Characteristics of the Amphisbaenians
Today’s living reptiles are placed within the class Reptilia which is further represented by three orders, the Chelonians (turtles and tortoises), Archosaura (the crocodilians) and the Squamata (lizards, snakes and worm lizards). Most people are familiar to some extent with virtually all of the aforementioned groups with the exception of one, the worm lizards. Worm lizards are a distinct group of reptiles that comprise the suborder amphisbaenia which are also referred to as “amphisbaenia”.
Despite their relative low species diversity this group of reptiles has as a fairly wide geographic distribution. In the Western Hemisphere amphisbaenia are present in southeastern United States, the Caribbean Islands, southwestern Mexico and most of tropical South America east of the Andes. In the Eastern Hemisphere amphisbaenia are occur in Spain, Portugal and Morocco, Iraq, Saudi Arabia, most of the southern half of continental Africa and Madagascar.
Of all the world’s reptiles this group is not only one of the least diverse (third to Crocodilians and Rhynchnocephalians), but the least understood. This is due in no small part that they lead a subterranean lifestyle thus making observations of their activities in nature a challenge to say the least. However, they are a wonderful, exciting and interesting group of animals nonetheless.
To begin with, the term “amphisbaenia” means “double moving”. This makes complete sense when taken into consideration just how similarity between the shape of the tail and head as well as the common name used throughout much of Latin America culebra con dos cabezas (snake with two heads).
All but one genus of amphisbaenia are limbless; the extraordinary Mexican mole lizards of the genus Bipes are the only amphisbaenia with limbs. These fascinating reptiles possess a well developed pair of front limbs used in assisting with locomotion as well as digging. Asides from this most amphisbaenia share a suite of distinctive characteristics.
The eyes are vestigial and located just beneath a layer of scales on the head, external ears are lacking and the nostrils are pointed backwards. The skin is lose and comprised of scales arranged in a series of complete and incomplete annuli. In general two annuli are present for every vertebra except for the Mediterranean amphisbaenia belonging to the genus Blanus (Pianka and Vitt, 2003). The lose skin moves independently of the body and when seen moving these creatures provide a distinctive impression of a combination of an accordion and worm.
Locomotion is often accomplished via rectilinear movement in which the amphisbaenia does not need to bend its body in order to move. Instead it can often keep its body straight while moving forward. However, other modes of movement are utilized by various species of amphisbaenia. Concertina movement is most often seen when the amphisbaenia is above the surface and in need of rapid movement. Mexican mole lizards (Bipes) will often demonstrate this mode of movement. Concertina movement is accomplished by bracing one part of the body against a contact point and pushing forward with the other portion of the body until it achieves a contact point useful to gain purchase while retracting the body from the original contact point. Essentially each part of the body undergoes alternating cycles of static contact and movement. Concertina movement can be effective as well within the confines of a burrow while the animal is advancing or resisting being pulled out by a predator.
The bones of the head are solidly fused together to facilitate burrowing and pushing through substrate. Well developed musculature compliments their dense heads which not only allows them to force their heads into substrates but to also deliver incredibly powerful and painful bites.
Unlike snakes the densely constructed skull of the amphisbaenia does not afford much kinetic movement to facilitate swallowing large prey items. Instead prey is securely grasped with a series of well developed interlocking teeth and appropriately sized chunks are torn away and swallowed. A medial tooth located on the top jaw fits within the teeth present on the lower jaw. This provides a scissor-like effect and allows sections of food items to be cut away.
Another modification present with the cranial morphology of many amphisbaenia is a columellar system consistingof a largely osseous columella (or stapes) and a cartilaginous extracolumella. The extracolumella is a long, slender rod connecting the head of the columella to a thickened skin layer far anteriorly and adjacent to the lower jaw. Its interruption reduces auditory sensitivity, from which it is evident that sound is transmitted from the facial plates of skin through the extracolumellar rod to the columella and finally via the cochlear fluids to the hair cells. However, the extracolumellar bone is lacking in Bipes and the Mediterranean worm lizard of the genus Blanus (Gans and Wever, 1975). Click here for a pdf file of a publication regarding the ears of the Amphisbaenians.
The ability to hear sound is used by the amphisbaenia to locate potential prey items as they move underground. Likewise it could also be useful in detecting the presence of a potential predator. However, hearing is not the only sense used in exploring and navigating the perpetually dark tunnels. Chemosensory functions similar to smell and taste are used as well. This is a shared characteristic among all amphisbaenians. The tongue is flicked out of the mouth and after being retracted into the mouth it comes into contact with the Jacobson’s organ located on the roof of the mouth. This organ facilitates the transfer of chemical cues which are interpreted by the brain.
Field studies involving the giant amphisbaena (Amphisbaena alba) revealed that they rely heavily on their chemosensory functions to locate and follow pheromone trails left behind leaf cutter ants as they journey to and from their nest (Riley, et al. 1986) in search of entrances to leaf cutter ant mounds.
Riley, J., Winch, J. M., Stimson, A. F. and Pope, R. D. (1986)'The association of Amphisbaena alba (Reptilia:
Amphisbaenia) with the leaf-cutting ant Atta cephalotes in Trinidad',Journal of Natural History,20:2,459 — 470