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

The Lepidosauria (/ˌlɛpɪdoʊˈsɔːriə/, from Greek meaning scaled lizards) is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes lizards and snakes.[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 the Mesozoic Era.[4] However, it is represented by only one living species: the tuatara (Sphenodon punctatus), a superficially lizard-like reptile native to New Zealand.[5][6]

Lepidosauria is a monophyletic group (i.e. a clade), containing all descendants of the last common ancestor of squamates and rhynchocephalians.[7] Lepidosaurs can be distinguished from other reptiles via several traits, such as large keratinous scales which may overlap one another. Purely in the context of modern taxa, Lepidosauria can be considered the sister taxon to Archosauria, which includes Aves (birds) and Crocodilia. Testudines (turtles) may be related to lepidosaurs or to archosaurs, but no consensus has been reached on this subject. Lepidosauria is encompassed by Lepidosauromorpha, a broader group defined as all reptiles (living or extinct) closer to lepidosaurs than to archosaurs.

Evolution

Lepidosauromorpha is thought to have split off from the ancestor of Archelosauria during the Permian period.[8] The earliest members of Lepidosauromorpha date the Early Triassic. Sophineta from the Early Triassic may be the oldest known lepidosaur, but its exact placement is uncertain.[9] The earliest rhynchocephalian, Wirtembergia, is known from the Middle Triassic.[10] While stem-group squamates may be known 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 the Cretaceous,[13] while rhynchcephalians declined during the same time period.[14]

Description

Extant reptiles are in the clade Diapsida, named for two pairs temporal fenestrae present 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] and molecular studies[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 facilitate cranial 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 the quadrate, allowing for the development of a tympanic membrane in the ear (a trait lost in the tuatara, but present in early rhynchocephalians), as well as the development of a subolfactory process on the frontal bones of the skull.[9][27]

Schematic skull of a squamate showing 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 and legless lizards have evolved the complete loss of their limbs. The upper jaw of Squamates is movable on the cranium, a configuration called kinesis.[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 the jugal and quadrate/quadratojugal bones, 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 of hemipenises 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 does not have a hemipenis, but instead has shallow paired outpocketings of the posterior wall of the cloaca.[16]

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

Second, most lepidosaurs have the ability to autotomize their tails. However, this trait has been lost on some recent species. In lizards and rhynchoc, 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 the epidermis, allowing them to be shed collectively, contrary to the scutes seen in other reptiles.[16] This is done in different cycles, depending on the species. However, lizards generally shed in flakes while snakes shed in one piece. Unlike scutes, lepidosaur scales will often overlap like roof tiles.

Biology and ecology

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

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 the female's body and Oviparous means that the female lays eggs. A few species within Squamata have the ability to reproduce asexually.[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 an egg tooth on the premaxillary that 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]

Because gular pumping is 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 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.[33]

Hunting and diet

A green crested lizard feeding on an invertebrate

Viperines can sense their prey's infrared radiation through bare nerve endings on the skin of their heads.[33] Also, viperines and some boids have thermal receptors that allow them to target their prey's heat.[33] 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.[33] Some snakes have fangs that produce venomous bites, which allows the snake to consume unconscious, or even dead, prey. Also, some venoms include a proteolytic component that aids in digestion.[33] 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.[33]

An eastern blue-tongued lizard preyed upon by an eastern brown snake

Within the subclass Lepidosauria there are herbivores, omnivores, insectivores, and carnivores. The herbivores consist of iguanines, some agamids, and some skinks.[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, Salvadora only eat lizards.[33] The remaining lizards are 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 the Nile monitor includes crocodile eggs, and small reptiles are preyed upon by larger ones.[33]

Conservation

Global range of lizard species

The geographic ranges of lepidosaurs are vast and cover all but the most extreme cold parts of the globe. Amphisbaenians exist in Florida, mainland Mexico, including Baja California, the Mediterranean region, the Middle East, North Africa, sub-Saharan Africa, South America, and the Caribbean.[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 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 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] 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 the rats' removal.[38] However, it may be difficult to keep these small mammals from reinhabiting these islands.

