Lepidosauria

Lepidosaurians Temporal range: Triassic - Holocene, 250–0Ma PreЄ Є O S D C P T J K Pg N
A Coachella Valley fringe-toed lizard
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Neodiapsida
Infraclass:	Lepidosauromorpha
Superorder:	Lepidosauria Haeckel, 1866
Orders
Rhynchocephalia Squamata

The foot of a skink, showing lepidosaurs' characteristic overlapping scales

The Lepidosauria (from Greek meaning scaled lizards) 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. The squamata includes snakes, lizards, and amphisbaenia. The Sphenodontidae are now only represented by two species of tuatara native to New Zealand although they were much more widespread and varied 200 million years ago. Lepidosauria is the sister taxon to Archosauria, which includes Aves and Crocodilia. The only surviving tuataras are the family Sphenodontidae, genus Sphenodon. Lizards and snakes are the most speciose group of Lepidosaurs and combined contain about 7,970 species.^[1] However, the tuatara, which is found in New Zealand, has only two remaining species. There are noticeable distinguishing morphological differences between lizards, tuataras, and snakes.

Diagnosis

The reptiles in the subclass Lepidosauria can be distinguished from other reptiles by a variety of characteristics. First, the males have evolved a hemipenis instead of a single penis with erectile tissue that is found in crocodilians, birds, mammals, and turtles. The hemipenis can be found in the base of the tail. The tuatara has not fully evolved the hemipenis, but instead has shallow paired outpocketings of the posterior wall of the cloaca that have been determined to be precursors to the hemipenis.^[2]

Second, most Lepidosaurs have the ability to autotomize their tails. However, this trait has been lost on some recent species. In lizards, 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 one 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.^[2] In snakes, the tail separates between vertebrae and some do not experience regrowth.^[2]

Third, the scales in lepidosaurs are horny (keratinized) structures of the epidermis, allowing them to be shed collectively, contrary to the scutes seen in other reptiles.^[2] This is done in different cycles, depending on the species. However, lizards generally shed in flakes while snakes shed in one piece. Unlike scutes, the ledpidosaur scales will often overlap like roof tiles.

General description

Extant reptiles are divided into two clades: the Anapsida and the Diapsida. These clades are named for their number of temporal fenestrations in the skull. The Anapsida lack any temporal fenestration, but the Diapsida have a pair of temporal fenestrations on each side of the skull.^[3] The Anapsida are represented only by turtles and their extinct relatives. Diapsida is composed Lepidosauria and their sister taxa Archosauria.^[2] The subclass Lepidosauria is then split into Squamata and Sphenodontidae.

The group Squamata includes snakes, lizards, and amphisbaenians. Squamata can be characterized by the reduction or loss of limbs. Snakes, some lizards, and most amphisbaenians have evolved the complete loss of their limbs. The skin of all squamates is covered in scales. The upper jaw of Squamates is movable on the cranium, a configuration called kinesis.^[4] This is made possible by a loose connection between the quadrate and its neighboring bones.^[5] Without this, snakes would not be able consume prey that are much larger than themselves. However, the tuatara does not share this characteristic with the other Lepidosauria. 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.^[1]

Sphenodontidae includes tuatara and their extinct relatives and can presently only be found on small islands off New Zealand. The tuatara has amphicoelous vertebrae, which means the vertebrae are hollowed out at both ends.^[4] The tuatara also have the ability to autotomize their tails. A well-developed median or pineal eye is present on the top of the head (parietal region) of the tuatara and their teeth are considered to be relatively large.^[2]

Fossil record

Snakes do not have an extensive fossil record, but the oldest known snake fossil is from between early and late Cretaceous period.^[2] There were tertiary fossil snakes that became extinct by the end of the Eocene period. The first colubrid also appeared in the Eocene period.^[2] Lizards first appeared in the middle Jurassic period, and this is when the scincomorph and the anguimorph lizards were first seen. The Gekkotans first appear in the late Jurassic period and the iguanians first appear in the late Cretaceous period.^[2] The lizards of the Cretaceous period represent extinct genera and species.^[6] The majority of amphisbaenians first appeared during the early Cenozoic period.^[1] The Sphenodontidae fossil record first appears in the Lower Triassic period, making it the earliest found fossil record of the Lepidosauria. The tuatara can now only be found on small islands off the New Zealand coast. However, fossil records show that it once lived on the main land New Zealand and was distributed globally.^[4]

