nerve cord. The

notochord may persist throughout the life cycle of some chordates or be displaced during embryological development as in most vertebrates by the vertebral column that forms around the nerve cord.

Con.• 3. Pharyngeal slits connect the pharynx, a muscular tube

that links the mouth cavity and the esophagus, with the outside. In terrestrial vertebrates, the slits do not actually connect to the outside and are better termed pharyngeal pouches. Pharyngeal pouches are present in the embryos of all vertebrates. They become slits, open to the outside in animals with gills, but disappear in those lacking gills. The presence of these structures in all vertebrate embryos provides evidence to their aquatic ancestry.

• 4. Chordates have a postanal tail that extends beyond the anus, at least during their embryonic development. Nearly all other animals have a terminal anus.

• All chordates have all four of these characteristics at some time in their lives. For example, humans have pharyngeal slits, a dorsal nerve cord, and a notochord as embryos.• As adults, the nerve cord remains while the notochord is

replaced by the vertebral column and all but one pair of pharyngeal slits are lost. This remaining pair forms the Eustachian tubes that connect the throat to the middle ear.

Nonvertebrate chordates have a notochord but no backbone

Tunicates The tunicates (subphylum Urochordata) are a group of about 1250 species of marine animals. Most of them are sessile as adults, the larvae having a notochord and nerve cord.

Con.The adults exhibit neither a major body cavity nor visible signs of segmentation.The pharynx is lined with numerous cilia, and the animals obtain their food by ciliary action. The cilia beat, drawing a stream of water into the pharynx, where microscopic food particles are trapped in a mucous sheet

Lancelets (Amphioxus)Lancelets are scale less, fishlike marine chordates a few centimeters long that occur widely in shallow water, partly buried, throughout the oceans of the world.The notochord runs the entire length of the dorsal nerve cord and persists throughout the animal’s life.

Con.Lancelets have many more pharyngeal gill slitsLancelets feed on microscopic plankton, using a current created by beating cilia that lines the oral hood, pharynx, and gill slits, they provide an exit for the water and are an adaptation for filter feeding.