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 it is related to the true monitor lizards.[3][4][5]
The earless monitor lizard was described in 1878 by Franz Steindachner.[6] The genus name Lanthanotus means "hidden ear" and the species name borneensis refers to its home island of Borneo.[7] The uniqueness of the species was recognized from the start 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.[7] 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.[7] 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.[8] More recent genetic evidence has found that the nearest relative of the earless monitor lizard is Varanidae.[9][10] Most authorities continue to recognize them as separate families as the divergence between them is deep,[6][9][10] but some have suggested that the earless monitor lizard should be included as a subfamily, Lanthanotinae, of the Varanidae.[11] 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.[9][10] The most recent common ancestor diverged in the mid-Cretaceous.[5][12]
The extinct Cherminotus known from Late Cretaceous fossil remains in Mongolia has been considered a member of Lanthanotidae, but this is disputed.[13]
The earless monitor lizard is endemic to the Southeast Asian island of Borneo. Here it is known from Sarawak in East Malaysia, as well as West and North Kalimantan in Indonesia.[14][15][16] Until late 2012, its known range in North Kalimantan was a part of East Kalimantan.[17] It is confirmed to be found in the Temburong District, Brunei, in 2022.[18] Prior to the discovery, it was believed that they may occur there and has been recorded c. 100 km (60 mi) from the border.[19] There are no records from Sabah, Central Kalimantan or South Kalimantan.[16][19] The species is easily overlooked.[7]
It is found in lowlands at elevations below 300 m (1,000 ft) near streams and marshes.[20][21] 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.[14][16][20] The streams it inhabits are often rocky.[14][16] Its habitat is tropical with air and water temperatures that generally are about 22–29 °C (72–84 °F),[15][16] and captives reportedly prefer 24–28 °C (75–82 °F).[22] At a site with a high density of earless monitor lizards the water was clear and had a neutral pH.[16] It is associated with the same microhabitat as Tropidophorus water skinks and in some places its range overlaps with T. brookei.[16]