Evolution

Lizards are originally split into two clades: the Iguania and the Scleroglossa. Snakes and amphisbaenians belong within the clade Scleroglossa. The analysis of teeth results in Iguania being made up of the sister taxa Chamaeleonidae and Agamidae.^[2] Snakes are actually a branch within the lizard group. In fact, some lizards, such as the Varanids, are more closely related to snakes that they are to other lizards. Varanids, which are semi-aquatic, large-bodied carnivorous lizards that reside in Australia.^[7]

Snakes currently have about 3070 extant species that are grouped into the scolecophidians and the alethinophidians.^[1] The scolecophidians are made up of about 370 species and are represented by small snakes with a limited gape size.^[1] The alethinophidians are made up of about 2,700 species and are represented by the more common snakes.^[1] As snakes evolved, their gape size increased from the narrowness of the scolecophidians. This allowed for the digestion of larger prey. There are about 2,500 species of poisonous snakes that all belong to the Caenophidia.^[1]

The amphisbaenians are mostly limbless, with the exception of three species that kept reduced forms of front limbs. Morphological data shows that species with front limbs form a sister group to those that are limbless. This means that the amphisbaenians’ loss of limbs occurred only once.^[1]

The Tuataras originated in the Triassic period and were distributed world wide. All species went extinct in the late Cretaceous, except for the two species of tuatara that now lives in New Zealand. This extinction correlated with the appearance of large mammals. The current Tuatara’s bone structure has evolved slightly from the species that existed in the Triassic period. Now, wild populations of the Tuatara can be found on thirty-two islands in addition to three islands in which populations have formed due to migration.^[8]

Ecology

Within the subclass Lepidosauria there are herbivores, omnivores, insectivores, and carnivores. The herbivores consist of iguanines, some agamids, and some skinks.^[6] Most lizards species and some snake species are insectivores. The remaining snake species, tuataras, and amphisbaenians are carnivores. Some snake species are generalist and eat a narrow range of prey. For example, Salvadora eat only lizards.^[6] The remaining lizards are omnivores and can consume plants or insects.

Viperines can sense their prey’s infrared radiation through bare nerve endings on the skin of their heads.^[6] Also, viperines and some boids have thermal receptors that allow them to target their prey’s heat.^[6] Many snakes are able to obtain their prey through constriction. 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.^[6] Some snakes have fangs that produce venomous bites. This allows the snake to consume an already dead or unconscious prey. Also, some venoms include a proteolytic component that aids in digestion.^[6] Chameleons grasp 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.^[6]

The major predators of reptiles are other reptiles. Smaller reptiles are preyed upon by larger reptiles. Also, reptile eggs are consumed by other reptiles. Furthermore, birds are a predator of reptiles. Raptors, wading birds, roadrunners, are all examples of birds that consume reptiles. Mammals are also known to consume reptiles.^[6]

The geographic ranges of snakes and lizards are vast and cover all but the most extreme cold parts of the globe. Amphisbaenians exist in Florida, mainland Mexico including Baja California, Mediterranean region, Middle East, North Africa, sub-Saharan Africa, South America, and the Caribbean.^[5] The tuatara is confined to only a few rocky islands of New Zealand. It digs burrows to live in and preys mostly on insects.^[4]

Life history and behaviour

Squamates are represented by viviparous, ovoviviparous, and oviparous species. Viviparous means that the female gives birth to live young. Ovoviviparous means that the egg will develop inside of the female’s body. Oviparous means that the female lays eggs. A few species within Squamata have the ability to reproduce asexually.^[7] The tuatara lays eggs that are usually about one inch in length. These eggs take about fourteen months to incubate.^[4]

