Comparative Anatomy - ZOO 420 2(1+1) Zoology Department Dr. Elsayed Younis

The skeleton of Reptiles - The skeleton of reptiles is built up on the same plan like that of the amphibians with some differences. The axial skeleton The skull :- is the skeleton of the head. It passes through three developmental stages ( membranocranium, chondro- cranium and osteocranium ). The skull is usually not completely ossified, where there are some membranous parts(side walls).

The skull consists of number of bones greater than those of the amphibians but less than of bony fishes. The skull is divided of two regions:- 1- neurocranium (cranium , sense capsules: olfactory or nasal capsules , optic capsules and audito or otic capsules ) 2- splancocranium (mandibular arch, hyoid arch, and branchial arches) The vertebral column is differentiated into five regions ( cervical, thoracic, lumbar, sacral and caudal vertebrae.

The vertebrae are all built up on the same plan, and resemble each other very closely. ( vertebra usually consists of centrum, neural arch, neural foramen, neural spine and transverse processes). Except the first two vertebrae (atlas and axis) which modified to help in the rotation of the head on the neck. The vertebrae are vary in number in the different taxa of reptiles. Most reptiles have seven mobile cervical vertebrae including atlas and axis.

The thoracic vertebrae carry well developed ribs, some of which articulate with the sternum Reptiles have 8-50 trunk vertebrae ( thoracic and lumber ). Reptiles have 2 to 3 sacral vertebrae with long lateral T. processes that articulate with the ilium. The caudal vertebrae are 12 or more. Some reptilian taxa including many lizards and a few snakes have the ability to shed the tail usually in response to attack and regenerate it later. The sternum of reptiles is mostly cartilaginous (cartilaginous plate) .

The appendicular skeleton pectoral girdle :- consists of two symmetrical halves, each half consists of ( coracoid, clavicle and scapula ) pelvic girdle :- consists of two symmetrical halves, each half consists of three parts ( ilium, ischium and pubis ) In turtles (characteristic of all turtles) the pectoral and pelvic girdles are located within the shell. Snakes have lost the pectoral girdle. Most species have vestige of the pelvic girdle (remnants of the ilium).

Fore-limbs :- consists of two bones ( humerus, radius and ulna which called radio-ulna ) and ended by the fore foot (hand) which have variable number of digits according to the taxa of reptiles. Hind-limbs: consists of three bones ( femur, tibia and fibula which called tibio-fibula) ended by the hind foot which have 5 digits. Snakes have lost all appendages. The fore-limbs of some species (turtles) are modified as flippers.