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Lepidosauria

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Lepidosaurs
Temporal range:
Middle Triassic-Present,[1]240–0Ma
Collage of five lepidosaurs. Clockwise from top left:tuatara,black mamba,green iguana,Smaug breyeriandreticulated python
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Lepidosauromorpha
Superorder: Lepidosauria
Haeckel,1866
Orders

TheLepidosauria(/ˌlɛpɪdoʊˈsɔːriə/,from Greek meaningscaled lizards) is a subclass or superorder ofreptiles,containing the ordersSquamataandRhynchocephalia.Squamata includeslizardsandsnakes.[2]Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day.[3]Rhynchocephalia was a formerly widespread and diverse group of reptiles in theMesozoic Era.[4]However, it is represented by only one living species: thetuatara(Sphenodon punctatus),a superficially lizard-like reptile native toNew Zealand.[5][6]

Lepidosauria is a monophyletic group (i.e. aclade), containing all descendants of thelast common ancestorof squamates and rhynchocephalians.[7]Lepidosaurs can be distinguished from other reptiles via several traits, such as largekeratinousscaleswhich may overlap one another. Purely in the context of moderntaxa,Lepidosauria can be considered thesister taxontoArchelosauria,which includesTestudines(turtles),Aves(birds) andCrocodilia(crocodilians). Lepidosauria is encompassed byLepidosauromorpha,a broader group defined as all reptiles (living or extinct) closer to lepidosaurs than toarchosaurs.

Evolution

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Lepidosauromorphais thought to have split off from the ancestor ofArchelosauriaduring thePermianperiod.[8]The earliest members ofLepidosauromorphadate the Early Triassic.Sophinetafrom the Early Triassic may be the oldest known lepidosaur, but its exact placement is uncertain.[9]The earliestrhynchocephalian,Wirtembergia,is known from theMiddle Triassic.[10]While the lepidosaurMegachirellamay represent astem-groupsquamate from the Middle Triassic[11]the earliest modern members of the group are known from the Middle Jurassic.[12]Squamates underwent a great radiation in theCretaceous,[13]while rhynchocephalians declined during the same time period.[14]

Description

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Extantreptiles are in thecladeDiapsida,named for two pairstemporal fenestraepresent on the skull behind the eye socket.[15]Until recently, Diapsida was said to be composed of Lepidosauria and their sister taxa Archosauria.[16]The subclass Lepidosauria is then split into Squamata[17]and Rhynchocephalia. More recent morphological studies[18][19]andmolecularstudies[20][21][22][23][24][25]also place turtles firmly within Diapsida, even though they lack temporal fenestrations.

The quadrate bone is particularly elongated in snakes, to facilitatecranial kinesis

The reptiles in the subclass Lepidosauria can be distinguished from other reptiles by a variety of characteristics.[26]Lepidosaurs are suggested to be distinguished from more primitive lepidosauromorphs by the development of a conch on thequadrate,allowing for the development of atympanic membranein the ear (a trait lost in the tuatara, but present in early rhynchocephalians), as well as the development of a subolfactory process on thefrontal bonesof the skull.[9][27]

Schematic skull of asquamateshowing the location of major dermal bones

The group Squamata[17]includes snakes, lizards, and amphisbaenians. Squamata can be characterized by the reduction or loss of limbs. Snakes andlegless lizardshave evolved the complete loss of their limbs. The upper jaw of Squamates is movable on thecranium,a configuration calledkinesis.[28]This is made possible by a loose connection between the quadrate and its neighboring bones.[29]Without this, snakes would not be able consume prey that are much larger than themselves. Amphisbaenians are mostly legless like snakes, but are generally much smaller. Three species of amphisbaenians have kept reduced front limbs and these species are known for actively burrowing in the ground.[30]The tuatara and some extinct rhynchocephalians have a more rigid skull with a complete lower temporal bar closing the lower temporal fenestra formed by the fusion of thejugalandquadrate/quadratojugalbones, similar to the condition found in primitive diapsids. However early rhynchocephalians and lepidosauromorphs had an open lower temporal fenestra, without a complete temporal bar, so this is thought to be a reversion rather than retention. The temporal bar is thought to stabilise the skull during biting.[31]

Male squamates have evolved a pair ofhemipenisesinstead of a single penis with erectile tissue that is found incrocodilians,birds,mammals,andturtles.The hemipenis can be found in the base of the tail. The tuatara does not have a hemipenis, but instead has shallow paired outpocketings of the posterior wall of thecloaca.[16]

The foot of askink,showing lepidosaurs' characteristic overlappingscales

Second, most lepidosaurs have the ability toautotomizetheir tails. However, this trait has been lost on some recent species. In lizards and rhynchocephalians, fracture planes are present within the vertebrae of the tail that allow for its removal. Some lizards have multiple fracture planes, while others just have a single fracture plane. The regrowth of the tail is not always complete and is made of a solid rod of cartilage rather than individual vertebrae.[16]In snakes, the tail separates between vertebrae and some do not experience regrowth.[16]

Third, the scales in lepidosaurs are horny (keratinized) structures of theepidermis,allowing them to be shed collectively, contrary to thescutesseen in other reptiles.[16]This is done in different cycles, depending on the species. However, lizards generally shed in flakes whilesnakesshed in one piece. Unlike scutes, lepidosaur scales will often overlap likeroof tiles.