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 dwellings because there is an abundance of rodent and insect prey. However, these reptiles are seen as pests and are often exterminated.[16]

Interactions with humans

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

Snakes are commonly feared throughout the world. Bounties were paid for dead cobras under the British Raj in India; similarly, there have 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.[16]

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 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 and stuffed.[16] Some humans are making a conscious effort to preserve the remaining species of reptiles, however.

References

  1. ↑ Jones, Marc EH; Anderson, Cajsa Lisa; Hipsley, Christy A; Müller, Johannes; Evans, Susan E; Schoch, Rainer R (25 September 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. PMC 4016551. PMID 24063680.
  2. ↑ Pyron, RA; Burbrink, FT; Wiens, JJ (2013). "A phylogeny and revised classification of Squamata, including 4,161 species of lizards and snakes". BMC Evolutionary Biology. 13: 93. doi:10.1186/1471-2148-13-93. PMC 3682911. PMID 23627680.
  3. ↑ Uetz, Peter (13 January 2010). "The original descriptions of reptiles". Zootaxa. 2334 (1): 59–68. doi:10.11646/zootaxa.2334.1.3.
  4. ↑ Jones, M.E.H. (2009). "Dentary Tooth Shape in Sphenodon and Its Fossil Relatives (Diapsida: Lepidosauria: Rhynchocephalia)". Frontiers of Oral Biology. 13: 9–15. doi:10.1159/000242382. ISBN 978-3-8055-9229-1. PMID 19828962.
  5. ↑ Hay, Jennifer M.; Sarre, Stephen D.; Lambert, David M.; Allendorf, Fred W.; Daugherty, Charles H. (June 2010). "Genetic diversity and taxonomy: a reassessment of species designation in tuatara (Sphenodon: Reptilia)". Conservation Genetics. 11 (3): 1063–1081. doi:10.1007/s10592-009-9952-7. hdl:10072/30480. S2CID 24965201.
  6. ↑ Jones, M.E.H.; Cree, A. (2012). "Tuatara". Current Biology. 22 (23): 986–987. doi:10.1016/j.cub.2012.10.049. PMID 23218010.
  7. ↑ Evans, S.E.; Jones, M.E.H. (2010). "The Origin, early history and diversification of lepidosauromorph reptiles". In Bandyopadhyay, S. (ed.). New Aspects of Mesozoic Biodiversity. Lecture Notes in Earth Sciences. Vol. 132. pp. 27–44. doi:10.1007/978-3-642-10311-7_2. ISBN 978-3-642-10310-0.
  8. ↑ Simões, T. R.; Kammerer, C. F.; Caldwell, M. W.; Pierce, S. E. (2022). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 8 (33): eabq1898. doi:10.1126/sciadv.abq1898. PMC 9390993. PMID 35984885.
  9. 1 2 Ford, David P.; Evans, Susan E.; Choiniere, Jonah N.; Fernandez, Vincent; Benson, Roger B. J. (2021-08-25). "A reassessment of the enigmatic diapsid Paliguana whitei and the early history of Lepidosauromorpha". Proceedings of the Royal Society B: Biological Sciences. 288 (1957): 20211084. doi:10.1098/rspb.2021.1084. ISSN 0962-8452. PMC 8385343. PMID 34428965.
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  11. ↑ Simōes, Tiago R.; Caldwell, Michael W.; Talanda, Mateusz; Bernardi, Massimo; Palci, Alessandro; Vernygora, Oksana; Bernardini, Federico; Mancini, Lucia; Nydam, Randall L. (30 May 2018). "The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps". Nature. 557 (7707): 706–709. Bibcode:2018Natur.557..706S. doi:10.1038/s41586-018-0093-3. PMID 29849156. S2CID 44108416.
