The Lepidosauria are reptiles with overlapping scales. This subclass includes Squamata and Rhynchocephalia. It is a monophyletic group and therefore contains all descendents of a common ancestor. Squamata includes snakes, lizards, and amphisbaenia. Rhynchocephalia was a widespread and diverse group 220-100 million years ago; however, it is now represented only by the genus Sphenodon, which contains a single species of tuatara, native to New Zealand. Lepidosauria is the sister taxon to Archosauria, which includes Aves and Crocodilia. Lizards and snakes are the most speciose group of lepidosaurs and, combined, contain over 9,000 species. There are many noticeable distinguishing morphological differences between lizards, tuataras, and snakes.
Squamata is the largest order of reptiles, comprising lizards, snakes and amphisbaenians, which are collectively known as squamates or scaled reptiles. With over 10,000 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish, and roughly equal in number to the Saurischia. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. Squamata is the most variably sized order of reptiles, ranging from the 16 mm (0.63 in) dwarf gecko to the 5.21 m (17.1 ft) green anaconda and the now-extinct mosasaurs, which reached lengths of over 14 m (46 ft).
Ophidia is a group of squamate reptiles including modern snakes and all reptiles more closely related to snakes than to other living groups of lizards.
Mosasaurs are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period, with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Bipes is a genus of amphisbaenians found only in Mexico, the sole living member of the family Bipedidae. Commonly known as ajolotes, they are carnivorous, burrowing reptiles, but unlike other species of amphisbaenians, they possess two stubby forelimbs placed far forward on the body. The shovel-like limbs are used to scrape away soil while burrowing, in a manner similar to a mole. Evidence for their occurrence in the United States is reviewed by Somma (1993).
Caenagnathidae is a family of bird-like maniraptoran theropod dinosaurs from the Late Cretaceous of North America and Asia. They are a member of the Oviraptorosauria, and close relatives of the Oviraptoridae. Like other oviraptorosaurs, caenagnathids had specialized beaks, long necks, and short tails, and would have been covered in feathers. The relationships of caenagnathids were long a puzzle. The family was originally named by Charles Hazelius Sternberg in 1940 as a family of flightless birds. The discovery of skeletons of the related oviraptorids revealed that they were in fact non-avian theropods, and the discovery of more complete caenagnathid remains revealed that Chirostenotes pergracilis, originally named on the basis of a pair of hands, and "Ornithomimus" elegans, named from a foot, were caenagnathids as well.
Rhineuridae is a family of amphisbaenians that includes one living genus and species, Rhineura floridana, as well as many extinct species belonging to both Rhineura and several extinct genera. The living R. floridana is found only in Florida no further north than the panhandle, but extinct species ranged across North America, some occurring as far west as Oregon. The family has a fossil record stretching back 60 million years to the Paleocene and was most diverse in the continental interior during the Eocene and Oligocene.
Blanus is a genus of amphisbaenians found in the Mediterranean region of Europe and North Africa. Like other amphisbaenians, Blanus are specialized for a subterranean existence, with a long, slender body, reduced limbs, and rudimentary eyes. The skull is powerfully constructed, allowing the animal to push through soil to create a burrow. The jaws are well-developed, with large, recurved teeth and a pair of canine-like teeth in the upper jaw.
Sineoamphisbaena is an extinct genus of squamate of uncertain phylogenetic placement. Its fossils are known from the Late Cretaceous deposits in Inner Mongolia, China. Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that it was the oldest known amphisbaenian; this, however, was challenged by other authors, such as Kearney (2003) and Conrad (2008), who instead assigned Sineoamphisbaena to the group of squamates variously known as Macrocephalosauridae, Polyglyphanodontidae or Polyglyphanodontia.
Scleroglossa is a clade of lizards that includes geckos, autarchoglossans, and amphisbaenians. Scleroglossa is supported by phylogenetic analyses that use morphological features. According to most morphological analyses, Scleroglossa is the sister group of the clade Iguania, which includes iguanas, chameleons, agamids, and New World lizards. Together, Scleroglossa and Iguania make up crown group Squamata, the smallest evolutionary grouping to include all living lizards and snakes.
Platynota is a group of anguimorph lizards and thus belongs to the order Squamata of the class Reptilia. Since it was named in 1839, it has included several groups, including monitor lizards, snakes, mosasaurs, and helodermatids. Its taxonomic use still varies, as it is sometimes considered equivalent to the group Varanoidea and other times viewed as a distinct group. It is phylogenetically defined as a clade containing Varanidae. It also includes many extinct species.
