Gnathostomata, Biology tutorial

Features of Superclass Gnathostomata (jaw- bearing animals):

  • The vertically biting tool known as jaws basically composed of two endoskeletal elements, palatoquadrate and Meckelian cartilage, and number of dermal elements known as teeth, at times joined to large dermal bones
  • Paired appendages paired pectoral and pelvic fins supported by the internal skeleton that supports more effective locomotion
  • Interventrals and basiventrals in backbone.
  • Gill arches, that support/hold gills, lie internally to gills and branchial blood vessels, unlike gill arches of all jawless craniates, that are external to gills and blood vessels
  • The horizontal semicircular canal in inner ear
  • Gnathostomes comprise rays, sharks, chimaeras, ray-finned fishes, lobe- finned fishes and land vertebrates comprising humans.

Characteristics of Class Placodermi (Extinct):

The class Placodermi is considered as first set of fish with jaws but is now extinct. Members of the class had the given characteristics:

  • Well-developed fins and armour plating
  • Dermal armour comprising of head armour and thoracic armour. In thoracic armour, primary dermal plates form the complete ring around the body and always comprise at least one median dorsal plate
  • Dwell in both freshwater and marine environment
  • Large size (up to 10 m)
  • Lower jaw and teeth present
  • Characteristics of the Class Chondrichthyes (Cartilaginous fish)

Cartilaginous fishes contain the given characteristics:

  • Internal skeleton is made up of cartilage (rather than bone)
  • Body scales are placoid (tooth-like) with bony base
  • Jaws suspended by 2 gill arches
  • Swim bladder or lung absent; contain oil-filled liver to give buoyancy
  • Claspers (modified pelvic fins) present in males for internal fertilization
  • Notochord present in young and slowly replaced by the backbone of cartilage in adult
  • Gills not covered by operculum

Class Chondrichthyes is separated in two extant subclasses:

Subclass: Elasmobranchii (sharks, rays and skates)

Subclass: Holocephali (chimaera, at times known as ghost sharks). Members of class Chondrichthyes contain backbone which is composed of cartilage. They are not fishes you come across frequently because of their marine.

Features of Class Osteichthyes (Bony fish)

Members of this class have the given features:

  • Bony endoskeleton
  • Body covered by cycloid scales (thin and round bony scales)
  • Paired pectoral and pelvic fins supported by bony rays
  • Bilaterally symmetrical tail fin
  • Visceral cleft as separate gill openings covered by a bony flap - the operculum.

The Osteichthyes are characterized by endochondral ("spongy") bone in endoskeleton, dermal fin rays composed by lepidotrichiae (altered, tile-shaped scales), and 3 pairs of tooth-bearing dermal bones lining jaws (dentary, premaxillary and maxillary). Osteichthyes comprise 2 main subclasses, Actinopterygii and Sarcopterygii.

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