Necturus Skull

By Michael Boone and Miranda White (2000)

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The Necturus maculosus, mudpuppy, skull is widely used as a representative of Class Amphibia, however, Necturus is neotenic and still possesses some larval characteristics not seen in other adult amphibians.  These larval features should not be confused with primitive adult characteristics.  The Necturus is a good model for ancestral tetrapods because it has retained more of the primitive features from its ancestors than mammals, but many fewer than other amphibians.  The chondrocranium, visceral skeleton, and dermal bones that are present in the head region of fishes are also present in amphibians.  However, only three bones remain in the chondrocranium (exoccipital, opisthotic, and prootic) and one in the visceral arches (quadrate).  The remaining bones of the skull are dermal in origin.  Some other differences between the Amia skull and the Necturus skull are the operculum found in most teleosts is lost and the visceral arches are also greatly reduced.  The selection pressures toward these changes include the reduction of the gills with the movement onto land and the formation of a neck.  Likewise, the trend throughout evolution towards a decrease in the number of bones in the skull is seen in the Necturus skull.  In the lower jaw, there is a reduction of bone with only three dermal bones covering the mandibular cartilage of Necturus.  The lower jaw articulates with the quadrate of the skull.  Other tetrapodâs lower jaws articulate with the squamosal bone of the skull to provide more strength needed for the exploitation of food resources available on land.  The teeth of Necturus are small, conical and homodont, making them essentially fishlike.  Teeth can be found on the premaxilla, vomer, and pterygoid bones of the upper jaw and the dentary, angular, and splenial bones of the lower jaw.

Dorsal View

1 Premaxilla  2 Frontal  3 Vomer  4 Pterygoid  5 Quadrate  6 Squamosal  7 Opisthotic

       8 Parietal

Ventral View

1 Premaxilla  2 Vomer  3 Pterygoid  4 Quadrate  5 Prootic  6 Parasphenoid 7 Opisthotic

     8 Exoccipital

Lower Jaw

Necturus Axial Skeleton

With the movement to land the vertebral column became more important in locomotion and in maintaining posture.  Thus, amphibians have more completely ossified vertebrae than do fishes.  Overlapping zygopophysis, which can be seen in the Necturus skeleton, holds these vertebrae together.  The postzygopophysis of one vertebra lies on top of the prezygopophysis of the next.  The centrum shape of the Necturusâ vertebrae is amphicoelous, the centrum on both sides of the vertebrae is concave.  The arrangement of the zygopophysis and the centrum shape allow for lateral bending but prevent vertical bending.   The movement from water onto land was the primary factor in regional specialization of the axial skeleton over evolutionary time.  As the first terrestrial vertebrates lost their gills and the bony connection between the skull and pectoral girdle developed, the evolution of the neck began.  The development of the neck in early tetrapopds is due, in large part, to the introduction of the cervical vertebrae, which allowed for increased mobility of the head.  This allowed them to scan the environment for predators and prey more easily.  The regional specialization continues with the development of the one sacral vertebra, which is a modification to withstand the push against the earth.  The trunk vertebrae stretch between the cervical and sacral vertebrae and serve as attachment sites for the ribs.  The Necturus ribs are short, primitive ribs.  The ribs do have two heads, the caput and tuberculum, which continues to be seen in later vertebrates as well.  The last type of ribs seen on the Necturus is the caudal vertebra, which lack ribs and bear hemal arches as seen in Squalus.


 Vertebrae (dorsal view)

1 Rib  2 Transverse process  3 Prezygapophyses 4 Vertebral arch  5 Postzygapophyses

Necturus Appendicular Skeleton

As evolution carried the vertebrate to land, the fin was evolved into a more active limb.  The forelimb consists of three segments:   the brachium (humerus), the antebrachium (radius and ulna), and the manus (carpals, metacarpals, phalanges). The hindlimb consists of the corresponding bones in the pelvic girdle: the os femoris (femur), the shank or crus (tibia and fibula), and the pes (tarsals, metatarsals, and phalanges).  There are many homologies between vertebrate appendages.  There are four general characteristics that transcend the vertebrae appendicular skeleton:  all bones are single and proximal, there are two distal bones, phalanges are present, and the same basic joints exist.  As these appendages became more important, the girdles became bigger and stronger. Since the Necturus is neotenic, many parts of its girdles are unossified.  Therefore, the basic plan of the appendicular skeleton shows very little change from Sarcopterygians because the Necturusâ body is still on the ground.   However, the basic plan and shape is representative of early terrestrial vertebrates.  In early amphibians and reptiles, the humerus and femur extend laterally and are held horizontally from the ground when the animal moves.  The distal part of the limbs is held vertically to the ground.

 Pectoral Girdle

1 Ulna  2 Radius  3 Metacarpals 4 Phalanges  5 Procoracoid process  6 Trunk vertebrae

     7 Humerus  8 Cervical vertebrae

Pelvic Girdle

1 Fibula  2 Tibia  3 Femur 4 Puboischiadic 5 Ilium