While in the egg, the Squamata embryo develops an egg tooth on the premaxillary that helps the animal emerge from the egg.^[6] A reptile will increase three to twentyfold in length from hatching to adulthood.^[6] There are three main life history events that Lepidosaurs reach: hatching/birth, sexual maturity, and reproductive senility.^[6]

Most Lepidosaurs rely on camouflage as one of their main defenses. Some species have evolved to blend in with their ecosystem, while others are able 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 the Echinosauria, have evolved the defense of feigning death.^[6]

Conservation

Boots made from snake skin, the three closest from diamondback rattlers

Snakes are commonly feared throughout the world. There have actually been advertised rattlesnake roundups in North America. Data shows that between 1959 and 1986 an average of 5,563 rattlesnakes were killed per year in Sweetwater, Texas, due to rattlesnake roundups, and these roundups have led to documented declines and local extirpations of rattlesnake populations, especially Eastern Diamondbacks in Georgia.^[2]

Habitat destruction is 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 dwelling because there is an abundance of rodent and insect prey. However, these reptiles are seen as pests and are often exterminated.^[2]

People have introduced species to the Lepidosaurs’ natural habitats that have increased predation on the reptiles. For example, mongooses were introduced to Jamaica from India to control the rat infestation in sugar cane fields. As a result, the mongooses fed on the lizard population of Jamaica, which has led to the elimination and decrease of many lizard species.^[2]

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.^[6]

Unfortunately, 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.^[2] In the year 1986, the World Resource Institute totaled that 10.5 million reptile skins were traded legally. This total does not even include the illegal trades of that year.^[2] Horned lizards are popular for being harvested and stuffed.^[2] Humans must make a conscious effort to preserve the remaining species of reptiles.

Climate change has led to the need for conservation efforts to protect the existence of the Tuatara. This is because it is not possible for this species to migrate to cooler climates. Conservationists are beginning to consider the possibility of translocating them to islands with cooler climates.^[9] The range of the Tuatara has already been minimized by the introduction of cats, rats, dogs, and mustelids.^[10] The eradication of the mammals from the islands where the Tuatara still survives has helped the species increase its population. An experiment observing the Tuatara population after the removal of the Polynesian Rat showed that the Tuatara expressed an island specific increase of population after rat’s removal.^[11] However, it may be difficult to keep these small mammals from reinhabiting these islands.

References

• Evans SE. 2003. At the feet of the dinosaurs: the origin, evolution and early diversification of squamate reptiles (Lepidosauria: Diapsida). Biological Reviews, Cambridge 78: 513–551. DOI: 10.1017/S1464793103006134
• Jones MEH. 2009. Dentary tooth shape in Sphenodon and its fossil relatives (Diapsida: Lepidosauria: Rhynchocephalia). Frontiers of Oral Biology 13: 9–15.
• Jones MEH, Tennyson AJD, Worthy JP, Evans SE, Worthy TH. 2009. A sphenodontine (Rhynchocephalia) from the Miocene of New Zealand and palaeobiogeography of the tuatara (Sphenodon). Proceedings of the Royal Society B 276: 1385–1390 DOI: 10.1098/rspb.2008.1785
• Evans SE, Jones MEH. 2010. The Origin, early history and diversification of lepidosauromorph reptiles. In Bandyopadhyay S. (ed.), New Aspects of Mesozoic Biodiversity, 27 Lecture Notes in Earth Sciences 132, 27-44. DOI 10.1007/978-3-642-10311-7_2
• Jones MEH, Anderson CL, Hipsley CA, Müller J, Evans S, Schoch R. 2013. Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara). BMC Evolutionary Biology 13:208 DOI:10.1186/1471-2148-13-208
• Pyron RA, Burbrink FT, Wiens JJ: A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Evolutionary Biology 13:93.

External links

• Animaldiversity.ummz.umich.edu
• Reptile taxonomy, Benton 2004
• Lepidosaur phylogeny