Tetrapod Skull Design

Skulls and jaws are not solid bone but are made up of many plates.  They can be loosely or tightly connected.  Biologists and paleontologists study the arrangement of the plates to determine relationships between similar animals.

During the late Carboniferous and Early Permian some important divisions in animal design occurred.  The most important is the amniotic egg, which frees terrestrial animal’s dependence on water to lay their eggs.  The second major division is in skull design.  The number of fenestrae (holes) behind the orbit (eye socket).  These holes are to attach large jaw muscles.  The transition from amphibian to reptilian was as slow as fish to amphibian.  But when the division between reptilian and proto-mammal-like happened in a relatively short amount of time.

[A] Anapsid skull - The most primitive form does not have any holes behind the orbit.  All the first terrestrial animals had this kind of skull.  Fish, amphibians and turtles still do.  Turtles are the only truly terrestrial representative of this group to persist beyond the Triassic.

[B] Synapsid scull - One fenestra (hole) behind the orbit: includes all Pelycosaurs and all their decedents, Therapsids and mammals.  (Feel the bone ridge next to your eye.  At the side of your face is a soft area where your jaw muscles attach.  That is your fenestra.)

Several other characteristics differentiate the Synapsids.  One is specialized regions of the tooth row: incisor, canine, and molar.  A second is the relationship between the jaw and ear.  The dividing attribute of mammals is that three rear jaw bones reduce in size and become part of the ear.  Third, their fore limbs tend to be strong.

[C] Diapsid skull - Two fenestra (holes) behind the orbit: all reptiles (Sauropsid) and all their decedents; Archosaurs, lizards, snakes, crocodiles, dinosaurs, birds.  Dinosaurs and birds have an additional fenestra (hole) in front of the orbit.

[D] Euryapsid skull - Marine reptiles have a skull with only one hole behind the orbit.  But the skull plate and teeth arrangements are not similar to synapsids (mammal-like).  The plate and teeth are instead similar to a diapsid.  It has been determined that the connecting arch of bone between the double fenestrae (holes) disappeared.

Tetrapod Feet

Plantigrade stance – entire foot (toes to ankle) lands on the ground (like most reptiles and humans)

Semi-digitigrade stance – only the ankle is held off the ground (like T. rex and dogs)

Digitigrade stance – metatarsals are held off the ground (walks on point) (like Sauropods and elephants)

Common Synapsid Characteristics

(mammals and mammal-like)

1. differentiation of the tooth row into specialized regions, incisor-like, canine-like, and molar-like

2. fore limbs are relatively large and powerful compared to the hind limbs

3. rear bones of the lower jaw have become intimately involved with the ear region

Common Sauropsid Characteristics

 (reptiles, dinosaurs, birds)

1. little tendency for the tooth row to develop specialized regions, a retained primitive feature

2. fore limbs tend to be relatively small and the hind limbs are often much larger

3. rear bones in the lower jaw tend not to become associated with  the ear, a retained primitive feature