''Ovoo gurvel'' is an extinct

varanid The Varanidae are a family of lizards in the superfamily Varanoidea and order Anguimorpha. The family, a group of carnivorous and frugivorous lizards, includes the living genus '' Varanus'' and a number of extinct genera more closely related ...

lizard from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

. It is one of the smallest and earliest monitor lizards. It was described in 2008. ''Ovoo'' possesses a pair of small bones in its skull that are not seen in any other lizard.

Description and history

''Ovoo'' is only known from a fossilized skull cataloged as

IGM Immunoglobulin M (IgM) is the largest of several isotypes of antibodies (also known as immunoglobulin) that are produced by vertebrates. IgM is the first antibody to appear in the response to initial exposure to an antigen; causing it to also ...

3/767 and designated the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

. The skull was discovered in 2001 near the rich Ukhaa Tolgod fossil site in a locality known as Little Ukhaa. The deposits at Little Uhhaa date back to the

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

stage of the Late Cretaceous. The generic name is derived from the name of a type of

cairn A cairn is a human-made pile (or stack) of stones raised for a purpose, usually as a marker or as a burial mound. The word ''cairn'' comes from the (plural ). Cairns have been and are used for a broad variety of purposes. In prehistory, t ...

called

ovoo Ovoo, oboo, or obo (, , , , Traditional Mongol: , "heap"; Chinese: 敖包 ''áobāo'', lit. "magnificent bundle .e. shrine) are cairns used as border markers or shrines in Mongolian folk religious practice and in the religion of other Mongol ...

s that are found along roads near Little Ukhaa. The specific name, ''gurvel'', comes from the Mongolian word for lizard. ''Ovoo'' was very small compared to living monitor lizards, with the exception of the

Short-tailed monitor The short-tailed pygmy monitor (''Varanus brevicauda'') is the second smallest living monitor lizard in the world with a maximum length of 25 cm. Distribution The short-tailed pygmy monitor ranges throughout central Australia, from the coas ...

. Nevertheless, the structures of its skull are very similar to those of living monitors. ''Ovoo'' shares many similarities with the extinct monitors ''

Aiolosaurus ''Aiolosaurus'' is an extinct genus of monitor lizard from the Late Cretaceous of Mongolia. The type and only species, ''Aiolosaurus oriens'', was named in 2000 from Ukhaa Tolgod, a rich fossil site in the Campanian-age Djadochta Formati ...

'' and ''

Cherminotus ''Cherminotus'' is an extinct genus of varanoid lizard from the Late Cretaceous of Mongolia. The type and only species, ''Cherminotus longifrons'', was named in 1984. Description and history ''Cherminotus longifrons'' was first described ...

'', which are also known from Little Ukhaa and Ukhaa Tolgod. Differences between these genera are seen in the shape of the bones in the skull. The distinguishing features of ''Ovoo'' include: *

Nasals In phonetics, a nasal, also called a nasal occlusive or nasal stop in contrast with an oral stop or nasalized consonant, is an occlusive consonant produced with a lowered velum, allowing air to escape freely through the nose. The vast majorit ...

, located behind the nostril openings, that are divided into two bones (in living monitors they are fused into one bone). *The large size of a pair of holes called

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

ry fenestrae in front of the nostril openings. *A bone called the septomaxilla that separates the premaxilla (the bone at the very tip of the snout) from the

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

(the bone that makes up the upper jaw). *The small size of a hole in the septomaxilla bone called the septomaxillary foramen. The most unusual feature of ''Ovoo'' is the presence of two small bones that are not present in any other lizard. When it was first described, these structures were called "mystery bones". The two bones are located between the eye sockets. They are positioned behind the nasal bones and in front of the

frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...

s. There are no homologous bones in any other animal, making their presence a mystery.

Classification

''Ovoo'' is one of many Late Cretaceous lizards belonging to a group called

Varanoidea Varanoidea is a superfamily of lizards, including the well-known family Varanidae (the monitors and goannas). Also included in the Varanoidea are the Lanthanotidae ( earless monitor lizards), and the extinct Palaeovaranidae. Throughout their ...

, which includes the living monitor and helodermatid lizards and the extinct

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...

s. Features linking it with these lizards include a rounded snout and a lack of contact between the maxillae and frontal bones. A phylogenetic analysis conducted with its first description placed it within the monitor subfamily Varaninae. ''Ovoo'' is the oldest known member of Varaninae. Of the other Mongolian varanoids, ''Aiolosaurus'' was placed as a basal member of

Varanidae The Varanidae are a family of lizards in the superfamily Varanoidea and order Anguimorpha. The family, a group of carnivorous and frugivorous lizards, includes the living genus '' Varanus'' and a number of extinct genera more closely related ...

(the family to which Varaninae belongs) and ''Cherminotus'' was placed in Lanthanotinae (another subfamily within Varanidae). Because of their age, ''Ovoo'', ''Aiolosaurus'', and ''Cherminotus'' may be representatives of the first radiation of monitor lizards. The Late Cretaceous varanoids '' Saniwides'' and ''

Telmasaurus ''Telmasaurus'' is an extinct genus of varanoid lizard from the Late Cretaceous of Mongolia. Fossils have been found from the Djadokha and Barun Goyot Formations that date between the early and middle Campanian stage from approximately 80 to 71 ...

'' are just as old as these lizards and have traditionally been viewed as the oldest monitors, but they were placed outside the monitor family in the 2008 analysis. Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from the analysis:

References

{{Taxonbar, from1=Q4046102, from2=Q7114202 Late Cretaceous lepidosaurs of Asia Monitor lizards Fossil taxa described in 2008