Chordates and Notochord Overview

Notochord and Nervous System

• In lower vertebrates, the notochord serves as the primary axial support throughout life, while in higher vertebrates, it is replaced by the vertebral column.
• Located on the ventral side of the neural tube, the notochord is critical for body support and development.
• The dorsal tubular nerve cord, unique to chordates, is positioned above the alimentary canal and forms a tube through ectodermic folding during embryonic development.
• The anterior portion of the nerve cord enlarges to form the brain, protected by a bony or cartilaginous cranium in vertebrates.
• Pharyngeal slits develop from the endodermic lining of the pharynx and function in filter feeding in primitive chordates, while in fish, they adapt for respiration.

Pharyngeal Slits and Endostyle

• Pharyngeal slits led to the evolution of structures like the Eustachian tube, middle ear cavity, tonsils, and parathyroid glands in tetrapods.
• The endostyle, often considered part of the thyroid gland, secretes mucus and iodinated proteins, crucial for trapping food particles.
• The post-anal tail, present in larval tunicates and amphioxus, is involved in locomotion and efficiency in movement, with mobility in mammals varying from lost (in humans as coccyx) to functional.

Evolutionary Origins

• Chordates likely share a common ancestor with echinoderms and hemichordates, emerging during the Cambrian period around 570 million years ago.
• The evolutionary connections between chordates and their primitive relatives are contested due to the lack of fossil evidence of early forms.
• Shared derived characteristics like pharyngeal slits suggest close relations with hemichordates, while calcichorda fossils propose links to early chordates.

Nervous and Sensory Systems

• As vertebrates transitioned from filter feeding to predation, their sensory systems adapted significantly, resulting in a tripartite brain.
• Key sensory organs evolved for vision (eyes), hearing and balance (inner ears), taste, and electroreception, aiding in effective predation.
• Neural crest and epidermal placodes in embryo development give rise to vital structures such as cranial features and sense organs.

Vertebrate Characteristics

• All vertebrates possess five chordate features plus additional structures: endoskeleton, muscular pharynx, muscular digestive tract, and specialized sense organs.
• Vertebrates generally have separate sexes with sophisticated reproductive structures, including gonads with ducts that connect to a cloaca or anal opening.
• Limbs supported by girdles and an appendicular skeleton are characteristic of most vertebrates.

Early Vertebrate Fossils

• Earliest vertebrate fossils (ostracoderms) trace back to the late Cambrian and Ordovician, but earlier fish-like vertebrates were discovered dating back to 530 million years ago.

Subclass Elasmobranchii

• Elasmobranchii within Chondrichthyes include sharks (Euselachii) and rays (Batoidei), distinguished by separate gill openings and unique sensory systems.
• Characteristics include a cartilaginous endoskeleton, placoid scales, urea retention mechanism, and claspers in males for internal fertilization.
• Orders within Elasmobranchii encompass diverse species, exhibiting specific traits such as nictitating membranes or adaptations for pelagic living.

Subclass Holocephali (Chimaeras)

• Chimaeras, also known as ratfish, diverged from shark lineage over 360 million years ago, with limited modern species (31).
• Their diet is varied, focusing on seaweed and invertebrates, and they possess unique flat jaws and a venomous spine for defense.
• Chimaeras inhabit temperate oceans and display a smooth, scale-less skin structure, further indicating their distinct evolutionary path.