thumb|227x227px|Earless monitor skull
The earless monitor lizard (Lanthanotus borneensis) is a semiaquatic, brown lizard native to the Southeast Asian island of Borneo. It is the only living species in the family Lanthanotidae and is related to the true monitor lizards.
Taxonomy
The earless monitor lizard was described in 1878 by Franz Steindachner. The genus name Lanthanotus means "hidden ear" and the species name borneensis refers to its home island of Borneo. The uniqueness of the species was immediately recognized and Steindachner placed it in its own family, Lanthanotidae. In 1899, George Albert Boulenger relegated it to the family Helodermatidae, together with the beaded lizards and Gila monster, on the basis of morphological similarities. Further studies were conducted in the 1950s where it was found that although it is related to Helodermatidae, this relationship is relatively distant. The similarity is in part the result of convergent evolution and they should be recognized as separate families. Both are part of a broader Anguimorpha, but the relationship among the various families has been a matter of dispute. Several earlier studies have placed the earless monitor lizard together with Helodermatidae and Varanidae (true monitor lizards) in Varanoidea. More recent genetic evidence has found that the nearest relative of the earless monitor lizard is Varanidae. Most authorities continue to recognize them as separate families as the divergence between them is deep, Together they form a clade and its sister group is Shinisauridae; at a higher level the sister groups of these three are Helodermatidae and others families in Anguimorpha.
The extinct Cherminotus known from Late Cretaceous fossil remains in Mongolia has been considered a member of Lanthanotidae, but this is disputed.
Distribution and habitat
The earless monitor lizard is endemic to the Southeast Asian island of Borneo, where it is known from Sarawak in East Malaysia, as well as West and North Kalimantan in Indonesia. Until late 2012, its known range in North Kalimantan was a part of East Kalimantan. It was also confirmed to be found in Brunei for the first time in 2022, in the country's Temburong District. Prior to the discovery, it was believed that they may occur there and has been recorded c. from the border. There are no records from Sabah, Central Kalimantan or South Kalimantan. These are typically in rainforests, but it is also found in streams flowing through degraded habitats such as agricultural land, mature fruit tree gardens and palm oil plantations, and reportedly may occur in rice paddies. At a site with a high density of earless monitor lizards the water was clear and had a neutral pH. It is associated with the same microhabitat as Tropidophorus water skinks and in some places its range overlaps with T. brookei. The species is extensively covered in osteoderms. Despite the name, they are capable of hearing, although lack a tympanum, an ear opening and other externally visible signs of ears. The upperparts are orangish-brown, and the underside is mottled dark brown and whitish, pale yellowish, ochre or rusty. Overall the sexes are alike, but males have a distinctly broader head and broader tail base than females. They sometimes oscillate the throat (similar to frogs) and the forked tongue is sometimes flicked (similar to snakes). They can make a gentle, squeaky vocalization. When hatching the total length of the young is about . When one year old, their total length is about . The record size of earless monitor is 55 cm.
Behavior
Earless monitor lizards are generally strictly nocturnal animals, although exceptional daytime observations in the open have been reported. When underwater, the semitransparent lower eyelids are generally closed, covering the eyes. and toxic compounds in the bite of this species. The main components are kallikreins (to a lesser degree CRiSP) with the primary effect being the cleaving of fibrinogen, which is important for blood clotting. However, this effect is quite weak in the venom of the earless monitor lizard compared to that of many other venomous reptiles, including some of the true monitor lizards.
Feeding
They typically feed on earthworms, crustaceans and fish. In one case, a single mating lasted 44 hours. In the wild mating has been seen in February, They are deposited on land. From May 2014 to October 2015 at least 95 earless monitor lizards appeared in the trade in Asia (Hong Kong, Indonesia, Japan and Malaysia), Europe (Czech Republic, France, Germany, Netherlands, Russia, Ukraine and the United Kingdom) and the United States. Collecting the species from the wild is illegal; the earless monitor lizard has been protected in Malaysia since 1971, in Brunei since 1978 and in Indonesia since 1980. Penalties range from a fine of US$1,600 and one year's imprisonment (Brunei) to $7,850 and three years' imprisonment (Malaysia), to $8,600 and five years' imprisonment (Indonesia). Unlike all other monitor species, the earless monitor lizard was not listed on CITES, which would restrict trade at an international level.
The first confirmed captive breeding was at a zoo in Japan in 2014. There have been other breeding reports by private keepers and captive bred individuals have been offered for sale, but some of these may not involve genuine cases of captive breeding.