D
Unfortunately for those obsessed with snakes, the fossil
record for these creatures is rather limited, due largely
ue in part to the breadth of life's diversity, the
to their delicate skeletal structure. In the transformation
collective scientific effort to understand evolutionary
from ancient lizard to early ancestral snake, a lot of bone
relationships between various species, a field known as
mass was done away with - not just limbs but also
systematics, is subject to constant change and debate.
shoulder and pelvic girdles. The size, number, and shape
Traditionally a field based on the physical, morphological of the remaining bones are also highly modified, and a
differences between groups of organisms, the rise of
complete understanding of the transition from lizard-like
molecular biology has seen genetics take a major role in
ancestor to snake remains elusive. Limblessness evolved
systematics, with many previously assumed relationships multiple times in squamate reptiles so it's clearly a useful
overturned by new evidence. Genetic information can be body plan. Whether this was an adaptation to an aquatic
particularly useful when studying closely-related species existence, where legs tucked against the body during
so similar that their species-level relationships are practi- sinusoidal swimming eventually gave way to selection for
cally impossible to define from external morphology.
reduced drag and were lost entirely, or a similar selective
scenario for a terrestrial, burrowing ancestor, squeezing its
Nevertheless, the subtle morphological variations between way through soil, leaf-litter and other debris, is uncertain.
species are hugely informative of both their evolutionary Some recent evidence, however, points towards a
and ecological history. Morphological data is particularly terrestrial ancestor for snakes.
interesting in terms of fossils, allowing a window of
inquiry into timescales where genetic evidence is partially Whatever the origin, the snake skeleton is rather
or entirely degraded beyond current sequencing
simplified. In brief, it consists of a skull, vertebrae and
techniques. A detailed fossil record, where available, is a ribs, with occasional vestiges of leg/hip bones. Starting
true snapshot of past phenotypic variation.
with the obvious, there are a LOT of vertebrae and ribs.
Humans have 33 vertebrae and 12 pairs of
ribs, whereas snakes can have anywhere
Coastal Taipan
from around 100 to over 400. Snakes’
(Oxyuranus scutellatus).
vertebrae are capable of 10-30 degrees
Left: annotated skull.
vertical rotation and a few degrees of
Below: full mount at Queensland
horizontal rotation, creating a flexible and
Museum, striking at a bandicoot.
mobile body plan. Although, at first glance,
Bones Identified:
the skeleton appears to be rather uniform,
1. Compound
closer investigation reveals that the spine
2. Dentary
may be divided into cervical, thoracic, and
3. Frontal
caudal regions. Specifically, we first find
4. Maxilla
two modified, rib-less vertebrae, the atlas
5. Palatine
and axis, supporting the skull and allowing
6. Pterygoid
effective articulation. These are followed by
7. Parietal
the cervical or neck vertebrae which are
8. Prefrontal
accompanied by small, flexible ribs (these
9. Premaxilla
have been lost in mammals). Next comes the
10. Postorbital
larger thoracic vertebrae of the trunk, all
11. Prootic
with ribs, which first increase in size and
12. Supratemporal
then taper down after the mid-body. Snakes
13. Quadrate
do not have a lumbar region; instead,
articulated ribs continue to appear along the
vertebrae into the cloacal region, homolo-
gous to the human sacrum, where we find
the fork-ended ‘cloacal’ ribs, otherwise
known as the lymphapophyses. This denotes
the end of the trunk, and on a living animal
coincides with the termination of the
digestive tract, waste excretion and openings
of the reproductive organs, all combined in
one multifunctional orifice, the cloaca. The
remaining vertebrae, which support the tail,
are tapering, small and devoid of ribs
(although they do possess pleurapophyses,
the lateral processes that form the start of the
ribs).
Returning to the skull, this is where
structural changes become much more