Varanoidea
Encyclopedia
Varanoidea is a superfamily
of lizard
s, including the well-known family Varanidae
(the monitors or goanna
). Also included in the Varanoidea are such extinct marine and semi-aquatic
forms as mosasaur
s and dolichosaurs, the venomous helodermatids (Gila monster
s and beaded lizard
s), the Lanthanotidae (earless monitor lizards), and the extinct Necrosauridae.
Throughout their long evolution
ary history, varanoids have exhibited great diversity both in habitat and form. This superfamily includes both the largest aquatic lizards known, the mosasaurs Tylosaurus
and Mosasaurus
(15+ meters in length), but also the largest-known terrestrial lizard, Megalania
(5-6 meters), and the largest extant lizard, the Komodo dragon
(Varanus komodoensis, 3+ meters).
Also known by the more archaic term "platynotans," the varanoids first appear in the fossil
record in the latter part of Early Cretaceous
, but possible varanoid ancestors have been traced back as far as Early Jurassic
times. Among the earliest known varanoids are the monitor-like necrosaurids Palaeosaniwa canadensis
from the Campanian
(roughly 71-82 mya) of North America
and Estesia mongoliensis
and Telmasaurus grangeri
, both from the Campanian of Mongolia
. Varanoids survived the Cretaceous–Tertiary extinction event and flourished worldwide during the Cenozoic Era. Current evidence strongly suggests that snakes evolved from an aquatic or burrowing varanoid ancestor, though much debate continues over precisely which lineage of varanoids.
Carroll characterizes the varanoids as "the most advanced of all lizards in achieving large size and an active, predaceous way of life." Some taxa, such as the Gila monster (Heloderma) and the extinct necrosaurs are armoured with osteoderms (bony deposits on the skin), and many forms have hinged jaws, allowing them to dislocate and distended their jaws during feeding.
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
of lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, including the well-known family Varanidae
Varanidae
Varanidae is a group of lizards of the superfamily Varanoidea. The family is a group of carnivorous lizards which includes the largest living lizard, the Komodo dragon, and the crocodile monitor. Varanidae contains the living genus Varanus and a number of extinct taxa...
(the monitors or goanna
Goanna
Goanna is the name used to refer to any number of Australian monitor lizards of the genus Varanus, as well as to certain species from Southeast Asia.There are around 30 species of goanna, 25 of which are found in Australia...
). Also included in the Varanoidea are such extinct marine and semi-aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
forms as mosasaur
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
s and dolichosaurs, the venomous helodermatids (Gila monster
Gila monster
The Gila monster is a species of venomous lizard native to the southwestern United States and northwestern Mexican state of Sonora...
s and beaded lizard
Beaded lizard
The Beaded lizard is a species of venomous lizard found principally in Mexico and southern Guatemala. Along with its congener, the Gila monster , it is the only lizard to have evolved an overt venom delivery system, and has long been considered the only venomous lizard...
s), the Lanthanotidae (earless monitor lizards), and the extinct Necrosauridae.
Throughout their long evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary history, varanoids have exhibited great diversity both in habitat and form. This superfamily includes both the largest aquatic lizards known, the mosasaurs Tylosaurus
Tylosaurus
Tylosaurus was a mosasaur, a large, predatory marine lizard closely related to modern monitor lizards and to snakes.-Paleobiology:...
and Mosasaurus
Mosasaurus
Mosasaurus is a genus of mosasaur, carnivorous, aquatic lizards, somewhat resembling flippered crocodiles, with elongated heavy jaws. The genus existed during the Maastrichtian age of the Cretaceous period , around 70-65 millions years ago in the area of modern Western Europe and North America...
(15+ meters in length), but also the largest-known terrestrial lizard, Megalania
Megalania
Megalania is a giant extinct goanna or monitor lizard. It was part of a megafaunal assemblage that inhabited southern Australia during the Pleistocene, and appears to have disappeared around 40,000 years ago...
(5-6 meters), and the largest extant lizard, the Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...
(Varanus komodoensis, 3+ meters).
Evolution
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record in the latter part of Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
, but possible varanoid ancestors have been traced back as far as Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
times. Among the earliest known varanoids are the monitor-like necrosaurids Palaeosaniwa canadensis
Palaeosaniwa
Palaeosaniwa is an extinct genus of varanoid lizard from the late Cretaceous of North America. The name, given by Charles Whitney Gilmore in 1928, means "ancient Saniwa."- Description :...
from the Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
(roughly 71-82 mya) of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Estesia mongoliensis
Estesia
Estesia is an extinct genus of Late Cretaceous helodermatoid lizard found in the Gobi Desert in Mongolia. It was discovered in June 1990 by a joint expedition made up of Mongolian and American palaeontologists, and described in 1992 by Mark Norell, Malcolm McKenna and Michael Novacek...
and Telmasaurus grangeri
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 75 million years ago. The type species T...
, both from the Campanian of Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
. Varanoids survived the Cretaceous–Tertiary extinction event and flourished worldwide during the Cenozoic Era. Current evidence strongly suggests that snakes evolved from an aquatic or burrowing varanoid ancestor, though much debate continues over precisely which lineage of varanoids.
Carroll characterizes the varanoids as "the most advanced of all lizards in achieving large size and an active, predaceous way of life." Some taxa, such as the Gila monster (Heloderma) and the extinct necrosaurs are armoured with osteoderms (bony deposits on the skin), and many forms have hinged jaws, allowing them to dislocate and distended their jaws during feeding.