Spade-headed wormlizards

(Trogonophidae)

Class Reptilia

Order Squamata

Suborder Amphisbaenia

Family Trogonophidae

Thumbnail description
Elongate, limbless, fossorial reptiles with scales arranged in annular rings, eyes greatly reduced or absent, no external ear openings, a spade-shaped head with sharp cutting edges, and an extremely short, pointed tail

Size
3.1–9.4 in (80–240 mm)

Number of genera, species
4 genera; 8 species

Habitat
Loose, sandy, or loamy soils

Conservation status
No species listed by the IUCN

Distribution
Trogonophids occur in northern Africa, the island of Socotra, and a disjunct distribution in the Middle East from western Iran to eastern Somalia

Evolution and systematics

Most recent phylogenetic analyses have placed amphisbaenians as one of three suborders of Squamates (the clade that includes snakes, lizards, and amphisbaenians), but their exact placement within that clade is not well understood. The interrelationships among the four amphisbaenian families are also poorly understood. A recent phylogenetic analysis indicates that trogonophids may be most closely related to the amphisbaenids among amphisbaenians. No significant fossil record exists for this family; a single fossil specimen from Africa is known. Trogonophis is probably the most primitive member of the Trogonophidae, with the highly specialized Agamodon and Diplometopon being the most derived members of the group. No subfamilies are recognized.

Physical characteristics

Certain features of trogonophids are common to most or all amphisbaenians. These include: a unique modification of the middle ear in which an elongated structure, the extracolumella, attaches to the stapedial bone of the middle ear extending forward to attach to tissue along the sides of the face and allowing the reception and transmission of vibrations to the inner ear; reduction or absence of the right lung; an enlarged, medial, premaxillary tooth; the periodic shedding of the skin in a single piece; a heavily ossified and robust skull; absence of eyelids and external ear openings; and a forked tongue.

Trogonophids are all limbless but retain both pectoral and pelvic girdle vestiges. The head is spade-shaped, with sharp lateral edges on the spade. The body shape of trogonophids is unique among amphisbaenians, being higher than wide, and resembling an upside down "U" in cross-section rather than the more common circular shape. The ventral surface of the body appears excavated, or concave. This shape is partially due to the elongation of the ribs, the ends of which dig into the ground, helping to balance the animal against the forces created during the oscillating motion that trogonophids use in burrowing. The dentition of trogonophids is acrodont, another unique feature among amphisbaenians. The tail is extremely short, pointed, and downward curving, and may be keeled in some species. Caudal autotomy is always absent in

these species. Striking pigmentation with spotting and checkerboard patterns occurs in some species. Beyond those easily recognizable features, trogonophids are also characterized by a number of unique internal conditions, including an enlarged, pectoral sternal plate and a greatly enlarged premaxilla in the facial portion of the skull.

Distribution

Trogonophids occur in northern Africa, Socotra Island, and the eastern Arabian peninsula.

Habitat

Spade-headed wormlizards are known to occur mainly in loose, sandy soils. Trogonophis wiegmanni, the least specialized member of this group, may be the most likely member of this family to be found above ground. Agamodon tends to be found in very fine, sandy soils and at greater depths below the surface.

Behavior

Trogonophids are unique among amphisbaenians in using mainly oscillating, rather than rectilinear, movements while tunneling. Oscillatory movements of the head consist of rotating the head alternatively left and right, which effectively shaves soil off the end of the tunnel via the sharp edges along the sides of the face, and simultaneously compacts the loose shavings onto the walls of the tunnel with the sides of the head. This behavior explains some of the specialized and unique anatomy of these forms. For example, the noncircular shape of the trunk in cross-section is useful in force resistance during oscillatory movements of the head, preventing the entire body of the animal from spinning. Trogonophids also use their exceptionally short tails as anchors to apply force when burrowing with their heads. Some trogonophids display interesting defensive behaviors when threatened, such as rolling over on their backs and freezing.

Feeding ecology and diet

Because amphisbaenians are fossorial, little is known of their behavior or ecology. Most species feed primarily on small arthropods, such as termites and beetle larvae. However, laboratory studies suggest that captive trogonophids are capable of preying on much bigger animals by biting off pieces from the prey animal, and their skull anatomy and dentition seem to support this ability. However, direct examination of stomach contents and field studies are rare for trogonophids, and what they actually eat in the wild is not known.

For trogonophids, chemical and auditory cues are the most important means used in locating prey. The uniquely adapted middle ear system allows prey movements to be detected, while the forked tongue and the Jacobson's organ allow the detection of chemical odors. Airborne sounds are picked up and transmitted to the inner ear along the specialized extracolumellar apparatus, which may also amplify the vibrations as well. This unique anatomy is consistent with behavioral studies conducted in laboratory experiments, which suggest that amphisbaenians can hear prey movements through the soil.

