Wikipedia

Sauropsida

Sauropsida(Greekfor "lizard faces" ) is acladeofamniotes,broadly equivalent to theclassReptilia,though typically used in a broader sense to also include extinctstem-grouprelatives of modern reptiles andbirds(which, astheropoddinosaurs, are nested within reptiles as more closely related to crocodilians than to lizards or turtles).[2]The most popular definition states that Sauropsida is thesibling taxontoSynapsida,the other clade of amniotes which includesmammalsas its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are recognized as a subgroup ofarchosaurianreptiles despite originally being named as a separate class inLinnaean taxonomy.

Sauropsids
Temporal range:Pennsylvanian(LateCarboniferous)-Present[1]
Clockwise from top left:Pareiasaurus(an extinctpareiasaurianparareptile),Mesosaurus(an extinctmesosaurianparareptile),Smaug breyeri(alizard),Dinemellia dinemelli(thewhite-faced buffalo-weaver),Crocodylus niloticus(theNile crocodile), andLabidosaurikos(an extinctcaptorhinideureptile)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Sauropsida
Watson,1956
Subclades

The base of Sauropsida forks into two main groups of "reptiles":Eureptilia( "true reptiles" ) andParareptilia( "next to reptiles" ). Eureptilia encompasses all living reptiles (including birds), as well as various extinct groups.Parareptiliais typically considered to be an entirely extinct group, though a few hypotheses for the origin of turtles have suggested that they belong to the parareptiles. The cladesRecumbirostraandVaranopidae,traditionally thought to belepospondylsand synapsids respectively, may also bebasalsauropsids. The term "Sauropsida" originated in 1864 withThomas Henry Huxley,[3]who grouped birds with reptiles based on fossil evidence.

History of classification

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Huxley and the fossil gaps

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TheBerlin specimenofArchaeopteryx lithographica,a historically important fossil which helped to establish birds as a component of the reptile family tree

The termSauropsida( "lizard faces" ) has a long history, and hails back toThomas Henry Huxley,and his opinion that birds had risen from thedinosaurs.He based this chiefly on the fossils ofHesperornisandArchaeopteryx,that were starting to become known at the time.[4]In theHunterian lecturesdelivered at theRoyal College of Surgeonsin 1863, Huxley grouped the vertebrateclassesinformally intomammals,sauroids, and ichthyoids (the latter containing theanamniotes), based on the gaps in physiological traits and lack oftransitional fossilsthat seemed to exist between the three groups. Early in the following year he proposed the names Sauropsida andIchthyopsidafor the two latter.[3]Huxley did however include groups on the mammalian line (synapsids) likeDicynodonamong the sauropsids. Thus, under the original definition, Sauropsida contained not only the groups usually associated with it today, but also several groups that today are known to be in the mammalian side of the tree.[5]

Sauropsids redefined (Goodrich, 1916)

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By the early 20th century, the fossils ofPermiansynapsids fromSouth Africahad become well known, allowing palaeontologists to trace synapsid evolution in much greater detail. The term Sauropsida was taken up byE. S. Goodrichin 1916 much like Huxley's, to include lizards, birds and their relatives. He distinguished them frommammalsand their extinct relatives, which he included in the sister group Theropsida (now usually replaced with the nameSynapsida). Goodrich's classification thus differs somewhat from Huxley's, in which the non-mammalian synapsids (or at least thedicynodontians) fell under the sauropsids. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ( "first lizards" ), which included somePaleozoicamphibiansas well as earlyreptilespredating the sauropsid/synapsid split (and thus not true sauropsids). His concept differed from modern classifications in that he considered a modified fifthmetatarsalto be anapomorphyof the group, leading him to placeSauropterygia,Mesosauriaand possiblyIchthyosauriaandAraeoscelidain the Theropsida.[5]

Detailing the reptile family tree

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In 1956,D. M. S. Watsonobserved that sauropsids and synapsids diverged very early in the reptilian evolutionary history, and so he divided Goodrich's Protosauria between the two groups. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively, making themparaphyletic,unlike Goodrich's definition. Thus his Sauropsida includedProcolophonia,Eosuchia,Protorosauria,Millerosauria,Chelonia(turtles),Squamata(lizards and snakes),Rhynchocephalia,Rhynchosauria,Choristodera,Thalattosauria,Crocodilia,"thecodonts"(paraphyleticbasalArchosauria), non-aviandinosaurs,pterosaursandsauropyterygians.However, his concept differed from the modern one in that reptiles without anotic notch,such asaraeoscelidsandcaptorhinids,were believed to betheropsids.[6]

This classification supplemented, but was never as popular as, the classification of the reptiles (according toRomer's classicVertebrate Paleontology[7]) into four subclasses according to the positioning oftemporal fenestrae,openings in the sides of the skull behind the eyes. Since the advent ofphylogenetic nomenclature,the termReptiliahas fallen out of favor with many taxonomists, who have used Sauropsida in its place to include amonophyleticgroup containing the traditional reptiles and the birds.

