Lanthanotidae includes a single species, Lanthanotus borneensis, that is restricted to Borneo. Very little is known of this species. It is a burrowing lizard that apparently also spends a good deal of time in water.
Earless monitors reach a maximum length of about half a meter, and are brown with many knobby scales that pick up and retain dirt particles, making them very hard to spot in their natural habitat. Lanthanotus gets its common name from its lack of external ear opening, and although the ear structures are fully developed underneath the skin, it does not seem to respond much to auditory stimulus. Its eyes are reduced, as are its nostrils, and it probably relies on its tongue for chemosensory reception as do other members of Varanoidea. Lanthanotus moves through dirt and water with serpentine twisting motions of its trunk, using its blunt head to push aside dirt, and using its rather weak limbs little if at all. Its tail may be somewhat prehensile. Lanthanotus is rather indolent, moving little in captivity, and is apparently nocturnal.
The natural food of Lanthanotus is unknown, although their pointed, recurved teeth hint at small soft-bodied prey. Captive specimens have eaten eggs, worms, and fish.
The behavior of Lanthanotus in the wild is very poorly known. Captive specimens have been rather lethargic, although they sometimes struggle energetically when handled and can cling tenaciously to food items.
Because of the few numbers of specimens obtained, in the mid-twentieth century it was thought the numbers of Lanthanotus must be very small and they were perhaps on the brink of extinction. More recently it has been thought that there may in fact be many Lanthanotus, but they are so difficult to observe in the wild or collect that their numbers have not been estimated.
Lanthanotidae belongs to Varanoidea, a group comprising Lanthanotidae, the Old World Varanidae, and the New World Helodermatidae. Varanoids in turn belong to the larger group Anguimorpha, which includes such forms as anguids (alligator lizards) and xenosaurids (knob-scaled lizards). Within Varanoidea, Lanthanotus is the sister taxon to Varanidae (Varanus), and is often considered to be a subfamily (Lanthanotinae) within Varanidae. Because of its movement and morphology, Lanthanotus had been thought to perhaps be the "missing link" between lizards and snakes, but this hypothesis is no longer accepted. Lanthanotus shares with Varanus the presence of nine cervical vertebrae, paired apical horns in the hemipenes, a smooth, deeply notched, retractile tongue, and several other characters associated with head morphology and musculature. Lanthanotus is distinguished from Varanus by contact of the prefrontal and the postfrontal above the orbits, a vertical suture between the angular and splenial on the medial side of the jaw, broad palatine shelves, and palatine teeth.
The only known Lanthanotus-like fossil is Cherminotus, found in Late Cretaceous beds in Mongolia.
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Jennifer C. Ast (author).