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Florida Wormlizards (Rhineuridae)

Florida wormlizards

(Rhineuridae)

Class Reptilia

Order Squamata

Suborder Amphisbaenia

Family Rhineuridae


Thumbnail description
Elongate, slender, limbless, fossorial reptiles with scales arranged in annular rings, eyes greatly reduced or absent, no external ear openings, a spatulate head covered with a keratinized shield, and a dorsoventrally flattened, short tail

Size
9.5–11 in (240–280 mm)

Number of genera, species
1 genus; 1 species

Habitat
Sandy soils, xerix scrub forest, mesic hammock forest

Conservation status
Not listed by the IUCN

Distribution
Rhineurids have a restricted distribution in northeastern and north-central Florida

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. Past studies disagree on whether rhineurids are the most primitive amphisbaenians or are derived within the group. A recent phylogenetic analysis suggests that rhineurids are derived amphisbaenians.

The family Rhineuridae is the only amphisbaenian family with a significant fossil record. This fossil record is entirely from the western and central United States, indicating that rhineurids were once widely distributed across the United States, whereas today a single relict species occurs in Florida. Rhineurids are found in the fossil record extending as far back as the Paleocene (about 60 million years ago). These fossils are already quite derived, exhibiting a shovel-headed cranial shape similar to that occurring in the extant species from Florida, but they also exhibit some intriguing primitive cranial features that are absent in most amphisbaenian species living today. For example, some fossil rhineurids have a complete orbit, enclosed posteriorly by the jugal bone, an element of the skull that is absent in all extant forms. The presence or absence of limbs in these fossil forms is not definitively known due to vagaries of fossil preservation, but no limb elements have been found so far associated with these fossils. If rhineurid fossils are ever found with preserved limb elements, this would significantly change our understanding of relationships within Amphisbaenia.

The taxonomy of this species is Rhineura floridana Baird, 1859, Micanopy, Florida.

Physical characteristics

Certain features of rhineurids 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.

Rhineurids exhibit a cylindrical body shape, similar to most amphisbaenians. The snout is dorsoventrally depressed, and the head bears a strong craniofacial angulation. The snout region may also exhibit a zone of keratinization over fused head scales, thus resembling a spatulate digging shield. The tail is short, flattened, and bears conical dorsal tubercles. Caudal autotomy is absent. The ventral throat scales are elongated. Rhineurids are usually pallid or pink in appearance. The dentition in rhineurids is pleurodont. The lower jaw is "countersunk" beneath the upper jaw, and the nostrils occur in a ventral position. A single, median, premaxillary tooth occurs on the premaxilla, sometimes flanked by two smaller teeth. There are four or five teeth on the maxillary and six teeth on the lower jaw.

Beyond those easily recognizable features, rhineurids are also characterized by a number of unique internal conditions, including the complete absence of any pectoral girdle elements and a uniquely modified pelvic girdle.

The Florida wormlizard has a body length ranging from 9.5–11 in (240–280 mm). The tail is less than 10% of the total length; it is flattened and covered dorsally by conical tubercles. The Florida wormlizard has a midbody diameter of approximately 0.5 in (12 mm).

Distribution

The single extant species, Rhineura floridana, ranges throughout northeastern and northcentral Florida. The recent discovery of the Florida wormlizard in southern Georgia means that this species is no longer considered a Florida endemic.

Habitat

This species lives in sandy hammocks; high pine; and dry, sandy, easily burrowed soils.

Behavior

Rhineurids are completely fossorial, using the spatulate head to tunnel into the soil. The shovel-shaped snout is an effective aid in penetrating the soil. Additionally, the snout bears a hard covering that forms a keratinous shield, thus hardening the penetrating edge even more. The keratin usually wears off with use and is continually replaced. The irregular, raised scales on the dorsal surface of the tail are used to plug the mouth of the tunnel, with soil particles blending into these raised tubercles and camouflaging both the tail and the tunnel entrance. When threatened, the Florida wormlizard retreats quickly into the soil and uses its tail to block the tunnel entrance.

Feeding ecology and diet

The natural history of rhineurids is very poorly known, and we have little information on diet for this species. Most amphisbaenians feed primarily on termites, beetle larvae, and ants, and presumably this is also true of rhineurids.

For rhineurids, 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 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 amplify the vibrations as well. This unique anatomy is consistent with behavioral studies conducted in laboratory experiments that suggest that amphisbaenians can hear prey movements through the soil.

Reproductive biology

Due to their secretive habits, little is known of the reproductive biology of rhineurids. Rhineurids are oviparous, laying a clutch that is usually comprised of two eggs in subterranean burrows. Detailed data are not available.

Conservation status

Rhineura floridana is not listed by the IUCN.

Significance to humans

The Florida wormlizard is of no economic significance to humans, but it may benefit humans ecologically by feeding on populations of ants and termites and potentially helping to keep these populations in check.


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

Baird, S. F. "Description of New Genera and Species of North American Lizards in the Museum of the Smithsonian Institution." Proceedings of the Academy of Natural Sciences of Philadelphia 1858 (1859): 253–256.

Berman, D. "A New Amphisbaenian (Reptilia: Amphisbaenia) from the Oligocene-Miocene John Day Formation, Oregon." Journal of Paleontology 50 (1976): 165–174.

——. "Hyporhina tertia, New Species (Reptilia: Amphisbaenia) from the Early Oligocene (Chadronian) White River Formation of Wyoming." Annals of the Carnegie Museum 44 (1972): 1–10.

——. "Spathorhynchus natronicus, a Middle Eocene Amphisbaenian (Reptilia) from Wyoming." Copeia 1973 (1973): 704–721.

Carr, A. "Notes on Eggs and Young of the Lizard Rhineura floridana." Copeia 1949 (1949): 77.

Cope, E. D. "Remarks on Reptiles." Proceedings of the Academy of Natural Sciences, Philadelphia 13 (1861): 75.

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

Gans, C., and E. Wever. "The Ear and Hearing in Amphisbaenia (Reptilia)." Journal of Experimental Zoology 179 (1972): 17–34.

Gilmore, C. W. "Fossil Lizards of North America." Memoirs of the National Academy of Science 22 (1928): 1–201.

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

Taylor, E. H. "Concerning Oligocene Amphisbaenid Reptiles." University of Kansas Science Bulletin 34 (1951): 521–579.

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

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