Cladistic definitions

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Sauropsida and the 19th-/20th-century conception of the classReptilia.Both are superimposed on a cladogram ofTetrapods,showing the difference in coverage.

The class Reptilia has been known to be anevolutionary graderather than a clade for as long asevolutionhas been recognised. Reclassifying reptiles has been among the key aims ofphylogenetic nomenclature.[8]The term Sauropsida had from the mid 20th century been used to denote abranch-basedcladecontaining all amniote species which are not on the synapsid side of the split between reptiles and mammals. This group encompasses all now-living reptiles as well as birds, and as such is comparable to Goodrich's classification. The main difference is that better resolution of the early amniote tree has split up most of Goodrich's "Protosauria", though definitions of Sauropsida essentially identical to Huxley's (i.e. including the mammal-like reptiles) are also forwarded.[9][10]Some later cladistic work has used Sauropsida more restrictively, to signify thecrown group,i.e. all descendants of the last common ancestor ofextantreptiles and birds. A number of phylogenetic stem, node and crown definitions have been published, anchored in a variety of fossil and extant organisms, thus there is currently no consensus of the actual definition (and thus content) of Sauropsida as a phylogenetic unit.[11]

Some taxonomists, such as Benton (2004), have co-opted the term to fit into traditional rank-based classifications, making Sauropsida and Synapsida class-level taxa to replace the traditional Class Reptilia, while Modesto and Anderson (2004), using thePhyloCodestandard, have suggested replacing the name Sauropsida with their redefinition of Reptilia, arguing that the latter is by far better known and should have priority.[11]

Cladistic definitions of Sauropsida include:

  • Sauropsida as thetotal groupof reptiles: "Reptiles plus all other amniotes more closely related to them than they are to mammals" (Gauthier, 1994).[2]This is a branch-based total group definition. Gauthier (1994) considered turtles to be descended from parareptiles, thus defining Reptilia as a more restricted crown group encompassing diapsids and parareptiles (apart from mesosaurs, which he considered to be the most basal branch of sauropsids).
    • Sauropsida as a total group, synonymous with Reptilia sensu lato: "The most inclusive clade containingLacerta agilisandCrocodylus niloticus,but notHomo sapiens"(Modesto & Anderson, 2004).[11]This total group definition leaves the question of turtle ancestry unresolved.
  • Sauropsida as a broadnode-basedgroup: "The last common ancestor of mesosaurs, testudines and diapsids, and all its descendants" (Laurin & Reisz, 1995).[12]Though formulated differently, this grouping was similar in scope and intention to the definition provided by Gauthier (1994).

Evolutionary history

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Main article:Evolution of reptiles
Mesozoic sauropsids: non-avialandinosaurs(Europasaurusandiguanodonts) alongside the early birdArchaeopteryxperched on the foreground tree stump.

Sauropsids evolved from basal amniotes approximately 320 million years ago, in theCarboniferousPeriod of thePaleozoicEra. In theMesozoicEra (from about 250 million years ago to about 66 million years ago), sauropsids were the largest animals on land, in the water, and in the air. The Mesozoic is sometimes called the Age of Reptiles. In theCretaceous–Paleogene extinction event,the large-bodied sauropsids died out in theglobal extinction eventat the end of the Mesozoic era. With the exception of a few species of birds, the entire dinosaur lineage became extinct; in the following era, theCenozoic,the remaining birds diversified so extensively that, today, nearly one out of every three species of land vertebrate is a bird species.