Biology and ecology

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Python(top) andrattlesnake(bottom) featuring pit organs for infrared sensing. Red arrows indicating pit organs and black arrows pointing to the nostrils

Squamatesare represented byviviparous,ovoviviparous,andoviparousspecies. Viviparous means that the female gives birth to live young, Ovoviviparous means that the egg will develop inside the female's body and Oviparous means that the female lays eggs. A few species within Squamata have the ability to reproduceasexually.[32]The tuatara lays eggs that are usually about one inch in length and which take about 14 months to incubate.[28]

While in the egg, the Squamata embryo develops anegg toothon thepremaxillarythat helps the animal emerge from the egg.[33]A reptile will increase three to twentyfold in length from hatching to adulthood.[33]There are three main life history events that lepidosaurs reach: hatching/birth, sexual maturity, and reproductive senility.[33]

Becausegular pumpingis so common in squamates, and is also found in the tuatara, it is assumed that it is an original trait in the group.[34]

Most lepidosaurs rely oncamouflageas one of their main defenses. Some species have evolved to blend in with their ecosystem, while others are able to change their skin color to blend in with their current surroundings. The ability to autotomize the tail is another defense that is common among lepidosaurs. Other species, such as theEchinosauria,have evolved the defense of feigning death.[33]

Hunting and diet

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Agreen crested lizardfeeding on an invertebrate

Viperinescan sense their prey'sinfrared radiationthrough bare nerve endings on the skin of their heads.[33]Also, viperines and someboidshavethermalreceptors that allow them to target their prey's heat.[33]Many snakes are able to obtain their prey throughconstriction.This is done by first biting the prey, then coiling their body around the prey. The snake then tightens its grip as the prey struggles, which leads to suffocation.[33]Some snakes have fangs that producevenomousbites, which allows the snake to consume unconscious, or even dead, prey. Also, some venoms include a proteolytic component that aids in digestion.[33]Chameleonsgrasp their prey with a projectile tongue. This is made possible by a hyoid mechanism, which is the contraction of the hyoid muscle that drives the tip of the tongue outwards.[33]

Aneastern blue-tongued lizardpreyed upon by aneastern brown snake

Within the subclass Lepidosauria there areherbivores,omnivores,insectivores,andcarnivores.The herbivores consist of iguanines, some agamids, and someskinks.[33]Most lizard species and some snake species are insectivores. The remaining snake species, tuataras, and amphisbaenians, are carnivores. While some snake species are generalist, others eat a narrow range of prey - for example,Salvadoraonly eat lizards.[33]The remaininglizardsare omnivores and can consume plants or insects. The broad carnivorous diet of the tuatara may be facilitated by its specialised shearing mechanism, which involves a forward movement of the lower jaw following jaw closure.[35]

While birds, including raptors, wading birds and roadrunners, and mammals are known to prey on reptiles, the major predator is other reptiles. Some reptiles eat reptile eggs, for example the diet of theNile monitorincludes crocodile eggs, and small reptiles are preyed upon by larger ones.[33]

Conservation

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Global range oflizardspecies

The geographic ranges of lepidosaurs are vast and cover all but the most extreme cold parts of the globe. Amphisbaenians exist inFlorida,mainlandMexico,includingBaja California,theMediterranean region,theMiddle East,North Africa,sub-Saharan Africa,South America,and theCaribbean.[29]The tuatara is confined to only a few rocky islands of New Zealand, where it digs burrows to live in and preys mostly on insects.[28]

Climate changehas led to the need forconservationefforts to protect the existence of thetuatara.This is because it is not possible for this species to migrate on its own to cooler areas. Conservationists are beginning to consider the possibility of translocating them to islands with cooler climates.[36]The range of the tuatara has already been minimized by the introduction of cats, rats, dogs, and mustelids to New Zealand.[37]Theeradicationof themammalsfrom the islands where the tuatara still survives has helped the species increase its population. An experiment observing the tuatara population after the removal of thePolynesian ratshowed that the tuatara expressed an island-specific increase of population after the rats' removal.[38]However, it may be difficult to keep these small mammals from reinhabiting these islands.

Habitat destructionis the leading negative impact of humans on reptiles. Humans continue to develop land that is important habitat for the lepidosaurs. The clear-cutting of land has also led to habitat reduction. Some snakes and lizards migrate toward human dwellings because there is an abundance of rodent and insect prey. However, these reptiles are seen as pests and are oftenexterminated.[16]

Interactions with humans

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Boots made from snake skin, the three closest fromdiamondback rattlesnakes

Snakes are commonly feared throughout the world. Bounties were paid for dead cobras under theBritish RajinIndia;similarly, there have been advertisedrattlesnakeroundups inNorth America.Data shows that between 1959 and 1986 an average of 5,563 rattlesnakes were killed per year inSweetwater, Texas,due to rattlesnake roundups, and these roundups have led to documented declines and local extirpations of rattlesnake populations, especiallyEastern Diamondbacksin Georgia.[16]

People have introduced species to the lepidosaurs' natural habitats that have increased predation on the reptiles. For example, mongooses were introduced toJamaicafromIndiato control the rat infestation in sugar cane fields. As a result, themongoosesfed on the lizard population of Jamaica, which has led to the elimination or decrease of many lizard species.[16]Actions can be taken by humans to help endangered reptiles. Some species are unable to be bred in captivity, but others have thrived. There is also the option of animal refuges. This concept is helpful to contain the reptiles and keep them from human dwellings. However, environmental fluctuations and predatorial attacks still occur in refuges.[33]

Reptile skins are still being sold. Accessories, such as shoes, boots, purses, belts, buttons, wallets, and lamp shades, are all made out of reptile skin.[16]In 1986, the World Resource Institute estimated that 10.5 million reptile skins were traded legally. This total does not include the illegal trades of that year.[16]Horned lizards are popularly harvested andstuffed.[16]Some humans are making a conscious effort to preserve the remaining species of reptiles, however.

References

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