Reproductive biology

The reproductive biology of trogonophids is very poorly known. All trogonophids are believed to be oviparous except for some species of Trogonophis, which bear live young (typically five neonates per litter).

Conservation status

No species of Trogonophidae are listed by the IUCN.

Significance to humans

Spade-headed wormlizards are of no economic significance to humans, but they may benefit humans ecologically by feeding on populations of ants and termites and potentially helping to keep these populations in check.

Species accounts

List of Species

No common name

Agamodon anguliceps

taxonomy

Agamodon anguliceps Peters, 1882, "Barava (African orientalis)" Brava, Somali Republic.

other common names

None known.

physical characteristics

This wormlizard has a steep, wedge-shaped head with sharp, raised edges. Its average body length is 4–8 in (100–180 mm). The tail is approximately 8% of total length. The species is pink ventrally, and it has dark blotches on a yellow background dorsally. Its dentition is acrodont, with a semifused row of teeth on both upper and lower jaws. The median premaxillary tooth is usually flanked by two other, smaller premaxillary teeth. It has three to four teeth on the maxillary, and six to eight teeth on the lower jaw.

distribution

Southcentral coast of Somali Republic, eastern Ethiopia.

habitat

This species inhabits loose sandy soils in sandy scrub forests and deserts.

behavior

Agamodon anguliceps is a highly derived trogonophid, exhibiting the most specialized anatomy and behavior within this group. Much of this specialization is related to the oscillatory locomotion that is used in the relatively uncompressible, sandy soils in which it lives. Agamodon anguliceps exhibits a fright reaction when disturbed, consisting of rolling onto its back, lying still, and exhibiting the pink underside of the body. Some evidence indicates that A. anguliceps exhibits vertical migratory movements through the soil corresponding to times of day. For instance, movements in the soil peak at dawn and dusk and individuals are generally closer to the surface around dusk, while they are found much deeper in the soil after dawn.

feeding ecology and diet

No studies examining the natural diet of Agamodon anguliceps exist. If we presume that the diet of A. anguliceps is similar to that of other amphisbaenians, then it would consist of small invertebrates such as termites, beetles, and beetle larvae. On the other hand, laboratory-based behavioral studies indicate that, when offered larger prey items, A. anguliceps is capable of attacking, killing, biting, and efficiently eating various vertebrates. This is not surprising considering the heavy jaws and chewing muscles exhibited in this species.

reproductive biology

No specific data exist on the reproductive behavior of Agamodon anguliceps. This species is believed to be oviparous.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Gans, C. Biomechanics: An Approach to Vertebrate Biology. Philadelphia: J. B. Lippincott Company, 1974.

Schwenk, K. "Feeding in Lepidosaurs." In Feeding: Form, Function, and Evolution in Tetrapod Vertebrates. San Diego: Academic Press, 2000: 175–291.

Vanzolini, P. E. Evolution, Adaptation and Distribution of the Amphisbaenid Lizards (Sauria: Amphisbaenidae). Ph.D. diss. Harvard University, 1951.

Periodicals

Gans, C. "The Characteristics and Affinities of the Amphisbaenia." Transactions of the Zoological Society of London 34 (1978): 347–416.

——. "Notes on a Herpetological Collection from the Somali Republic. I. Introduction and Itinerary." Mus. Roy. Afrique Centrale, Ann. 8, no. 134 (1965): 1–14.

——. "Studies on Amphisbaenids (Amphisbaenia: Reptilia). I. A Taxonomic Revision of the Trogonophidae and a Functional Interpretation of the Amphisbaenid Adaptive Pattern." Bulletin of the American Museum of Natural History 119 (1960): 129–204.

Gans, C., and E. Wever. "The Amphisbaenian Ear: Blanus cinereus and Diplometopon zarudnyi." Proceedings of the National Academy of Sciences 72 (1975): 1487–1490.

——. "The Ear and Hearing in Amphisbaenia (Reptilia)." Journal of Experimental Zoology 179 (1972): 17–34.

Kearney, M. "The Appendicular Skeleton in Amphisbaenians." Copeia 2002, no. 3 (2002): 719–738.

Peters, W. "Über eine neue Art und Gattung der Amphisbaenoiden, Agamodon anguliceps, mit eingewachsenen Zähnen, aus Barava (Ostafrica) and über die zu den trogonophides gehörigen Gattungen." Math. Nat. Mitteil. Sitzber. K. Preussische Akad. Wiss. Berlin, no. 3 (1882): 321–326; Sitzber.

Zangerl, R. "Contributions to the Osteology of the Postcranial Skeleton of the Amphisbaenidae." American Midland Naturalist 33 (1945): 764–780.

——. "Contributions to the Osteology of the Skull of the Amphisbaenidae." American Midland Naturalist 31 (1944): 417–454.

Maureen Kearney, PhD