Phylogeny

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Thecladogrampresented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[13]Allgeneticstudies have supported the hypothesis that turtles (formerly categorized together with ancientanapsids) are diapsid reptiles, despite lacking any skull openings behind their eye sockets; some studies have even placed turtles among thearchosaurs,[13][14][15][16][17][18]though a few have recovered turtles as lepidosauromorphs instead.[19]The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[13]

Amniota

Synapsida(mammalsand their extinct relatives)

Sauropsida/
†Parareptilia
Eureptilia
Reptilia(total group)

Laurin & Piñeiro (2017) and Modesto (2019) proposed an alternate phylogeny of basal sauropsids. In this tree, parareptiles include turtles and are closely related to non-araeoscelidian diapsids. The familyVaranopidae,otherwise included inSynapsida,is considered by Modesto a sauropsid group.[20][21]

In recent studies, the "microsaur"cladeRecumbirostra,historically consideredlepospondylreptiliomorphs, have been recovered as early sauropsids.[22][23]

A 2024 study definesCaptorhinidaeandAraeoscelidiaas sister groups that split off before the formation of crown amniota (synapsids and sauropsids). The same study also considers parareptiles to be polyphyletic, with some groups being closer to the crown group of reptiles than others.[24]

Structure difference with synapsids

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The last common ancestor of synapsids and Sauropsida lived at around 320mya during Carboniferous, known asReptiliomorpha.

Thermal and secretion

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The earlysynapsidsinherited abundant glands on their skins from their amphibian ancestors. Those glands evolved into sweat glands in synapsids, which granted them the ability to maintain constant body temperature but made them unable to save water from evaporation. Moreover, the way synapsids discharge nitrogenous waste is throughurea,which is toxic and must be dissolved in water to be secreted. Unfortunately, the upcomingPermianandTriassicperiods were arid periods. As a result, only a small percent of early synapsids survived in the land from South Africa to Antarctica in today's geography. Unlike synapsids, sauropsids do not have those glands on the skin; their way of nitrogenous waste emission is throughuric acidwhich does not require water and can be excreted with feces. As a result, sauropsids were able to expand to all environments and reach their pinnacle. Even today, most vertebrates that live in arid environments are sauropsids, snakes and desert lizards for example.

Brain structure

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Different from howsynapsidshave their cortex in six different layers of neurons which is calledneocortex,the cerebrum of Sauropsida has a completely different structure. For the corresponding structure of the cerebrum in the classic view, the neocortex of synapsids is homology with only theArchicortexof the avian brain. However, in the modern view appeared since the 1960s, behavioral studies suggested that avianneostriatumandhyperstriatumcan receive signals of vision, hearing, and body sensations, which means they act just like the neocortex. Comparing an avian brain to that to a mammal,nuclear-to-layered hypothesisproposed by Karten (1969), suggested that the cells which form layers in synapsids' neocortex, gather individually by type and form several nuclei. For synapsids, when one new function is adapted in evolution it will be assigned to a separate area of cortex, so for each function, synapsids will have to develop a separate area of cortex, and damage to that specific cortex may cause disability.[25]However, for Sauropsida functions are disassembled and assigned to all nuclei. In this case, brain function is highly flexible for Sauropsida, even with a small brain, many Sauropsida can still have a relatively high intelligence compared to mammals, for example, birds in the family Corvidae. So, it is possible that some non-avian dinosaurs, likeTyrannosaurus,which had tiny brains compared to their enormous body size, were more intelligent than previously thought.[26]

