Four changes are evident in the evolution of reptiles:

1. evolution of stronger, more effective jaw mechanisms

2. evolution of more effective locomotion on land

3. development of amniotic egg

4. development of scales

These changes separate the reptiles from the amphibians.

#1, the evolution of stronger, more effective jaw mechanisms, is the subject of this lecture and one topic in last week’s lab. The other changes will be discussed in later lectures.

The skull is a skeletal structure found in head region.

In the Chondrichthyes, the skull consists of chondrocranium and upper jaw. In the Osteichthyes through mammals, the skull is a fused unit in which the braincase is added to and welded together by a series of dermal bones.

Skull of early amphibians - "labyrinthodonts" - consisted of three major units:

1. Chondrocranium (brain case)

Formed originally as cartilage. If ossified, called the neurocranium

2. Palatal complex: ossification of palatoterygoquadrate cartilage (upper jaw) and some dermally derived membrane bones forming the roof of the mouth

The anterior portion of this complex forms the palatal plate with gaps for the internal nares.

Posteriorly, the palatal complex on either side is separated from the edge of the dermal skull by openings called the subtemporal fossae. The temporal muscles that close the jaws descend through these openings.

3. Dermal skull roof or dermatocranium

A shield of membrane or dermal bone covering the top and sides of the head and extending down to the jaw rims, with a marginal row of teeth.

In amphibians & stem reptiles, the dermal skull roof is unbroken except for openings for eyes & nasal openings. In these animals, the temporal muscles which close the jaw are shut inside the solid dermal roof of the cheek or temporal region of the skull. When the jaws close, the muscles shorten and expand in breadth. There is little room for expansion under the solid dermal shield. This limited the size of the muscle.

Very early in reptile history, changes to ancestral condition occurred in the nature of muscle attachment to the lower jaws and the skull. These changes had to do with the development of stronger and more efficient jaw muscles.

In several reptile lineages (except anapsids), openings developed in the solid dermal skull roof. Jaw muscles go through these temporal openings and attach to outer surface of the cranium, permitting development of stronger jaw muscles & more efficient opening mechanisms.

These openings evolved independently several times in different reptile lineages, resulting in openings that are in different places on the skull in different groups.

1. Anapsid condition:

Absence of openings in the bones that roof over the temporal region of the skull: no temporal fossae in skull

Turtles are related to the anapsid stem reptiles (‘cotylosaurs’) and have no true temporal fossae.

Sea turtles demonstrate this condition well but other turtles have formed an emargination into the roof of the skull from behind.

Otic notch - anapsid condition slightly modified

2. Synapsid condition

One temporal opening (low on side of skull) in the bones that roof over the temporal region of the skull. This condition originated in the reptilian ancestors of mammals and continues in mammals today.

3. Diapsid condition

Two temporal openings

Diapsid condition is very clear in alligators and crocodiles, and characterizes dinosaurs.

In most lizards, bar closing the lower fossa has been lost in lizards, leaving a single opening and an arch.

In snakes, the upper bar has been lost as well, leaving the whole cheek clear of dermal bone.

Icthyosaurs and plesiosaurs have a modified diapsid condition referred to in the lab as euryapsid, with the loss of one of the two temporal fossae.