  12. ↑ Rage, Jean-Claude (December 2013). "Mesozoic and Cenozoic squamates of Europe". Palaeobiodiversity and Palaeoenvironments. 93 (4): 517–534. doi:10.1007/s12549-013-0124-x. ISSN 1867-1594. S2CID 128588324.
  13. ↑ Herrera-Flores, Jorge A.; Stubbs, Thomas L.; Benton, Michael J. (March 2021). "Ecomorphological diversification of squamates in the Cretaceous". Royal Society Open Science. 8 (3). doi:10.1098/rsos.201961. ISSN 2054-5703. PMC 8074880. PMID 33959350.
  14. ↑ Anantharaman, S.; DeMar, David G.; Sivakumar, R.; Dassarma, Dilip Chandra; Wilson Mantilla, Gregory P.; Wilson Mantilla, Jeffrey A. (2022-06-30). "First rhynchocephalian (Reptilia, Lepidosauria) from the Cretaceous–Paleogene of India". Journal of Vertebrate Paleontology. 42 (1). doi:10.1080/02724634.2022.2118059. ISSN 0272-4634.
  15. ↑ Romer, Alfred Sherwood; Parsons, Thomas Sturges (1986). The Vertebrate Body. Saunders. ISBN 978-0-03-058446-6.
  16. 1 2 3 4 5 6 7 8 9 10 11 Pough, F. Harvey; Andrews, Robin M.; Cadle, John E.; Crump, Martha L.; Savitzky, Alan H.; Wells, Kentwood D. (1998). Herpetology. Prentice Hall. ISBN 978-0-13-850876-0.
  17. 1 2 Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood, Perry L.; Sites, Jack W.; Wiens, John J. (24 March 2015). "Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa". PLOS ONE. 10 (3): e0118199. Bibcode:2015PLoSO..1018199R. doi:10.1371/journal.pone.0118199. PMC 4372529. PMID 25803280.
  18. ↑ Rieppel, O.; DeBraga, M. (1996). "Turtles as diapsid reptiles" (PDF). Nature. 384 (6608): 453–5. Bibcode:1996Natur.384..453R. doi:10.1038/384453a0. S2CID 4264378.
  19. ↑ Müller, Johannes (2004). "The relationships among diapsid reptiles and the influence of taxon selection" (PDF). In Arratia, G; Wilson, M.V.H.; Cloutier, R. (eds.). Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag Dr. Friedrich Pfeil. pp. 379–408. ISBN 978-3-89937-052-2.
  20. ↑ Mannen, Hideyuki; Li, Steven S. -L. (Oct 1999). "Molecular evidence for a clade of turtles". Molecular Phylogenetics and Evolution. 13 (1): 144–148. doi:10.1006/mpev.1999.0640. PMID 10508547.
  21. ↑ Zardoya, R.; Meyer, A. (1998). "Complete mitochondrial genome suggests diapsid affinities of turtles". Proc Natl Acad Sci U S A. 95 (24): 14226–14231. Bibcode:1998PNAS...9514226Z. doi:10.1073/pnas.95.24.14226. ISSN 0027-8424. PMC 24355. PMID 9826682.
  22. ↑ Iwabe, N.; Hara, Y.; Kumazawa, Y.; Shibamoto, K.; Saito, Y.; Miyata, T.; Katoh, K. (2004-12-29). "Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins". Molecular Biology and Evolution. 22 (4): 810–813. doi:10.1093/molbev/msi075. PMID 15625185.
  23. ↑ Roos, Jonas; Aggarwal, Ramesh K.; Janke, Axel (Nov 2007). "Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary". Molecular Phylogenetics and Evolution. 45 (2): 663–673. doi:10.1016/j.ympev.2007.06.018. PMID 17719245.
  24. ↑ Katsu, Y.; Braun, E. L.; Guillette, L. J. Jr.; Iguchi, T. (2010-03-17). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes". Cytogenetic and Genome Research. 127 (2–4): 79–93. doi:10.1159/000297715. PMID 20234127. S2CID 12116018.