References

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  1. ^Marjanović, D. (2021)."The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates".Frontiers in Genetics.12.521693.doi:10.3389/fgene.2021.521693.PMC8149952.PMID34054911.
  2. ^abGauthier J.A. (1994):The diversification of the amniotes.In: D.R. Prothero and R.M. Schoch (ed.) Major Features of Vertebrate Evolution: 129–159. Knoxville, Tennessee: The Paleontological Society.
  3. ^abHuxley, Thomas Henry (1864)."The Structure and Classification of the Mammalia".Medical Times and Gazette.Huxley Archives.Retrieved2023-03-16.
  4. ^Huxley, Thomas Henry(1877)."Lectures on Evolution".Collected Essays IV.Retrieved2023-03-16.{{cite book}}:|website=ignored (help)
  5. ^abGoodrich, E.S. (1916). "On the classification of the Reptilia".Proceedings of the Royal Society of London.89B(615): 261–276.Bibcode:1916RSPSB..89..261G.doi:10.1098/rspb.1916.0012.
  6. ^Watson, D.M.S. (1957). "On Millerosaurus and the early history of the sauropsid reptiles".Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.240(673): 325–400.Bibcode:1957RSPTB.240..325W.doi:10.1098/rstb.1957.0003.
  7. ^Romer, A.S. (1933).Vertebrate Paleontology.University of Chicago Press.,3rd ed., 1966.
  8. ^Gauthier,.A., Kluge, A.G & Rowe, T. (1988). The early evolution of the Amniota. Pages 103–155 in Michael J. Benton (ed.): The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst. Ass. Spec. Vol. 35A. Clarendon Press, Oxford.
  9. ^Laurin, Michel; Gauthier, Jacques (January 1996)."Amniota. Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives".Tree of Life Web Project.Archived from the original on 10 April 2006.Retrieved2023-03-16.{{cite web}}:CS1 maint: unfit URL (link)
  10. ^Pearse, A.S. (ed, 1947): Zoological Names: a List of Phyla, Classes, and Orders. Prepared forSection F, American Association for the Advancement of Science.Second edition. Durham, North Carolina, U.S.A., pp. 1-22
  11. ^abcModesto, S.P.; Anderson, J.S. (2004)."The phylogenetic definition of Reptilia".Systematic Biology.53(5): 815–821.doi:10.1080/10635150490503026.PMID15545258.
  12. ^Laurin, Michel; Reisz, Robert R. (1995)."A reevaluation of early amniote phylogeny".Zoological Journal of the Linnean Society.113(2): 165–223.doi:10.1111/j.1096-3642.1995.tb00932.x.
  13. ^abcLee, M.S.Y. (2013)."Turtle origins: Insights from phylogenetic retrofitting and molecular scaffolds".Journal of Evolutionary Biology.26(12): 2729–2738.doi:10.1111/jeb.12268.PMID24256520.S2CID2106400.
  14. ^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.PMID10508547.
  15. ^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.ISSN0027-8424.PMC24355.PMID9826682.
  16. ^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.PMID15625185.
  17. ^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.PMID17719245.
  18. ^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.PMID20234127.S2CID12116018.
  19. ^Lyson, Tyler R.; Sperling, Erik A.; Heimberg, Alysha M.; Gauthier, Jacques A.; King, Benjamin L.; Peterson, Kevin J. (2012-02-23)."MicroRNAs support a turtle + lizard clade".Biology Letters.8(1): 104–107.doi:10.1098/rsbl.2011.0477.PMC3259949.PMID21775315.
  20. ^Laurin, Michel; Piñeiro, Graciela H. (2017)."A reassessment of the taxonomic position of mesosaurs, and a surprising phylogeny of early amniotes".Frontiers in Earth Science.5:88.Bibcode:2017FrEaS...5...88L.doi:10.3389/feart.2017.00088.hdl:20.500.12008/33548.ISSN2296-6463.
  21. ^Modesto, Sean P. (January 2020)."Rooting about reptile relationships".Nature Ecology & Evolution.4(1): 10–11.doi:10.1038/s41559-019-1074-0.ISSN2397-334X.PMID31900449.S2CID209672518.Retrieved29 December2020.
  22. ^Pardo, Jason D.; Szostakiwskyj, Matt; Ahlberg, Per E.; Anderson, Jason S. (June 2017)."Hidden morphological diversity among early tetrapods".Nature.546(7660): 642–645.Bibcode:2017Natur.546..642P.doi:10.1038/nature22966.hdl:1880/113382.ISSN1476-4687.PMID28636600.S2CID2478132.
  23. ^Mann, Arjan; Pardo, Jason D.; Maddin, Hillary C. (2019-09-30)."Infernovenator steenae, a new serpentine recumbirostran from the 'Mazon Creek' Lagerstätte further clarifies lysorophian origins".Zoological Journal of the Linnean Society.187(2): 506–517.doi:10.1093/zoolinnean/zlz026.ISSN0024-4082.
  24. ^Successive climate crises in the deep past drove the early evolution and radiation of reptiles
  25. ^Karten, H. J. in Comparative and Evolutionary Aspects of the Vertebrate Central Nervous System (ed. Pertras, J.) 164–179 (1969).
  26. ^Jarvis, Güntürkün, O., Bruce, L., Csillag, A., Karten, H., Kuenzel, W., Medina, L., Paxinos, G., Perkel, D. J., Shimizu, T., Striedter, G., Wild, J. M., Ball, G. F., Dugas-Ford, J., Durand, S. E., Hough, G. E., Husband, S., Kubikova, L., Lee, D. W.,... Butler, A. B. (2005). "Avian brains and a new understanding of vertebrate brain evolution".Nature Reviews.Neuroscience, 6(2), 151–159.doi:10.1038/nrn1606.
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