Nothing in Biology Makes Sense
Except in the Light of Evolution
At the end of the Devonian period, for the first time, vertebrates from amphibious habitats
were invading a new environment significantly dissimilar from their past habitat, where they lived
for millions of years. This was for sure a huge step that opened up a broad spectrum of avenues for
evolution over a tremendous range of adaptations. The problems they faced during their deviation
from normal habit to complete land once exposed, led the amphibians to a great changeover in their
overall structure and a change in the emphasis of evolutionary trend. The aftermath of these
changes were primarily directed along the lines of adaptation for life on land and in air, eventually.
During the Carboniferous times, the advent of reptilians was witnessed. The transformation of gill
arches into jaws, and the adaptation towards amniotic egg, etc., freed vertebrates from their
dependence on water, and facilitated their transition towards adapting to a completely terrestrial
habitat during their life history.
A mixture of amphibians and reptilian characters as seen in Seymouria (see below
illustration) is indicative of the gradual transition that took place between either of the classes
(amphibians and reptilian) during the vertebrate evolution. As these changes were gradual than
abrupt, it is hence difficult to clearly draw a distinction between amphibians and reptilians where all
the fossil records were taken into consideration. However, there still exist certain characters that are
typical of reptilians and their close relatives. Structurally, reptilians are generally characterised by a
rather deep skull than the amphibians, the primitive Otic notch (Otic notches are invaginations in the
posterior margin of the skull roof, one behind each orbit and the presence or absence of the otic
notches is a distinguishing trait used to separate the amniotes from the amphibian grade tetrapods)
is much suppressed in the reptilian skull, and most important is the trait characterised by extreme
reduction of post-parital elements in the skull, so that bones behind the partials are small or shifted
from the roof of the skull to the Occiput (back of the head), or even completely suppressed.
Perhaps the most practical approach towards the classification of reptilians is by the structural varieties of the temporal region, as shown by reptilians during their development. Though this system of classification based upon a single character is not perfect, it provides a useful arrangement for the study of reptilian orders. Reptilian are classified thus – Kingdom: Animalia, because they are motile and heterotrophic; Phylum: Chordata, because at some point of their life, they exhibit a notochord which gives rise to the backbone, a dorsal nerve cord etc., Class: Reptilia, because they possess a body covered with scales, and embryos covered in amniotic membrane. Their class can further divided into sub-classes depending on the temporal opening in the skull. These include Anapsida, Synapsida, and Diapsida. These are explained as below:
Anapsida: No temporal opening in the skull behind the eye.
Synapsida: A single lower temporal opening above Post-orbital and Squamosal bone.
Diapsida: Both upper and lower temporal openings are separated by Post-orbital and Squamosal bone.
Mosasaurs, Sphenodontans, Snakes, Lizards, and a vast majority of today’s reptilians are Lepidosaurs – a large successful group of diapsid reptiles. Lizards and snakes are amongst the most numerous and diverse of modern reptiles – there are about 25 living species of Crocodilians, some 400 species of turtles, about 3800 species of lizards, and an estimated 3000 species of snakes known till date! This preponderance of the squamates (snakes and lizards belong to Order-Squamata) among the reptilians has existed since the end of the Age of Dinosaurs. Lizards are quadrapedal reptiles where in, the fourth toe of the hind foot is greatly elongated. During their run, the elongated toe provides a strong lateral push which serves as the chief propulsive force to sprint. Furthermore, some lizards like the collared lizard of North America rise up on their hind legs while running. Other characters include adaptation of teeth for an insectivore diet, development of ears from the shoulder girdle, and general development of limbs and feet which are typical of a lizard. It is therefore appropriate to say that a very special, truly unexpected adaptation marked the initial stages in squamate evolution. From this beginning in the late Permian and Triassic times, lizards became established during the Jurassic period and underwent an adaptive radiation that marked the evolution through the middle and late Mesozoic and Cenozoic history. Today’s lizards share the distinction for being the most successful in multiplication of species, geographical distribution, and range of adaptation along with their cousins: the snakes.
In the reptilian evolution, snakes are the last amongst all major groups to evolve. They are highly modified lizards whose ventral scales are wide, spanning across the entire width of the ventral surface, facilitating in their leg-less locomotion. The absence of limbs stands the ground towards the adaptation of an entirely new type of locomotion over land. Locomotion is accomplished by various movements of the body like side-to-side movements, straight forward motion – in which the body muscles act in rhythmic waves, pulling the snake along, and even a kind of ‘Corkscrew looping motion’ known as ‘side winding,’ as witnessed in some Rattle snakes. The body and the tail is greatly elongated by an enormous multiplication of vertebrae, and with such a body shape comes numerous adaptations in the structure and arrangement of internal organs. The elongated body shape and the skull structure with a long, movable quadrate bone, give snakes a sort of double joint to the back of the jaw, enabling it to open wide and swallow prey exceeding the snakes’ own diameter. Furthermore, the pre-maxillae in the skull and the anterior end of the dentaries in the lower jaw are not coalesced, rather they are loosely connected by ligamental bonds so that they can be spread apart to a considerable angle. Thus when a snake hunts down a prey, it engulfs it as a whole, spreading its jaws to swallow the prey in whole, the latter often being larger than the snake’s diameter.
As if having inspired the quote “Snake in the Grass,” these limbless beings hunt by stealth – attacking with a lightning quick lunge. If they miss their mark, they rarely get a second chance. Consequently, some snakes have developed highly toxic types of venom that are capable of quickly subduing their prey. These venom types have originated separately in various genera, for instance –Vipers are ‘Hemotoxic,’ Cobras ‘Neurotoxic,’ and so on.
In the Mesozoic era when the dinosaurs and other dominant reptiles were extinct, the surviving reptiles were forced to compete with tetrapods, which include birds and mammals. Lizards managed to survive in the protection of dense foliage & rocks then, and snakes, their evolved counterparts were successful in thriving during the Age of Mammals, taking advantage of this very foliage and rocks that offered a safe haven for their ancestors.
Deepak Tarun is a post graduate in Zoology. Not one to embrace anything at its face value, Deepak believes in really digging to the roots, before arriving at a decision. A budding wildlife biologist and passionate knowledge seeker, Deepak likes to read, and watch stand-up comedy in his spare time.