  25. ↑ Tyler R. Lyson; Erik A. Sperling; Alysha M. Heimberg; Jacques A. Gauthier; Benjamin L. King; Kevin J. Peterson (2012-02-23). "MicroRNAs support a turtle + lizard clade". Biology Letters. 8 (1): 104–107. doi:10.1098/rsbl.2011.0477. PMC 3259949. PMID 21775315.
  26. ↑ Evans, S.E. (2003). "At the feet of the dinosaurs: the early history and radiation of lizards" (PDF). Biological Reviews. 78 (4): 513–551. doi:10.1017/S1464793103006134. PMID 14700390. S2CID 4845536. Archived from the original (PDF) on 2019-02-19.
  27. ↑ Evans, Susan E. (2016), Clack, Jennifer A.; Fay, Richard R; Popper, Arthur N. (eds.), "The Lepidosaurian Ear: Variations on a Theme", Evolution of the Vertebrate Ear, Cham: Springer International Publishing, vol. 59, pp. 245–284, doi:10.1007/978-3-319-46661-3_9, ISBN 978-3-319-46659-0, retrieved 2024-01-08
  28. 1 2 3 Bellairs, Angus d'A (1960). Reptiles life history, evolution, and structure. Harper. OCLC 692993911.
  29. 1 2 Benton, M. J (1988). The Phylogeny and classification of the tetrapods. Oxford. OCLC 681456805.
  30. ↑ Vidal, Nicolas; Hedges, S. Blair (February 2009). "The molecular evolutionary tree of lizards, snakes, and amphisbaenians". Comptes Rendus Biologies. 332 (2–3): 129–139. doi:10.1016/j.crvi.2008.07.010. PMID 19281946. S2CID 23137302.
  31. ↑ Simões, Tiago R.; Kinney-Broderick, Grace; Pierce, Stephanie E. (2022-03-03). "An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny". Communications Biology. 5 (1): 195. doi:10.1038/s42003-022-03144-y. ISSN 2399-3642. PMC 8894340. PMID 35241764.
  32. ↑ Smith, James G (2010). "Survival estimation in a long-lived monitor lizard: radio-tracking of Varanus mertensi". Population Ecology. 52: 243–247. doi:10.1007/s10144-009-0166-0. S2CID 43055329.
  33. 1 2 3 4 5 6 7 8 9 10 11 12 13 Zug, George R. (1993). Herpetology: An Introductory Biology of Amphibians and Reptiles. Academic Press. ISBN 978-0-12-782620-2.
  34. ↑ Functional morphology and evolution of aspiration breathing in tetrapods
  35. ↑ Jones, M.E.H.; O'Higgins, P.; Fagan, M.; Evans, S.E.; Curtis, N. (2012). "Shearing mechanics and the influence of a flexible symphysis during oral food processing in Sphenodon (Lepidosauria: Rhynchocephalia)". The Anatomical Record. 295 (7): 1075–1091. doi:10.1002/ar.22487. PMID 22644955. S2CID 45065504.
  36. ↑ Besson, A. A.; Cree, A. (2011). "Integrating physiology into conservation: an approach to help guide translocations of a rare reptile in a warming environment". Animal Conservation. 14: 28–37. doi:10.1111/j.1469-1795.2010.00386.x. S2CID 84015883.
  37. ↑ Nelson, Nicola J.; et al. (2002). "Establishing a new wild population of tuatara (Sphendon guntheri)". Conservation Biology. 16 (4): 887–894. doi:10.1046/j.1523-1739.2002.00381.x. S2CID 85262510.
  38. ↑ Towns, David R (2009). "Eradication as reverse invasion: lesions from Pacific Rat (Rattus exulans) removals on New Zealand islands". Biol Invasions. 11 (7): 1719–1733. doi:10.1007/s10530-008-9399-7. S2CID 44200993.
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