Rhinoceros Viper
(Bitis nasicornis).
Annotated skull with bones
labelled as follows:
1. Compound
2. Dentary
3. Frontal
4. Maxilla
5. Palatine
6. Pterygoid
7. Parietal
8. Prefrontal
9. Premaxilla
10. Postorbital
11. Pootic
12. Supratemporal
13. Quadrate
Image courtesy Scott Eipper.
interesting. With so many functions, from housing the
brain, the delicate eyes and ocular nerves, as well as the
mechanisms behind other senses such as taste and smell,
not to mention being indispensible for activities such as
hunting, feeding, and drinking, the skull is by necessity a
finely orchestrated and complicated structure. Reductions
to the skull during snake evolution might seem limiting,
however it was the loss and modification of certain
structures in their lizard-like ancestors that led to their
incredible ability to swallow massive prey items.
The first members of the class Reptilia appeared in the
late Carboniferous Period, some 312 million years ago;
sauropsid turtles and crocodiles had emerged by the
As lizards developed greater biting ability, the temporal
fenestrae became larger to accommodate more muscle.
The infratemporal arch, the bottom boundary of the lower
or inferior temporal fenestra, was lost. With further
reductions in the outer skull, the quadrate bone became
detached and elongated, and was co-opted into
articulating the jaw.
For the purpose of comparison, let's jump for a moment to
the mammals - more specifically ourselves. The human
lower jaw is made up of the mandible, a single, large arch
of the dentary bone, fused at the chin. The articular and
quadrate bones, now called the malleus and incus, have
migrated to create the mammalian middle ear. The upper
‘The requirements for mobility and a powerful bite began to
outweigh the need for a heavy, bony outer skull.’
Triassic, early birds and lizards by the Jurassic, and
snakes followed in the Cretaceous. Early reptiles (the
Sauropsida) possessed an anapsid skull with a domed roof
formed by sutured bony plates, broken only by holes for
the eyes, the nostrils, and the photoreceptive parietal or
pineal eye (located towards the rear of the head on the
dorsal surface). As the requirements for mobility and a
powerful bite came to outweigh the need for a heavy,
bony outer skull, small openings began to appear in the
casing behind the eye. These ‘temporal fenestrae’ (using
the Latin word for windows) reduce weight and create
space for the jaw muscles to move. In the ancestral
mammalian clade (Synapsida), two additional openings
expanded to become one large window in the side of the
head. Sauropsid reptiles underwent a similar process,
developing a diapsid skull characterized by two temporal
fenestrae on each side. Some diapsid lineages later
evolved a single larger opening similar to that of the
synapsid skull.
jaw and cheekbones are in fact composed of the maxillary
(upper jaw) and squamosal bones fused together and into
the cranium, forming the zygomatic arch to which our jaw
abductor muscles attach, and allowing for clenching and
chewing by lifting and manoeuvring the mandible. This
movement of the mandible is all the kinesis our jaws are
capable of.
If you've ever watched a snake feed, something very
different is going on. The top, bottom, left and right all
seem to move independently but in remarkably cohesive
and co-operative fashion to drag massive prey items ever
further down, down, down the hatch. Snakes achieve this
incredible cranial kinesis by a number of means. When
the early snakes broke away the early lizards, some 150
million years ago in the late Jurassic or early Cretaceous,
they already possessed the reduced outer skull and, impor-
tantly, the mobile quadrate bone of Squamate reptiles. The
quadrate links the squamosal bone to the small articular
bone, from which ligaments suspend the entire jaw