Obamadon is an extinct genus of polyglyphanodontian lizards from the Late Cretaceous of North America. Fossils have been found in the Hell Creek Formation of Montana and the Lance Formation of Wyoming. Researchers describe it as being distinguished by its "tall, slender teeth with large central cusps separated from small accessory cusps by lingual grooves." The type species was named Obamadon gracilis after United States president Barack Obama, "in reference to the tall, straight teeth, and the manner in which Mr. Obama has acted as a role model of good oral hygiene for the world." According to Nicholas R. Longrich of Yale University, the creature "was probably a foot long, [and] with these tall, slender teeth it used to eat insects and plant matter."
Polyglyphanodontia is an extinct clade of lizards from the Cretaceous that includes around a dozen genera. Polyglyphanodontians were the dominant group of lizards in North America and Asia during the Late Cretaceous. Most polyglyphanodontians are Late Cretaceous in age, though the oldest one, Kuwajimalla kagaensis, is known from the Early Cretaceous Kuwajima Formation (Japan). Early Cretaceous South American taxon Tijubina, and possibly also Olindalacerta, might also fall within Polyglyphanodontia or be closely allied to the group, but if so, they would be two of only three Gondwanan examples of an otherwise Laurasian clade. They produced a remarkable range of forms. Chamopsiids, including Chamops, were characterized by large, blunt, crushing teeth, and were most likely omnivores. Macrocephalosaurus, from the Gobi Desert, was a specialized herbivore; it grew to roughly a meter long and had multicusped, leaf-shaped teeth like those of modern iguanas. Polyglyphanodon, from the Maastrichtian of Utah, was another herbivore, but its teeth formed a series of transverse blades, similar to those of Trilophosaurus. Peneteius had remarkable, multicusped teeth, similar to those of mammals. The polyglyphanodontids first appear in the latter part of the Early Cretaceous in North America, and became extinct during the Cretaceous-Paleogene extinction event. Polyglyphanodontians closely resembled the teiid lizards, and purported teiid lizards from the Late Cretaceous appear to be polyglyphanodontians.
Bipedidae is a family of amphisbaenian lizards that includes the extant genus Bipes represented by four species from Mexico and the extinct genus Anniealexandria represented by one species that lived in what is now Wyoming during the earliest Eocene approximately 55 million years ago. Phylogenetic analysis indicates that Bipedidae is most closely related to the family Blanidae, which includes the living genus Blanus.
Hoburogekko is an extinct genus of gecko that includes a single species, Hoburogekko suchanovi, from the Early Cretaceous of Mongolia. It is known from two fossil specimens, one preserving the front part of the skull and the other preserving part of the lower jaw. Hoburogekko is one of four known Mesozoic geckos or gecko-like lizards, the others being Cretaceogekko from the Early Cretaceous of Burma, AMNH FR21444, an undescribed specimen from a slightly older deposit in Mongolia, and Gobekko from the Late Cretaceous of Mongolia. Hoburogekko is the third oldest known gecko behind AMNH FR21444 and Cretaceogekko.
The Lacertoidea is a group of lizards that includes the Lacertidae, Teiidae, Gymnophthalmidae, and the burrowing Amphisbaenia.
Archaerhineura was a genus of amphisbaenian lizards in the family Rhineuridae that is now extinct. The only species is Archaerhineura mephitis, named in 2015 on the basis of a single fragment of the lower jaw from the Polecat Bench Formation in Park County, Wyoming, which dates to the late Paleocene. Archaerhineura is one of the oldest amphisbaenians and was part of an evolutionary radiation of Rhineuridae in the Paleocene several million years after the Cretaceous–Paleogene extinction event. This rhineurid radiation coincided with the radiation of another group of amphisbaenians, Amphisbaeniformes, which includes the still-extant families Blanidae and Amphisbaenidae. The presence of Archaerhineura and other Paleocene rhineurids in the western United States indicates that amphisbaenians, which would later have a nearly global distribution, originated in North America.
Anniealexandria is an extinct genus of amphisbaenian lizard known by the type species Anniealexandria gansi from the earliest Eocene of Wyoming. Anniealexandria is the only known member of the family Bipedidae in the fossil record, which otherwise only includes the extant genus Bipes from Mexico. It was named in 2009 in honor of Annie Montague Alexander, founder of the University of California Museum of Paleontology. Remains of Anniealexandria are known only from a single fossil locality in the Bighorn Basin called Castle Gardens, but within the locality its fossils are common in the Willwood Formation, usually consisting of isolated jaw bones and vertebrae. Anniealexandria seems to have been a common component of a paleofauna that included fifteen other lizard species and existed in western North America during a period of global warming in the latest Paleocene and earliest Eocene.
Pariguana is an extinct genus of iguanid lizard from the Late Cretaceous of western North America. It is known from a single type species, Pariguana lancensis, named in 2012 on the basis of a partial lower jaw from the Lance Formation in eastern Wyoming. This jaw bone comes from a layer dated approximately 650,000 years before the Cretaceous–Paleogene extinction event. Pariguana is the oldest definitive iguanid from North America, and may represent the first stage of the iguanian evolutionary radiation from Asia into North America.
