Reptiles are ectothermic animals whose body is covered by epidermal scales. They possess monocondylic skull that rests on a long neck made of atlas, axis and other cervical vertebrae. They have two sacral vertebrae, which are fused together to transfer the weight of body onto the hind limbs. Pentadactyle limbs bear sharp claws which help the animal in creeping and climbing. Reptiles were the first vertebrates that laid a large, shelled, cleoid egg that could develop on land. The egg has three membranes, amnion, allantois and yolk sac which help in the development of the embryo on land in dry conditions. Generally in reptiles with big jaws and large head, the skull becomes too heavy for the neck to support it. Therefore vacuities or fossae developed in reptiles to lighten the skull and also to provide space for accommodating powerful jaw muscles.
The skull names come from the word “apse,” which means “arch.” The number and types of arches, or enclosed fenestrae or windows, give the skull types their names. These openings actually give the skull extra space for the attachment of jaw muscles, allowing animals to snap their jaws more forcefully.
Temporal fenestrae have been used to classify amniotes by Osborn, 1903. Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestrae are large holes on the sides of the skull. The function of these holes has long been debated (Case, 1924). Many believe that they allow muscles to expand and lengthen, resulting in greater bulk of jaw musculature. The longer muscle fibres allow an increase in the gape of the jaw to handle larger prey (Pirlot, 1969).
ANAPSID SKULL (found in Cotylosauria and Chelonia)
This is the most primitive type of skull of reptiles that occurred in primitive labyrinthodont amphibians and then in primitive reptiles like Seymoria. The anapsids were the first reptiles to appear in the Carboniferous Period (345 to 280 million years ago). “Anapsid” means “withoutarch“, meaning that they have no fenestrae in their skulls. The anapsids were first represented by small, herbivorous Procolophonids, and the much larger, herbivorous Pareiasaurs.
Chelonia that today includes tortoises and turtles, still possess this type of skull, which has complete skull roofing, without temporal vacuities or fossa. As the skull is completely covered by bones, this type of skull is heavy but strong. Turtles have small head and jaws without teeth and hence the weight of the skull can be borne by the neck, but other reptile that evolved massive jaws and head, must evolve lighter skulls so that it could be supported by the neck and also larger and stronger jaw musculature. This is achieved by developing vacuities or empty spaces in the temporal area of the skull and sometimes before the eye orbit.
EURYAPSID SKULL (found in Plesiosaurus)
Euryapsid means “widearch“. They have one fenestra high on both sides of the skull. The euryapsids are represented by the marine reptiles such as the icthyosaurs, plesiosaurs, and placodonts, which are all completely extinct groups. The euryapsids arose from the diapsids which had two fenestrae, but sealed one of the openings to strengthen their skulls for life under water.
Plesiosaurus was aquatic giant reptile with a long neck, small skull and fish-eating jaws. Its limbs were modified into paddles for swimming and there was no tail fin but a small tail. Skull had a single pair of temporal vacuities bordered by parietal, postorbital and squamosal bones. Maxilla and premaxilla had sharp teeth for fish catching.
PARAPSID SKULL (found in Ichthyosaurus)
This type of skull evolved in Ichthyosaurus, which included dolphin-like aquatic reptiles that preyed upon fishes or other aquatic animals. This skull also had only one pair of temporal vacuities on the upper side, guarded by two additional bones, namely, postfrontal and supratemporal, which push the postorbital and squamosal bones towards the lower side. The vacuities are guarded by parietal bones above. The condition found in ichthyosaurs is distinguished from the euryapsid condition because their temporal fenestrae are only bordered by the parietal, postfrontal, and supratemporal (Pirlot, 1969). This condition has been called parapsid, but it only represents a minor variation from the euryapsid pattern.
DIAPSID SKULL (found in Sphenodon, snakes, lizards, crocodiles and dinosaurs)
This type of skull has two temporal vacuities on either side of the skull. The superior temporal vacuity is surrounded by parietal above and postorbital and squamosal below. The inferior temporal vacuity is guarded by the postorbital and squamosal above and jugal and quadratojugal below. This type of skull is lighter and has more space for the attachment of jaw muscles. It is found in a large number of living reptiles and also in extinct dinosaurs. Dinosaurs and crocodiles also have a pair of preorbital vacuities anterior to the eye orbits.
SYNAPSID SKULL (found in Pelycosauria and Therapsida)
This type of skull was found in Dimetrodon (Pelycosauria) and the mammal-like therapsid reptiles (Cynognathus), in which there was only one inferior temporal vacuity on each side of the skull but it was guarded by postorbital and squamosal bones above and jugal and quadratojugal below. “Synapsid” means “together arch.” The synapsids were the dominant land vertebrates from the Late Carboniferous Period (280 to 230 million years ago) to the end of Triassic Period (230 to 195 million years ago). Although the syanpsids were reptiles, they later gave rise to mammals.